As of today, Earth is home to more than 7.6 billion humans spread over 200 countries across 510 million km^2 of landmass. Every country’s human population is constantly changing, which attributes to immigration, mass migration, birth and deaths rates. In the modern era, the invention of automobiles, public transportation, aircraft, railways, sea ferries, ship and boats minimised journey time for enormous travel distances between distant countries. According to Flight tracker Overview, about 10,710 aircraft are airborne servicing 636,859,278 flights carrying more than 11 million passengers every day. However, these complex and convenient machines didn’t exist before the invention of the wheel and rotary turbine. So how did the earliest humans cover such large distances before roads existed? The answer is walking. Note that the earliest humans didn’t have access to any comfortable shoes that modern humans wear today, so they had to walk barefoot. You can imagine how painful walking barefoot on rough, barren and uneven surfaces covered with sharp rocks, tree branches and fallen leaves would be. Furthermore, the soil they walk on is home to billions of bacteria like E. Coli, faecal matter, non-winged insects and camouflaged predators, which means the earliest humans were susceptible to predatory ambushes, bacterial and viral infections or toxic bites. So how did some of our ancestors have the motivation, resilience and endurance to go the distance and explore all of Earth’s landmass despite the treacherous terrain, harsh weather conditions, and survive being mauled by carnivorous predators? Let’s find out.
Human migration is defined as the movement of humans from 1 location to another with the intention of permanent or temporary settlement in a novel location.
This diagram shows the net migration rate worldwide.
According to the World Bank’s 2011 Migration and Remittances Factbook, it estimated an exponentially increasing trend of immigrants from 215.8 million in 2010 to nearly 250 million in 2015. A 2013 International Migration wall chart estimated a large proportion of immigrants were female, often migrating along or with their family members and community. Countries with substantial immigration rates include the United States, Russia, Germany, Saudi Arabia, Canada, UK, France, Australia and India. A majority of immigrants originate from India, Mexico, China, Ukraine, Bangladesh, Pakistan, Philippines, South Korea, Syria, Turkey and the UK. The busiest migration corridors worldwide include:
— Libra-European Union
— Mexico-United States
— Morocco-European Union
— Russia-Ukraine
— Ukraine-Russia
— Bangladesh-India
— Nepal-India
— Turkey-Germany
— South Asia-GCC Countries
— Algeria-France
— Kazakhstan-Russia
— Ukraine-Poland
— Russia-Kazakhstan
— Cuba-United States
— China-Northern America
— India-Northern America
— Philippines-Northern America
— Vietnam-Northern America
— South Korea-Northern America
— China-Australia
— Mainland China-Hong Kong
— Vietnam-Australia
— Hong Kong-Canada
Why do humans migrate?
Several laws of social science have laid out numerous reasons to explain human migration. In the 1880s, Ravenstein’s laws proposed:
— Every migration flow generates a return or counter migration.
— A majority of migrants move a short distance.
— Migrants who move longer distances tend to choose big-city destinations.
— Urban residents are often less migratory than inhabitants of rural areas.
— Families are less likely to make international moves than young adults.
— Most migrants are adults.
— Large towns grow by migration rather than natural increase.
— Migration occurs in stages.
— Differences between urban and rural migrations.
— Technology influences migration.
— Economic circumstances stimulates migration.
In 1966, Everett S. Lee proposed laws that categorised factors causing migrations into either push or pull factors. Push factors refer to unfavourable conditions in the area that a person lives in, whereas pull factors refer to attractive conditions in foreign areas.
(a) Push factors:
— Inadequate jobs
— Lack of employment or entrepreneurial opportunities
— Inadequate conditions
— Desertification
— Famine or drought
— Slavery or forced labour
— Abysmal medical care
— Loss of wealth
— Natural disasters
— Death threats
— Desire for political or religious freedom
— Lack of political or religious rights
— Threat of arrest or punishment
— Pollution
— Appalling housing
— Landlord / tenancy issues
— Bullying
— Mentality
— Discrimination
— Persecution or intolerance based on race, religion, gender or sexual orientation
— Slim chances of marriage
— Condemned housing (radon gas, etc.)
— War
— Shortage of farmland
— Difficult to start new farms
— Oppressive legal or political conditions
— Struggling or Failing economy
— Military draft, warfare or terrorism
— Cultural clashes and feuds
— Expulsion by armed force or coercion
— Overpopulation
— Low wages
(b) Pull factors:
— Vast job opportunities
— Favourable living conditions
— Political or religious freedom
— Enjoyable lifestyle
— Access to education
— Access to advanced medical care
— Attractive climates
— Tighter security
— Family links i.e. chain migration
— Large industries
— Higher chances of marriage
— Cheap purchase of farmland
— Quick and/or substantial wealth
— Prepaid travel
— Access to improved welfare programmes
— Building a new nation
— Building specific cultural or religious communities
— Cultural opportunities
— Easygoing to across the boundaries
A 1999 article by Davin Dalton suggested humans migrate seeking for a lifestyle that was more desirable, habitable, luxurious and fulfilling to themselves and their families compared to their current deplorable, arduous, tiresome, unrewarding and unhygienic lifestyles. Some people migrate for economic purposes seeking for vast, secure and profitable economic opportunities. This ‘economic migration’ is triggered by push factors such as poverty, inadequate economic opportunities, land shortage, and low home living standards, as well as pull factors such as prosperity, vast opportunities, available employment and higher living standards. Those living in developing countries who considered migrating to foreign developed countries had to factor in potential costs and benefits of migration such as plane tickets, accommodation, taxi rides, chances of successfully landing a job, and the prevailing wage rates relative to their former job. Other factors swaying towards migration include existing contacts with relatives and friends, and the potential effect of leaving behind their household. In this modern day and age, social media communications allows the sharing of knowledge of updated conditions in the countries people consider migrating to. Social media apps such as Facebook, Instagram, WeChat, as well as documentaries, tv shows showcasing popular holiday destinations and anecdotes from returnees can provide such valuable information.
A 2012 article suggested the successive waves of Eurasian nomadic movement throughout history may be caused by climatic cycles that expanded or contracted pastureland in Central Asia, especially Mongolia and Altai to the west. It’s known that people back then were displaced from their home patch by other tribes searching for fertile land that can be grazed by essential flocks. Each group forced the next further southwards and westwards, into the highlands of Anatolia, the Pannonian Plain, into Mesopotamia, or southwards, into the rich pastures of China. Bogumil Terminski described this process in the context of the Sea People invasion as the “migratory domino effect”.
— A 2008 Idyorough article proposed that migrated occurs due to individuals’ scavenge hunt for food, sex and security outside their usual habitation. It suggested that towns and cities were established from human struggle in an attempt to obtain food, sex and security. Early humans may have necessarily moved out of their usual habitation and enter into indispensable social relationships that may be cooperative or antagonistic, in order to produce food, security and reproduction. To achieve this, humans developed the handy tools and equipment that could interact with nature.
— Zipf’s Inverse Distance Law
— Gravity model of migration
— Friction of distance
— Radiation law for human mobility
— Buffer theory
— Stouffer’s 1940 theory of intervening opportunities
— Zelinsky’s 1971 Mobility Transition Model
— Bauder’s 2006 regulation of labour markets: It suggested international migration of workers was necessary for the survival of industrialised economies.
Immigration is defined as the international movement of people into a destination country of which they aren’t natives, citizens, permanent residents or foreign / migrant workers. The word ‘immigration’ was coined in the 17th century, referring to non-warlike population movements between the emerging nation states. This often confuses with migration and emigration. The words ‘migrant’ or ‘immigrant’ originate from the Latin word migrare, meaning wanderer. They are defined as people crossing national borders during their migration, according to the perspective of the foreign country which they enter. The words ‘emigrant’ or ‘outmigrant’ are defined as people who leave according to the perspective of the country they leave the border.
According to a 2015 article published by the United Nations, more than 250 million people have migrated worldwide, which reflects a 41% increase since the beginning of the millennium. 1/3 of the world's international migrants are currently living in just 20 countries. A majority of international migrants live in the United States (19%), with Germany and Russia (12 million each) not too far behind. These countries are followed by Saudi Arabia (10 million), UK (9 million) and United Arab Emirates (8 million). Overall, Asia is the largest recipient of international migrants than any other continent in the world, with about 26 million migrants. Behind it is Europe with about 20 million migrants. Migration often occurs between countries situated within the same continent or between neighbouring continents. Furthermore, about 37 million international migrants were younger than 20 years of age and 177 million migrants were aged between 20 and 64. The youngest international migrants emigrated from Africa (median age of 29), followed by Asia (35 years), and Latin America/Caribbean (36 years), while Northern American (42 years), European (43 years) and Oceanian (44 years) migrants tended to be older. About 43% of emigrants were Asian, 25% of emigrants were European and 15% of emigrants were Latin American.
This diagram shows the number of migrants and migrant workers per country in 2015.
Albert Einstein once said “life is like riding a bicycle. To keep your balance, you must keep moving.” Or should you?
What motivates ancient humans to move from their old home and another home?
Why did they choose to move in a certain direction over any other direction?
Was this choice random or did some environmental marker stimulated an instinctual decision?
In 2005, archeologists discovered fossil remains in Kenya’s East African Rift Valley that contained osteological elements of both chimpanzees and humans dating between 545 and 284 kyr (thousand years, radiometric). This lead to hypotheses that the chimpanzee-human last common ancestor (CHLCA) shared physiological, physical, anatomical and biological features with Homo (human) and Pan (chimpanzee and bonobo) genera of the Hominini taxon.
This model illustrates the estimated speciation of the Hominini and Gorillini over the past 10 million years. The hybridisation process within Hominini is indicated as ongoing roughly 8-6 million years ago.
In the first episode of Mind Field Season 3, VSauce (Michael Stevens) explained some CHLCA’s were banished from the safe habitat of the trees, possibly due to weak climbing skills, and ventured into the dangerous savannah where many of its predators lived such as tigers, cheetahs, and snakes. In order to survive in the treacherous savannah for an indefinite period of time, they had to develop novel communication techniques like verbal language, design new craftwork like tools and weapons, and adjust to unfamiliar novel environments like lack of trees, hills, and vast open spaces. This may have lead to neuroanatomical changes in their brain which sacrificed brain regions tasked in short-term memory for new functions in verbal and non-verbal language, craftsmanship, combat and warfare. This is known as the “Cognitive Tradeoff Hypothesis”. You can watch the episode by clicking on the link below.
— Daniel Wolpert: “We have a brain for one reason and one reason only — and that’s to produce adaptable and complex movements.”
— Ido Portal: “ The body will become better at whatever you do, or don’t do. You don’t move? The body will make you better at NOT moving. If you move, your body will allow more movement.”
— Erwan Le Corre: “To wild animals, movement is not a chore, not a temporary punishment for being physically lazy and out of shape, not an optional activity just for better looks.”
These above names are teachers of the Ben Medder Coaching Unit. They all claim that humans were meant to move physically. Besides obvious physical attributes such as strength and endurance, they argue the human body contributes to emotions, learning and relationships, as well as cognitive processes, understanding and decision making. This indicates the inseparable connections between the mind and the body, from the endocrine system to then enteric system, i.e. “The body is your brain.”
There are concerns that most of the human population seek “anti-fragile” experiences, but this often constitutes extreme inactivity, minimal exertion or inappropriate intensities that severely stresses the organism. In a fitness context, “the dose makes the poison” and each individual’s equilibrium is different. Although exercise is exerted movement, repetitions and overloaded weight exercises would still lead you to a sense of denial of a delusional lifestyle. Even you successfully complete a “work0out” to achieve an external goal, if you didn’t enjoy that process, the goals you actually seek are short-sighted and unsustainable. In 1998, Gabbard claims that early movement experiences are beneficial to optimal brain development in early childhood, suggesting all learning is physical no matter its abstractness.
A study found declining physical activities in the UK (by 20%) and China (by 45%) within the last 2 generations due to advancing technologies, machines and vehicular transport doing the movement for humans. Humans’ use of leisure time doesn’t compensate for the lost physical inactivity. It seems living a sedentary lifestyle has become the norm in modern human society. A modern human with access to luxury and survival necessities don’t have the incentive to run, climb or jump because they are not forced to move to catch their prey or avoid being prey to vicious predators. This generates a disconnection between the mind and the body. Most of the human population are employed in careers that don’t require physical body movement, such as office desktop jobs where they sit at their desks clicking the mouse buttons and typing on the keyboards. It seems modern humans have forgotten or ignored the narrative and relationship between their bodies and their movement. We only notice our bodies when we feel pain, cramp, injury, rigidity or abnormal movements. Humans have effectively betrayed their own bodies, ignored and dishonoured them by utilising them as “locomotive devices” to transport their head containing the brain.
You may ask “exercise is optional, movement is essential, but what is the difference?” Exercise is regarded as a modern invention, an obligation or chore to escape the cramping, sedentary and mundane routines to allow our bodies to let loose physically. But this motivation to exercise is driven by pain rather than pleasure-seeking. In addition, exercise is generally focused on specificity of muscle movements in certain directions, which lacks real skill development. Most exercise regimes uses machines and isolated exercises that seems to expand our fitness repertoire and experts in impractical and limited movements.
On the other hand, movement is more ancient, historical, traditional and unique. This included hunting and gathering, dancing around the bonfire, walking, climbing, running, jumping, crawling, lifting, swimming, fighting, and sexual intercourse. The human body was created to enact all these movements. Recent public health research are investigating the need for more movement in our lives, as opposed to exercise, in order to remain physically healthy.
A few decades ago, the rise of bodybuilding attributes to the visual impact and imagery of exercise, which seems to influence on the history of the current “fitness” paradigm. Undoubtedly, bodybuilders are dedicated to the sport but unfortunately they post a negative effect on the fitness culture as a whole was that of isolation purely for cosmetic motivation. However, it is promising to see a positive shift in paradigms. More people realising the need for more than just exercise for aesthetic reasons, but rather “functional training” i.e. movement. They believe training should “train movement, not muscles.” Movement may be meaningful, which allows people to be aware of and enjoying the present (now).
For many generations, scores of humans have moved from place to place, particularly different countries, intending to settle temporarily or permanently in a new location. This involves traversing over long distances and from 1 country or region to another.
Studies theorised that pre-modern migration began:
— About 1.75 million years ago, pre-modern migration began with the Homo Erectus migrating from Africa to Eurasia.
— About 150,000 years ago, Homo sapiens occupied most of Africa.
— About 70,000 - 125,000 or 270,000 years ago, some members of Homo sapiens moved out of Africa to, possibly, Asia.
— By around 40,000 BC, other members of Homo sapiens migrated across Australia, Asia and Europe.
— Around 15,000 - 20,000 years ago, Homo sapiens migrated to the Americas (North & South America).
— Around 2,000 years ago, humans established settlements in the Pacific Islands.
Notable major population movements associated with the Neolithic Revolution and with the Indo-European expansion. Traces of the Turkic expansion as part of the Early Medieval Great Migrations remain. Tatjana et al. (2002) found that the migration of relatively small elite populations culturally transformed countries such as Turkey and Azerbaijan. Weale et al. (2002) theorised that substantial migration of Anglo-Saxon Y chromosomes into Central England (contributing 50%–100% to the gene pool at that time) contributed to the elite-migration parallels in the Roman and Norman conquests of Britain, though this is widely debated.
Climate change, landscape change, and inadequate food-supply were some of the many factors that contributed to early human migration. Historians suggested about 8000 years ago, ancestors of the Austronesian peoples spread from the South Chinese mainland to the island of Taiwan. Historical linguistics suggested that seafaring people migrated from Taiwan, in distinct waves separated by millennia, to an entire region by the Austronesian languages, around 6000 years ago. Around the Middle to Late Bronze Age, contemporary with the Late Harappan phase in India (around 1700 to 1300 BC), the Indo-Aryan migration from the Indus Valley to the plain of the River Ganges in Northern India may have occurred. Beginning around 180 BC, a series of invasions from Central Asia followed in the northwestern Indian subcontinent, including those led by the Indo-Greeks, Indo-Scythians, Indo-Parthians, and Kushans.
From 728 BC, the Greeks began expanding by colonising in several places, including Sicily and Marseille. Those major migration movements include the Celtic group in the first millennium BC, followed by the Migration Period of the first millennium AD from the North and East.
— Around 9th century AD, a sub-migration of Magyans into Pannonia (modern-day Hungary) occurred.
— Between 6th and 11th century AD, the Turkic migrated from their homeland in modern Turkestan across most of Central Asia into Europe and the Middle East.
— Around the 5th and 6th centuries AD, Madagascar may have been uninhabited until Austronesian seafarers arrived from Indonesia. Subsequent migrations from both the Pacific and Africa caused the emergence of the Malagsy people.
This map shows the chronological dispersion of Austronesian people across the Indo-Pacific.
It is thought the southern half of Africa was populated by Pygmies and Khoisan speakers prior to expansion of speakers of the Banju languages. These speakers’ descendants today occupy the arid regions around the Kalahari Desert and the forests of Central Africa.
— By around 1000 AD, Bantu peoples migrated to the modern-day Zimbabwe and South Africa.
— Between the 11th and 13th centuries, the Arab Bedouin tribes consisting of Banu Hilal and Banu Ma’qil migrated westwards from the Arabian Peninsula via Egypt. This lead to the Arabic and Islamic influence on the western Maghreb, until it was then dominated by Berber tribes. Ostsiedlung was the medieval eastward migration and settlement of Germans.
— During the 13th century, Mongol and Turkic migrates occurred across Eurasia.
Between 11th and 18th centuries, many Asian migrations occurred.
— In the 13th century, the Vatsayan Priests migrated from the eastern Himalaya hills to Kashmir during the Shan invasion. They may have settled in the lower Shivalik Hills to sanctify the manifest goddess.
— In the 11th century, the Vietnamese began expanding southward in the Ming occupation, known as “nam tiến” (Vietnamese for southward expansion).
— During the early Qing Dynasty (starting in 1636), Inner Willow Palisade separated Manchuria from China proper, restricting the movement of the Han Chinese into Manchuria. This occurred due to the area being off-limits to the Han, before the Qing’s colonisation of the area with them (late 18th century) later on in the dynasty's rule.
Since Early Modern times, migration seemed to gather at accelerating pace with the Age of Exploration and European colonialism.
— In the 16th century, American ports were estimated to be infiltrated with 240,000 European migrants.
— In the 19th century, over 50 million Europeans migrated to the Americans alone.
— The influx of incoming settlers numerically overwhelmed the local populations or tribes, such as the Aboriginal people in Canada, Brazil, Argentina, Australia, Japan and United States.
This map shows the migratory movements of the Migration Period between the 4th and 6th centuries.
What are the prehistoric migrations?
Prior to the end of the Last Glacial Maximum, paleolithic migration of anatomically modern humans occurred throughout Afro-Eurasia and to the Americas. During the Holocene climatic optimum, formerly isolated populations started shifting and merging, leading to the pre-modern distribution of the world's major language families. Following the population movements of the Mesolithic, the Neolithic revolution occurred, followed by the Indo-European expansion in Eurasia and the Bantu expansion in Africa. Population movements of the proto-historical or early historical period include the Migration period, followed by (or connected to) the Slavic, Magyar Norse, Turkic and Mongol expansions of the medieval period. Around the 1st millennium AD, the Pacific Islands and the Arctic were the last regions to permanently settled. Migration on the intercontinental scale has gathered at an accelerating pace since the beginning of both the Age of Exploration and the Early Modern period and its emerging colonial empires.
— Urheimat
(a) Neolithic to Chaocolithic
This map illustrates the Neolithic expansions from the 7th to the 5th millennium BC.
Historians believed agriculture was first practiced around 10,000 BC in the First Crescent. This lead to the wave-like propagation across Europe, reaching northern Europe around 5 millennia ago, which is supported by Archaeogenetics. Busby et al. (2016) suggested admixture originated from an ancient migration in Eurasia into parts of Sub-Saharan Africa. Ramsay et al. (2018) found evidence of ancient Eurasians migrating into Africa with the Eurasian admixture in modern Sub-Saharan Africans ranging from 0% to 50%. It is higher ) in the Horn of Africa and parts of the Sahel Zone.
(b) Bronze Age
The Indo-European migration is dated to the end of the Neolithic, the early Neolithic and the late Palaeolithic eras.
- Neolithic cultures included the Corded Ware culture, Yamna culture and Kurgan culture.
- Early Neolithic cultures included the Starčevo-Körös & Linearbandkeramic.
- Late Palaeolithic culture is based on the Palaeolithic Community Theory.
This diagram illustrates the scheme of Indo-European migrations from c. 4000 to 1000 BC according to the Kurdan hypothesis. The purple area corresponds to the assumed Urheimat (Samara culture, Sredny Stog culture). The red area corresponds to the area which may have been settled by Indo-European-speaking peoples up to c. 2500 BC; the orange area to 1000 BC.
Speakers of the Proto-Indo-European language may have originated from the northern part of the Black Sea (which is today’s Eastern Ukraine and Southern Russia). From this point, they may have migrated and spread their language by cultural diffusion to Anatolia, Europe, and Central Asia Iran and South Asia around the beginning of the Neolithic period. Colin Renfrew hypothesised the development of such languages occurred in Anatolia, and agricultural revolution in the early Neolithic lead to the culture spread and Indo-European expansion.
Not much is known about the inhabitants of pre-Indo-European “Old Europe”. So far, it is known the Basque language and the indigenous languages of the Caucasus remained from the era. As part of the Uralic languages, the Sami languages dispersed into Europe around the same period as the Indo-European languages. It is argued that since that period speakers of other Uralic languages such as the Finns and the Estonians may have contacted with other Europeans more often, which lead to increased genetic admixture.
Based on available sources, the earliest migrations that can reconstructed are dated around 2nd millennium BC. From circa 2000 BC the Proto-Indo-Iranians dispersed, with the Rigveda documenting the presence of early Indo-Aryans in the Punjab from the late 2nd millennium BC. Assyrian sources placed Iranian tribes in the Iranian plateau from the 9th century BC. In the Late Bronze Age, the Aegean and Anatolia were overrun by moving populations called the “Sea Peoples”, which led to the collapse of the Hittite Empire and ushering in the Iron Age.
(c) Austronesian Expansion
During the circa 1600 BC and AD 1000, the islands of the Pacific were becoming populated. Originating from Austronesia, probably New Guinea, the Lapita people, named after an archaeological site in Lapita, New Caledonia, were thought to have migrated to the Solomon Islands around 1600 BC, and later to Fiji, Samoa and Tonga. By the beginning of the 1st millennium BC, a majority of Polynesia became home to thriving cultures that settled on the islands' coasts and lived off the sea. Colonisation of Micronesia was complete by 500 BC and around 1000 AD New Zealand was the last region of Polynesia to be colonised.
(d) Bantu Expansion
Oliver Roland (1966) described the Bantu expansion as the major prehistoric migratory pattern that shaped the ethno-linguistic composition of Sub-Saharan Africa. The Bantu people branched from the Niger-Congo phylum, which originated from West Africa around the Benue-Cross rivers area in southeastern Nigeria. Around the start of the 2nd millennium BC, the Bantu migrated to Central Africa. 1000 years later, they migrated in a southeastern direction, spreading pastoralist and agriculture values. Around 2000 years later, they populated the southern part of Africa, which displaced the Khoisan languages indigenous to Central and Southern Africa with Bantu languages.
(e) Arctic peoples
The Arctic is the last region settled by the humans, which is the Dorset culture between around 500 BC and 1500 AD. Bruce Rigby (2006) and Shannon Raye Wood (2009) suggested the Inuit, descendants of the Thule culture, emerged from from western Alaska around AD 1000 and gradully dispersed the Dorset culture.
What are the proto-historical and early historical migrations?
Landnahme, the German word for “land-taking”, describes a migration event associated with a founding legend. Examples include:
— The conquest of Canaan in the Hebrew Bible
— The Indo-Aryan migration and expansion within India according to the Rigveda
— The Invasion traditions in the Irish Mythological Cycle, which accounted for the arrival of Gaels in Ireland and Franks in Austrasia during the Migration period.
— The Anglo-Saxon invasion of Britain
— The settlement of Iceland in the Viking Age
— The Slavic migrations
— The Hungarian conquest
(i) Iron Age
The Greek Dark Ages began upon the Dorian invasion of Greece. The Urartians were displaced by Armenians, and the Cimmerians and the Mushki migrated from the Caucasus into Anatolia. A Thraco-Cimmerian connection associates these migrations to the Proto-Celtic world of central Europe. This lead to the introduction of Iron to Europe and the Celtic expansion to western Europe and the British Isles around 500 BC.
This diagram illustrates the Celtic expansion in Europe, 6th–3rd century BC.
(ii) Migration period
— Migration Period
— Turkic Migration
This period of migrations refers to the separation of Antiquity from the Middle Ages in Europe as the ‘Migration Period’, which is further divided into 2 phases.
1. From 300 to 500 AD, migration Germanic, Sarmanitan and Hunnic tribes began before ending with their settlement in the areas of the former Western Roman Empire. Examples of tribes include Ostrogoths, Visigoths, Burgundians, Suebi, Alamanni and Marcomanni.
2. From 500 to 900 AD, Slavic, Turkic and other tribes migrated before ending with their resettlement in Eastern Europe, which made it predominantly Slavic. Furthermore, more Germanic tribes migrated within Europe during this period, including the Lombards (to Italy), and the Angles, Saxons, and Jutes (to the British Isles), as well as the Avars, Bulgars, Huns, Arabs, Vikings and Varangians. The final phase of the migrations involved the Hungarians moving to the Pannonian plain.
Historians linked the European migration period with the simultaneous Turkic expansion, which involved the displacement of other peoples towards the west. By the High Medieval times, the Seljuk Turks themselves reached the Mediterranean.
(iii) Early medieval period
— Muslim conquests
— Turkic expansion
— Mongol invasions
The medieval period was a time when people were dispersing throughout Europe in spite of limited human mobility and slow social change. The Vikings migrated from Scandinavia, raided all over Europe from the 8th century and settled in many places, including Normandy, northern England,
Scotland and Ireland. Meanwhile, the Normans later conquered the Saxon Kingdom of England, most of Ireland, southern Italy and Sicily.
In the 8th century, Muslim Arabs, Berbers and Moors invaded Iberia, which lead to the founding of Kingdoms such as al Andalus. This brought with them a wave of settlers from North Africa. The linguistic, cultural Arabisation of the Maghreb was mainly due to a warlike Arab Bedouin tribe called Banu HIlal invading North Africa.
What migrations occurred in the Late Middles Ages?
During the 12th and 14th centuries, many Germans migrated into East Central and Eastern Europe. The Ostsiedlung settlements followed territorial gains of the Holy Roman Empire, but areas beyond were settled. At the end of the Middle Ages, the Romani (gypsies) migrated from the Middle East to Europe. It’s thought they originated in India, an offshoot of the Domba people of Northern India who had left for Sassanid Persia around the 5th century.
What migrations occurred in the Early Modern Period?
(A) Early Modern Europe
During the Early Modern period, migrations within Europe began to accelerate. This included the recruitment of landless labourers for monarchs to settle depopulated or uncultivated regions and the religious persecution leading to a series of forced migration. Examples include:
— The expulsion of Jews from Spain in 1492
— Mass migration of Protestants from the Spanish Netherlands to the Dutch Republic after the 1580s
— The expulsion of the Moriscos (descendants of former Muslims) from Spain in 1609
— The expulsion of the Huguenots from France in the 1680s.
Since the 14th century, the Serbs began migrating northwards from areas of their medieval Kingdom and Empire overran by the Ottoman Turks to today’s Vojvodina (northern Serbia), which was ruled by the Kingdom of Hungary at the time. The Serbs were encouraged by the Hadsburg monarchs of Austria to settle on their frontier with the Turks and provide military service through by granting them free land and religious toleration. The Great Migrations (Exoduses) of the Serbs occurred in 1690 and 1737. (More details of it below).
Other instances of labour recruitments include the Plantations of Ireland and the recruitment of Germans by Catherine the Great of Russia to settle the Volga region in the 18th century. The Plantations of Ireland involved the settling of Ireland with Protestant colonists from England, Scotland and Wales in the period 1560–1690.
This painting entitled ‘Migration of the Serbs’ (Seoba Srba) was illustrated by Serbian painter Paja Jovanović.
The Great Migrations of the Serbs refer to the 2 large migrations of Serbs from various territories under the rule of Ottoman Empire to regions under the rule of Habsburg Monarchy, during the 17th and 18th century.
i. 1st migration
The First Great Migration occurred during the Habsburg-Ottoman War (1683-1699) under Serbian Patriarch Arsenije III Crnojević, which resulted in the Habsburg retreat and Ottoman reoccupation of southern Serbian regions. Those regions were temporarily held by the Habsburgs between 1688 and 1690. There were extreme radicalisations of the relations between Muslims and Christians in the European provinces of the Ottoman Empire during the Austro-Turkish war (1683-1699). As a result of the lost rebellion and suppression, Serbian Christians and their church leaders, headed by Serbian Patriarch Arsenji III sided with the Austrians in 1689. Noel Malcolm claimed Albanian Catholics were also part of the exodus. Maroš Melichárek (2017) found they settled mainly in the southern parts of the Kingdom of Hungary in cities such as Szentendre, Buda, Mohács, Pécs, Szeged, Baja, Tokaj, Oradea, Debrecen, Kecskemét and Satmár.
In 1690, Emperor Leopold I allowed the refugees gathered on the banks of the Sava and Danube in Belgrade to cross the rivers and settle in the Habsburg Monarchy.
This is a painting of Serbian Patriarch Arsenjie III, leader of the First Great Serb Migration.
This map shows the main territory settled by Serbs during the Great Serb migration in 1690 (represented with blue colour)
ii. 2nd migration
The Second Great Migration also occurred during the Habsburg-Ottoman War (1737-1739), under the Serbian Patriarch Arsenije III Crnojević, which paralleled with the Habsburg withdrawal from Serbian regions. Between 1718 and 1739, those Serbian regions was known as the Kingdom of Serbia. In 1737, Crnojević sided with Habsburgs and supported the rebellion of Serbs in the region of Raška against Ottomans. However, the Habsburg armies and the Serbian Militia failed to succeed, which lead to their subsequent retreat. By 1739, Ottomans overtook the entire territory of the Habsburg Kingdom of Serbia. After the retreat of Habsburg armies and the Serbian Militia, a large portion of Christian population from the region of Raška and other Serbian lands migrated towards the north. They settled in Syrmia and neighbouring regions, within the borders of the Habsburg Monarchy.
This drawing illustrates the Serbs crossing the river for Austrian territory.
This is a painting of Serbian Patriarch Arsenjie IV, leader of the Second Great Serb Migration.
iii. Aftermath
The migrating Serbs settled in parts of present-day Hungary, Vojvodina, and Croatia (probably as north as Szentendre in Hungary and Komarno in Slovakia. Between the 14th and 18th centuries, the Serbs migrated from Balkans to the Pannonian plain. The 2 Great Migrations contributed to the provision of privileges that regulated the status of Serbs within Habsburg Monarchy.
(B) Colonial Empires
Between the 16th and 20th centuries, European Colonialism imposed European colonies in many regions of the world, particularly in the Americas, South Asia, Sub-Saharan Africa, and Australia. Major human migrations before the 18th century were largely directed by the state. For instance, Spanish emigration to the New World was limited to settlers from Castile with the intention of acting as soldiers or administrators. Labour shortage in Europe (with Spain being the worst affected of a depopulated Europe) lead to the discouragement of mass migration.
Since tropical diseases accounted for the majority of European deaths in the New World during this period, England, France and Spain used slaves as free labor in their American possessions.
In less tropical regions of North America's east coast, scores of religious dissidents, mostly English Puritans, settled there during the early 17th century. Meanwhile, Spanish restrictions on emigration to Latin America were revoked and the English colonies in North America experienced an influx of settlers seeking cheap or free land, economic opportunity and religious toleration.
Between 1620 and 1676, various early English colonies demonstrated prevailing self-rule from the time of the Plymouth colony's founding. After 1688, the English colonies gradually came more directly under King William III’s royal governance, which affected the type of emigration executed. During the early 18th century, a large number of non-English seekers of greater religious and political freedom were allowed to settle within the British colonies. This included:
— Protestant Palatine Germans displaced by French conquest.
— French Huguenots disenfranchised by an end of religious tolerance
— Scotch-Irish Presbyterians
— Quakers who were often Welsh
— Presbyterian and Catholic Scottish Highlanders seeking a new start after a series of unsuccessful revolts
Economic necessity increasingly motivated the English colonists to emigrate during the early 18th century. Petty criminals and indentured servants desperate to pay off their debts settled heavily in colonies like Georgia. By 1800, the demographic character of the American continent had been transformed by European emigration, due to the devastating effect of European diseases and warfare on Native American populations. Meanwhile, South Asia and Africa had less European settlers as it was limited to a handful of administrators, traders and soldiers.
This map illustrates colonial empires throughout the world in 1754, prior to the Seven Years’ War.
This map illustrates the putative migrations in and out of Africa as well as the locations of major ancient human remains and archeological sites (López et al.2015).
Who were the early humans before the homo sapiens?
After about 3 million years ago, the earliest humans descended from the Australopithecinc ancestors most likely originated from Eastern Africa, specifically in the area of the Kenyan Rift Valley, where the oldest known stone tools were discovered. Stone tools also discovered in ShangChen, China were estimated to be made about 2.12 million years ago, which claimed to be the earliest known evidence of hominins outside Africa, surpassing Dmanisi in Georgia by 300,000 years.
Between 2.1 and 0.2 million years ago (Mya), several migrations of archaic human populations (genus Homo) out of Africa and throughout Eurasia occurred in the Lower Paleolithic, and then into the beginning Middle Paleolithic, These migrations are collectively known as “Out of Africa I”. Migrations of Homo Sapiens into Eurasia that occurred shortly after 0.2 million years ago are known as “Out of Africa II”. Historians estimated the earliest presence of Homo outside of Africa dates back to approximately 2 million years ago.
This map illustrates the early expansions of hominins out of Africa. Yellow shaded areas illustrate successive dispersals of Homo Erectus. Dark yellow shaded areas illustrate dispersals of Homo neanderthalensis (ochre). Red shaded areas illustrate dispersals of Homo sapiens.
When did the early dispersals occur?
The discovery of remains of Graecopithecus and Ouranopithecus in Greece and Anatolia are both estimated to be around 8 million years old. This lead to hypotheses that these species demarcated the pre-Homo hominin expansion out of Africa. They may be related to the Trachilos footprints found in Crete, estimated to be 6 million years ago.
About 5 million years ago, Australopithecina emerged in East Africa around the Afar Depression. 1 million years later, Gracile australopithecines (Australopithecus afarensis) emerged in the same region.
Discovered in Lomekwi, Kenya were earliest known retouched tools, which is estimated to be 3.3 Mya, in the late Pilocene. Semaw (2000) proposed that they might be the product of Australopithecus garhi or Paranthropus aethiopicus, as these hominins are known to be contemporary with the tools. Around 2 Mya, the emergence of Homo may have occurred with the discovery of Homo habilis remains at Lake Turkana, Turkey, dated to approximately 2.1 Mya.
A 2018 study magnetostratigraphically dated the lowest layer containing stone artefacts in Shangchen, central China, as early as 2.12 Mya.
Recent evidence suggested Homo floresiensis descended from an early expansion of the Homo. There is debate whether to label the earliest hominins leaving Africa Homo habilis, or a form of early Homo or late Australopithecus closely related to Homo habilis, or a very early form of Homo erectus. Dembo et al. (2015) implied that morphology of H. floresiensis is closely related to the Australopithecus sediba, Homo habilis, and Dmanisi Man, which indicated that the ancestors of H. floresiensis left Africa before the appearance of H. erectus. A 2017 phylogenetic analysis evinced that H. floresiensis descended from a species (presumably Australopithecine) ancestral to Homo habilis, which implied a sororal ancestral relationship either to H. habilis or to a minimally habilis-erectus-ergaster-sapiens clade, and its line is older than H. erectus itself. Researchers postulated that H. floresiensis represents a hitherto unknown and very early migration out of Africa occurring before 2.1 Mya.
1. Homo Erectus
This is a forensic reconstruction of ad adult male Homo Erectus.
Between 2 and 3 million years ago, Homo genus spread throughout East Africa and to Southern Africa (Telanthropus capensis). Around 1.9 million years ago, Homo Erectus migrated out of Africa to Eurasia via the Levantine corridor and Horn of Africa, which is thought to associate with the operation of the Saharian pump. Homo Erectus then dispersed throughout the Old World, travelling as far as Southeast Asia. About 1.3 million years ago, the Oldowan lithic industry traced its steps to as far as north as the 40th Parallel (Xiaochangliang). This early migration out of Africa is postulated after the discovery of key sites in Riwat, Pakistan (~ 2 Mya), Ubeidiya in the Levant (1.5 Mya), and Dmanisi in the Caucasus (1.81 ± 0.03 Mya). Based on stone artifacts discovered in the Nihewan Basin, China may have been populated as early as 1.66 Mya. The earliest recorded use of fire by Homo erectus may have been as early as 1.27 million years ago after the discovery of an archeological site of Xihoudu (西侯渡) in the Shaanxi province. Around 1.7 million years ago, Meganthropus reached Southeast Asia (Java). About 500,000 years later, Atapuerca reached Western Europe.
Van Arsdale (2013) theorised that Homo erectus emerged just after 2 Mya, which they may have cohabited with H. herbals in East Africa for nearly 500,000 years. Fossils of H. Erectus discovered in both Africa and the Caucasus were estimated to be around 2 million years old. Skulls found in Dmanisi demonstrated signs of ageing with the loss of all but 1 tooth years before death, decreasing the likelihood of this Homo Erectus dying alone. However, evidence is lacking to support the theory that H. Erectus demonstrated care for the old. The earliest known evidence for African H. erectus, dubbed Homo ergaster, is a single occipital bone (coded KNM-ER 2598), estimated to be approximately 1.9 million years old (contemporary with H. rudolfensis). The next available fossil is a skull coded KNM-ER 3733, estimated to be 1.6 million years old, however there are no remains found between 1.6 & 1.9 million years old.
2. After H. Erectus — Clactonian, Micoquien, Mousterian, Neanderthal extinction
After its dispersal 1 million years ago, H. erectus was in the midst of diverging into a new species. Since it is a chronospecies that never went extinct, its "late survival" is a matter of taxonomic convention.
Late forms of H. erectus are thought to have survived about 0.5 million ago to 143,000 years ago.
Derived forms of H. antecessor existed in Europe around 800,000 years ago, while forms of H. heidelbergensis existed in Africa around 600,000 years ago. In turn, H. heidelbergensis spread across East Africa (H. rhodesiensis) and to Eurasia, where it gave rise to Neanderthals and Denisovans.
This map illustrates the Spread of Denisovans and Neanderthals after 500,000 years ago.
Movements of the H. heidelbergensis, Neanderthals and Denisovans headed northwards beyond the 50th parallel via Eartham Pit, Boxgrove (500,000 years ago), Swanscombe Heritage Park (400,000 years ago), and Denisova Cave (50,000 years ago). Ewen Callaway (2011) suggested that late Neanderthals reached the boundary of the Arctic by c. 32,000 years ago, which coincided with their displacement by the H. Sapiens from their previous habitats. This proposal was based on excavations at the site of Byzovaya in the Urals (Komi Republic) conducted in 2011.
Despite the sparse fossil records, other archaic human species may have migrated throughout Africa by this time. This assumption is based on traces of admixture with modern humans found in the genome of African populations. First discovered in South Africa in 2013, fossil remains of Homo Naledi, estimated to be approximately 300,000 years old, suggested the presence of such archaic human species.
This map illustrates the known Neanderthal range with separate populations in Europe and the Caucasus (blue), the Near East (orange), Uzbekistan (green), and the Altai region (purple).
Theories suggest that Neanderthals migrated across the Near East and Europe, and Denisovans migrated across Central and East Asia and to Southeast Asia and Oceania. Lopez, van Dorp & Hellenthal (2016) uncovered evidence that the overlapping of habitats may have caused interbreeding between Denisovans and Neanderthals.
3. Homo Heidelbergensis
Bar-Yosef, O. & Belfer-Cohen, A. (2001) classified archaic humans located in Europe around 800,000 years ago as a separate species derived from H. Erectus, known as Homo Heidelbergensis. From about 400,000 years ago, H. Heidelbergensis developed its own characteristic industry, known as Clactonian. Known to be in southern Africa by around 300,000 years ago, Homo rhodesiensis share anatomical similarities with the H. heidelbergensis.
This is a model of a forensic interpretation of an adult male Homo rhodesiensis based on the Kabwe skull.
Ewan Callaway (2017) postulated that before 300,000 years ago, Homo sapiens emerged in Africa from a lineage closely related to early H. heidelbergensis. Genetic studies conducted in 2016 & 2019 indicated that a later migration wave of H. sapiens (from .07-.05 Ma) from Africa may account for a majority of the ancestry of current non-African populations.
What migratory routes did our ancestors take out of Africa?
The Bab-el-Mandeb route stretches from Africa to West Asia, connecting the Horn of Africa with Arabia. It may have allowed dry passage for our ancestors during certain periods of the Pleistocene. Other candidate routes include Levantine corridor and the Strait of Gibraltar.
Bab-el-Mandeb is a 30 km strait between East Africa and the Arabian Peninsula, with a small island named Perim located 3km off the Arabian bank. Because this location brings East Africa in direct proximity with Eurasia, it may have been a major route of migration as there was no requirement to hop from one water body to the next across the North African desert.
This is a satellite image of the Bab-el-Mandeb strait.
Redfield, Wheeler and Often (2003) suggested the land connection with Arabia disappeared in the Pliocene, followed by the Red Sea’s evaporation and corresponding increases in salinity. This would have left traces in the in the fossil record after just 200 years and evaporite deposits after 600 years, which unfortunately neither have been detected. This may be due to strong current flows from the Red Sea into the Indian Ocean, making crossing without a land connection. Chauhan (2009) reported the discovery of oldawan grade tools on Perim Island, which implied the strait may have been arguably navigated in the Early Pleistocene.
The Strait of Gibraltar is regarded as the Atlantic entryway to the Mediterranean, where the distance between the Spanish and Moroccan banks are 14 km. Although glaciation reduced sea levels in the Pleistocene, this was unlikely to reduce the crossing distance to less than 10 km. Since deep currents push westwards, strong surface water flows back into the Mediterranean. The entrance into Eurasia via the Strait of Gibraltar would elucidate the hominin remains discovered at Barranco León in southeastern Spain (dated 1.4 Mya) and Sima del Elefanta in northern Spain (1.2 Mya). Meanwhile, Arzerello’s et al. (2007) discovery of remains at Pirro Nord in southern Italy (estimated between 1.3 & 1.7 Mya) suggested our ancestors contentiously arrived from the East, however more evidence is required to prove or disprove this theory. The discovery of Sicilian Oldowan grade tools in Sicily in 1973 lead to an hypothesis that our ancestors may have walked across the Strait of Sicily. Villa (2001) argued the artefacts may have been crafted during the Middle Pleistocene as a land bridge existing during the Pleistocene was improbable.
Why did our ancestors disperse out of Africa?
i. Climate Change & Hominin flexibility
For a given species in a given environment, the finiteness and limited availability resources curbs the survival rates indefinitely, known as the ‘carrying capacity’. When individuals approach this threshold, gathering resources in the poorer yet less exploited peripheral environment is considered unchallenging than in the preferred habitat. Researchers suggested that prior to their migration into the the peripheries (such as encroaching into the predatory guild), Homo habilis may have developed some baseline behavioural flexibility. Marean (1989) suggested that natural selection may have positively selected and amplified this behavioural flexibility, inducing Homo erectus’ ability to adapt to the peripheral open habitats. Eldredge & Gould (1997) surmised the possibility of a new and environmentally flexible hominin population returning to the old niche and subsequently replacing the ancestral population. Studies in the 1990s concluded that the carrying capacity's pressure to adapt to the open grounds may have been strained by the step-wise shrinkage of the woodland and associated reduction of hominin carrying capacity in the woods around 1.8 Mya, 1.2 Mya, and 0.6 Mya. Lahr (2010) hypothesised that the way to the Levantine corridor may have been opened, sporadically, in the Early Pleistocene due to the Homo erectus' new environmental flexibility and favourable climate fluxes.
ii. Chasing fauna
Shipman (1984) used lithic analysis to argue that Oldowan hominins were not predators. Nevertheless, there is evidence to support the theory that Homo erectus likely followed animal migrations to the north during wetter periods to find scavenged food. The scavenged remains of the sabre-tooth cat Megantereon’s prey were likely an abundant food source for hominins, especially during in glacial periods, since its teeth are unable to disintegrate bone marrow.
iii. Coevolved zoonotic diseases
The absence of zoonotic diseases outside the hominin’s original habitat may have contributed to the hominin’s successful survival within Eurasia once out of Africa. Zoonotic diseases usually keep its host alive (i.e. hominins) long enough to transmit itself from animals to humans. However they won’t necessarily do so as human factor is optional in completion of their life cycle. Nonetheless, since zoonotic diseases evolved alongside them, these infections are well accustomed to human presence. Goodall (1986) evaluated that 55% of chimps at the Gombi reserve die of mostly zoonotic diseases. This implies that a higher African ape population directly increases the diseases’ survival and transmission rates. Fortunately, a significant number of zoonotic diseases are restricted to hot and damp African environments, meaning hominins that moved into drier and colder habitats of higher latitudes increased their survival chances.
iv. Physiological traits
Ruff (2009) theorised that the long arms of Homo habilis indicated arboreal adaptation. Steudel (1996) suggested that the longer legs and shorter arms of Homo erectus indicated a transition to terrestrial locomotion, though how this relative leg lengthening posed an evolutionary advantage remains unclear. Furthermore Steudel (1994) argued that sheer body size lead to increased walking energy efficiency and endurance. Wheeler (1992) postulated that Homo erectus with larger body sizes had slower dehydration rates, which enabled it to cover greater distances before facing thermoregulatory limitations. Klein (1999) asserted that effective colonisation of Eurasia would not have been possible without the capability of endurance walking at a normal pace.
4. Homo Sapiens
— Dispersal throughout Africa
Based on thermoluminescence dating of artefacts and remains from Jebel Irhoud, Morocco, Homo sapiens (anatomically modern humans) are thought to have emerged about 300,000 years ago. Recent studies on a skull found in Florisbad, South Africa, is estimated to be about 259,000 years ago, classifying them as early Homo sapiens. In September 2019, CT studies of a virtual skull shape of the last common human ancestor to Homo sapiens found it represented the earliest modern humans. This indicated that modern humans arose between 260,000 and 350,000 years ago through a merging of populations in East and South Africa. In July 2019, remains of a 210,000 year old H. Sapien and a 170,000 year old H. neanderthalensis were discovered by anthropologists in Apidima Cave in southern Greece, which is more than 150,000 years older than previous H. sapiens finds in Europe.
After their emergence, early modern humans expanded to Western Eurasia, Central, Western and Southern Africa. Hershkovitz et al. (2018) & Lopez and van Dorp (2016) didn’t find evidence of early Eurasian expansions persisting, however Southern and Central African migrations lead to deep temporal divergent patterns in living human populations. Eleanor Scerri (2017) argued early modern human expansion in sub-Saharan Africa contributed to the end of late Acheulean (Fauresmith) industries at about 130,000 years ago, despite claims of late coexistence of archaic and early modern humans in West Africa as late as 12,000 years ago.
It’s possible ancestors of the modern Khoi-San expanded to Southern Africa between 260,000 and 150,000 years ago. This meant there were 2 ancestral population clusters in Africa co-existing by the beginning of the MIS 5 “megadrought” 130,000 years ago. Those population clusters were the bearers of mt-DNA haplogroup L0 in southern Africa (ancestors of the Khoi-San), and bearers of haplogroup L1-6 in central/eastern Africa (ancestors of remaining humans). Between 120,000 and 75,000 years ago, bearers of L0 were thought to back-migrate towards eastern Africa. Studies in 2009 hypothesised that ancestors of the Central African forager populations (African Pygmies) migrated to Central Africa at least 60,000 years ago, possibly up to 130,000 years ago.
Because the fossil remains were discovered in sparse locations, it is difficult to interpret the possible migratory paths in Western Africa. So far, it is known Homo sapiens reached the western Sahelian zone by 130,000 years ago, and then tropical West African sites less than 130,000 years ago. Archaic MSA sites may have persisted until the Holocene boundary (about 12,000 years ago), which might be instrumental in late survival of archaic humans, and late hybridisation with H. sapiens in West Africa.
This diagram illustrates early modern human migrations based on the distributions of mitochondrial haplogroups.
— Early northern African dispersal
It’s estimated that between 185,000 and 115,000 years ago, populations of H. Sapiens migrated to the Levant and to Europe. Researchers theorised that these early migrations led to temporary colonisations, leading to recessions by about 80,000 years ago.
Theories suggested 2 different routes would have been taken out of Africa by modern humans at least 125,000 years ago.
(1) Through the Nile Valley heading to the Middle East, at least into modern Israel (Qafzeh: 120,000–100,000 years ago)
(2) Present day Bab-el-Mandeb strait on the Red Sea crossing to the Arabian Peninsula and settling into present-day United Arab Emirates (125,000 years ago), Oman (106,000 years ago), and the Indian Subcontinent (Jwalapuram: 75,000 years ago).
Despite the lack of remains in the UAE, Oman and Indian Subcontinent, the stone tools found at Jebel Faya, Jwalapuram and some African locations shared apparent similarities. Bruce Bower (2011) suggested that the tool’s creators were all modern humans. This provides evidence for the theory that modern humans migrated from Africa to southern China around 100,000 years ago based on Zhiren Cave, Zhirendong, Chongguo City (~100,000 years old), and the Liujiang hominid (Liujiang County ~ 110,000 - 139,000 years old). A 2019 study on teeth found in Lunadong (Bubing Basin, Guangxi, southern China), including a right upper second molar and a left lower second molar, estimated it to be at least 126,000 years old. Previous migratory routes out of Africa didn’t leave any traces according to genetic analyses of Y-chromosome and MtDNA, indicating those modern humans didn’t survive in large numbers and were assimilated by our major antecessors. This extinction coincided with the Toba eruption that occurred about 74,000 years ago, though others argue it scarcely impacted human population.
Based on available fossil records, researchers postulated a centre of origin and dispersal for the mtDNA haplogroup L3 in Asia. This is based on similar coalescence dates of L3 and its Eurasian-distributed M and N derivative clades (~71,000 years ago), long distances between the the oldest subclades of M and N in Southeast Asia, and the comparable age of the paternal haplogroup DE.
Around 125,000 years ago, early anatomically modern humans initially migrated out of Africa. However, 55,000 years later, fully modern human L3-carrying females back-migrated with males bearing the paternal haplogroup E from Eurasia to Africa. Vicente Cabrera (2017) suggested these new Eurasian lineages may have replaced the old autochthonous male and female African lineages.
Recent studies theorised that between 70,000 and 50,000 years ago a single Out-of-Africa migration lead to the emergence of modern Eurasian descendants, which associated with the origin and expansion of maternal haplogroup L3 from Eastern Africa. This meant earlier waves of modern human migration out of Africa predating 70,000 years ago mostly became extinct, which contributed about 2% to the ancestry only of some Oceanian peoples such as Papuans.
— Coastal Migration
This map is an overview of the people migrations of the world by early humans during the Upper Paleolithic, following to the Southern Dispersal paradigm.
Between 70,000 and 50,000 years ago, the “recent dispersal” of modern humans throughout the world occurred. Zhivotovsky et al. (2003) and Stix (2008) suggested a small group from East Africa fewer than 1000 individuals, bearing the mitochondrial haplogroup L3, crossed the Red Sea strait at Bab el Mandib, now called Yemen, after about 75,000 years ago. The theory that those modern humans migrated along the northern route through Sinai/Israel/Syria (Levant) is supported by evidence. This meant their descendants dispersed about the coastal route around Arabia and Persia to the Indian subcontinent before 55,000 years ago. Recent studies supported a theory that between 70,000 and 50,000 years ago, modern humans migrated out of Africa along the coast, associating with the mitochondrial haplogroups M and N, both derivative of L3.
The discovery of a jawbone fragment containing 8 teeth in Misliya Cave, Israel is estimated to around 185,000 years old. Archeologists also shed layers made between 140,000 and 250,000 years old, uncovering tools of the Levallois type. Researchers such as Pallab Ghosh (2018) estimated the date of the first migration to be earlier than 250,000 years ago if there is evidence of an association with the modern human jawbone discovery. Kay Prüfer et al. (2013) determined that 0.2% of mainland Asian and Native American DNA were Denison DNA, which supported the theory that H. sapiens interbred with Neanderthals and Denisovans along way.
Known as the “Southern Dispersal” hypothesis, the early migration of modern humans of recent African origin was theorised to have occurred along the southern coast of Asia, from the Arabian peninsula via Persia and India to Southeast Asia and Oceania. Other researchers referred it as the "southern coastal route” or "rapid coastal settlement”, as later descendants of those migrations then colonised the rest of Eurasia (including Europe), the remainder of Oceania, and the Americas.
Several theorists hypothesised that the initial dispersal to the Arabian peninsula, India, Southeast Asia, New Guinea, Australia, Near Oceania, coastal China, and Japan began around 70,000 to 50,000 years ago, referred as ‘the coastal route theory”. This associates with the presence and dispersal of mtDNA haplogroup M and haplogroup N, and the specific distribution patterns of Y-DNA haplogroup C and haplogroup D, in these regions.
Moreover, it proposed that between 70,000 and 50,000 years ago, early modern humans (bearers of mtDNA haplogroup L3) crossed the Bab-el-Mandeb straits from East Africa to the Arabian Peninsula. Zhivotovsky et al. (2003) estimated about 7.5 - 20% of Africans crossed the Red Sea. Spencer Wells, Posth et al. & Haber et al. suggested this small group of Africans spent a few thousand years travelling along the coastal route around Arabia and Persia to India relatively rapidly, beginning from India then dispersing to Southeast Asia (Sundaland) and Oceania (Sahul).
This map represents the coastal migration model. It indicates later development of mitochondrial haplogroups from 3 population centres in the Near East, India and East Asia.
i. Near Oceania
Around 50,000 years ago, modern humans migrated along the Asian coast to Southeast Asia and Oceania, eventually colonising Australia. The arrival in Australia marked the first time H. sapiens expanded its habitat beyond that of H. erectus. Denisovan ancestry is shared by Australian Aborigines, Melanesians, and small groups scattered across Southeast Asia, such as the Mamanwa, a Negrito people in the Philippines. Studies suggested that interbreeding took place in Eastern Asia where the Denisovans lived. Studies conducted in 2013 theorised that Denisovans may have crossed the Wallace Line, with Wallacea serving as their last refugium. Even though H. Erectus crossed the Lombok gap reaching as far as Flores, they never made it to Australia.
During this era, sea level was much lower and most of Maritime Southeast Asia formed one land mass known as “Sunda”. Modern humans migrated southeastwards on the coastal route to the straits between Sunda and Sahul, the continental land mass of present-day Australia and New Guinea. It’s estimated the gaps on the Weber Line spanned 90km, which meant seafaring skills were required to migrate from New Guinea to Australia. Based on the pattern of mitochondrial haplogroups descended from haplogroup M, and in Y-chromosome haplogroup C, migration is thought to then continued along the coast eventually turning northeast to China finally reaching Japan before turning inland.
This map illustrates the probable extent of land and water at the time of the last glacial maximum 20,000 years ago. Back then the sea level was probably more than 110 m lower than today.
Sequencing of one (West Australian) Aboriginal genome from an old hair sample revealed evidence of the individual’s descending from people who migrated into East Asia between 62,000 and 75,000 years ago. Rasmussen et al. (2011) suggested this single migration into Australia and New Guinea occurred before the arrival of Modern Asians (between 25,000 and 38,000 years ago) and their later migration into North America. Other researchers argued the same migration occurred around 50,000 years ago, before rising sea levels separated Australia from New Guinea approximately 8,000 years ago. Bowler et al. (2003) and Clarkson et al. (2015) proposed the oldest evidence of settlement in Australia is dated 50,000–60,000 years old, as well as the oldest human remains dated around 40,000 years old, and the earliest human artefacts dated at least 65,000 years old. Tim Flannery (2002) argued the extinction of the Australian megafauna by humans occurred between 46,000 and 15,000 years ago, similar to the occurrence of the American human megafauna. There is still debate on the continued use of stone tools found in Australia.
— Dispersal throughout Eurasia
- Upper Paleolithic, Mammoth steppe, Archaic human admixture with modern humans, Mousterian
Around 60,000 to 50,000 years ago, the initial coastal migration to South Asia remained there for a period of time before dispersing throughout Eurasia. At the beginning of the Upper Paleolithic, the spread of early humans gave rise to the major population movements that make up the Old World and the Americas.
Several millennia ago, Upper Paleolithic populations in the West spread throughout Asia and Europe, followed by a back-migration of M1 to o North Africa and the Horn of Africa. These populations are associated with mitochondrial haplogroup R and its derivatives.
As early as 43,000 years ago, humans were present in Europe, which rapidly replaced the Neanderthal population. Sankararman et al. (2012) argued that substantial interbreeding between early humans and Neanderthals ceased before 47,000 years ago, i.e. took place before modern humans entered Europe, since contemporary Europeans have Neanderthal ancestry. Harpending and Cochran (2009) found evidence of modern humans passing their mitochondrial DNA through at least 1 genetic feedback, in which genome diversity was drastically reduced. It is proposed that approximately 100,000 years ago, humans spread from a geographically restricted area through a geographic bottleneck. Then, 50,000 years later, a significant growth amongst geographically dispersed populations occurred firstly in Africa, and thence spreading elsewhere. Conclusions from climatological and geological studies support the bottleneck hypothesis. There are suggestions that a millennium-long cold period followed the largest volcanic eruption of the Quaternary period, Toba. This would have potentially reducing human populations to a few tropical refugia, estimated to be as few as 15,000. This meant maximisation of the genetic drift and founder effects. Stanley Ambrose (1998) believed that the greater diversity amongst African genomes reflected the extent of African refugia during the Toba incident. However, a 2016 review underscored the inconsistency of the single-source hypothesis of non-African populations compared to multiple sources with genetic mixing across Eurasia, according to ancient DNA analysis.
i. Europe
— European early modern humans, Aurignacian, Gravettian, Art of the Upper Paleolithic,
Stephan Oppenheimer (2012) theorised that cultural adaption to big game hunting of sub-glacial steppe fauna caused the spread of anatomically modern humans reached Europe around 40,000 years ago from Central Asia and the Middle East. It is known that Neanderthals were still present both in the Middle East and in Europe before the arrival of anatomically modern humans (also known as “Cro-Magnon” or European early modern humans). In various regions such as the Iberian peninsula and the Middle East, there is overlap between modern humans and Neanderthal, where interbreeding between the two populations took place to a certain extent. This interbreeding may have contributed Neanderthal genes to palaeolithic and ultimately modern Eurasians and Oceanians.
A 2001 study noted that the northern latitude differentiated Europe from other parts of the inhabited world, which suggested humans, whether Neanderthal or Cro-Magnon, reached sites in Arctic Russia around 40,000 years ago.
Around 50,000 years ago, the first anatomically modern humans to set foot in Europe (Eurasia) were the Cro-Magnon by the Zagros Mountains (near present-day Iran and eastern Turkey). 1 group is known to settle coastal areas around the Indian Ocean, while the other group migrated north to the steppes of Central Asia. Modern human remains discovered in Italy, Britain and European Russian Arctic were estimated to be 43 - 45,000 and 40,000 years old respectively.
Hoffecker (2006) suggested humans faced adaptive challenges during their colonisation of the environment west of the Urals. In winter, average temperatures ranged from -20 to - 30 degrees C (-4 to -22 degrees F) and fuel and shelter were scarce. Since the wheel hadn’t been invented yet, those humans had to travel on foot and rely on hunting highly mobile herds such as deer for food. During this time, humans smartly conjured up technological innovations to overcome their environmental challenges. Those innovations include:
— Tailoring clothing from the pelts of fur-bearing animals
— Constructing shelters with hearths using bones as fuel
— Digging “ice cellars” into the permafrost to store meat and bones
This is a representation of modern humans migrating into Europe, based on simulation by Currat & Excoffier (2004); (YBP = Years before present)
Caramelli et al. (2003) identified mitochondrial DNA sequences of 2 Cro-Magnons discovered in the Paglicci Cave, Italy, i.e. Paglicci 52 and 12, to be Haplogroup N typical of the latter group, estimated to be between 23,000 and 24,000 years old. Maca-Meyer et al. (2001) and Currat (2004) suggested that around 45,000 years ago, the modern human population spread across Europe for around 15 - 20,000 years. This would have coincided with the displacement of the Neanderthals. Reasons for the extended period of time for modern humans to occupy European territory was the competing resistance from the Neanderthals. At that time, Neanderthals were larger in size and had larger craniums, with a more robust or heavily built frame. This meant Neanderthals demonstrated more physical strength than Homo sapiens. Their adaptation to freezing weather would have contributed to their elongated European lifespan, around 200,000 years. Because the Cro-Magnons were anatomically modern, developed widespread trade networks, superior technology and evolved bodies more suited to running, they eventually completely displaced the Neanderthals, whose last refuge was in the Iberian Peninsula. The last known fossil record of the Neanderthals is dated after approximately 25,000 years ago, which demarcates extinction. The last known population to live around a cave system on the remote south-facing coast of Gibraltar is estimated to be around 30,000 to 24,000 years ago.
From the extent of linkage disequilibrium, the last instance of Neanderthal gene flow into early ancestors of Europeans occurred 47,000–65,000 years before present. A 2012 study suggested interbreeding occurred somewhere in Western Eurasia, possibly the Middle East, based on available archaeological and fossil evidence. Other studies evaluated a higher Neanderthal admixture in East Asians than in Europeans. They found a similarity between North African groups and Neanderthals as non-African populations in terms of amount of derived alleles. On the other hand, Sub-Saharan African groups are the only modern human populations with no substantial Neanderthal admixture. Some Nomad pastoralist groups in the Sahel and Horn of Africa, who are associated with northern populations, were found to contain the Neanderthal-linked haplotype B006 of the dystrophin gene. Since it is present on the northern and northeastern perimeter of Sub-Saharan Africa, this suggests their genes originated outside of Africa.
ii. East and North Asia
— Mongoloid & Genetic research, Ancient North Eurasians
In 2017, Yang et al. studied the DNA of “Tianyuan Man”, who lived in China c. 40,000 years ago. They found his DNA demonstrated substantial Neanderthal admixture associated with modern Asian and Native American populations. A 2013 study discovered the chromosome 3p21.31 region (HYAL region) of East Asians contained Neanderthal introgression of 18 genes. From 45,000 years ago, the introgressive haplotypes were positively selected in only East Asian populations leading to a steady increase. 40,000 to 42,000 years later, there was a sudden increase of growth rate of these haplotypes. Ding et al. (2014) found that they occur at higher frequencies among East Asian populations compared to other Eurasian populations (e.g. European and South Asian populations). This suggests that this Neanderthal introgression occurred within the ancestral population shared by East Asians and Native Americans.
Jeong et al. (2016) analysed the population genetics of the Ainu people of northern Japan and discovered it played a role in the early peopling of East Asia. They found the Ainu represented a more basal branch than the modern farming populations of East Asia. This suggested the existence of an ancient (pre-Neolithic) connection with northeast Siberians. Different 2013 studies conducted by Kamberov and Wade found an association between several phenotypical traits and Mongoloids with a single mutation of the EDAR gene, estimated about 35,000 years old.
About 50,000 years ago, Mitochondrial haplogroups A, B and G emerged, followed by subsequent colonisation of Siberia, Korea and Japan 15,000 years later. It’s known portions of these populations migrated to North America during the Last Glacial Maximum.
— Last Glacial Maximum (LGM)
i. Eurasia
— Solutrean, Magdalenian
Around 20,000 years ago, about 5,000 years after the Neanderthal extinction, the Last Glacial Maximum (LGM) drove the migration of northern hemisphere inhabitants to several shelters (refugia) until the end of this period. It’s presumed the resulting populations resided in such refuges during the LGM to ultimately reoccupy Europe, where archaic historical populations are considered their descendants. Further migrations, such as the Neolithic expansion from the Middle East and the Chaocolithic population movements associated with Indo-European expansion, transformed the composition of European populations. Above the Arctic Circle on the Yana River, Siberia, at 71°N lies a Paleolithic site that dates back to 27,000 radiocarbon years before present, which coincides with glacial times. Pitulko et al. (2004) demonstrated the site was evidence of humans adapting to the harsh, high-altitude, Late Pleistocene environment much earlier than previously thought.
This schematic illustration shows the Beringia migration according to matrilineal genetics: Arrival of Central Asian populations to the Beringian Mammoth steppe c. 25,000 years ago.
ii. Americas
— Settlement of the Americas, Genetic history of indigenous peoples of the Americas
Spencer Wells and Mark Read (2002) suggested Paleo-Indians originated from Central Asia, crossed the Beringia land bridge between eastern Siberia and present-day Alaska, before the settlement of the Americas. Several studies estimated by the end of the last glacial period, humans lived throughout the Americas no earlier than 23,000 years before present.
Fitzhugh et al. (2009) theorised that the earliest migrants moved during the Quarternary glaciation, significantly reducing sea levels. This followed herds of now-extinct pleistocene migrations along ice-free corridors that stretched between the Laurentide and Cordilleran ice sheets. Fladmark (1979) proposed early migrants travelled along the Pacific coast to South America as far as Chile, either on foot or using primitive boats. The Centre for Climate Systems Research believed rising sea levels would have submerged any archaeological evidence of coastal occupation during the last Ice Age. Studies in 2015 uncovered indigenous Australasian genetic markers in Amazonia that supports the coastal route hypothesis.
— Holocene Migration
- Pre-modern human migration, Mesolithic, Urheimat
Following the Last Glacial Maximum, the Holocene is estimated to have occurred around 12,000 years ago. 3,000 years later, the Holocene climate optimum involved mass migration of humans that were geographically confined to refugia. This meant a large proportion of Earth’s landmass were settled by Homo sapiens.
This period marks the transition from the Mesolithic to the Neolithic stage throughout the temperate zone. The Neolithic stage subsequently transitions to the Bronze Age in Old World cultures, as well as the emergence of the historical record in the Near East and China beginning around 4,000 years ago.
These mass migrations during the transitional Mesolithic to Neolithic era are thought to had given rise to the pre-modern distribution of the world's major language families such as Niger-Congo, Niho-Saharan, Afro-Asiatic, Uralic, Sino-Tibetan, or Indo-European phyla. The suggestion of the major language families of Eurasia (excluding Sino-Tibetan) may have derived from a single proto-languages spoken at the beginning of the Holocene period is known as “the Nostratic theory”.
This diagram shows the theorised prehistoric migration routes for Y-chromosome Haplogroup N lineage following the retreat of ice sheets after the Last Glacial Maximum (22–18,000 years ago).
i. Eurasia
— Neolithic Revolution, Indo-European Expansion, Proto-Uralic homeland hypotheses
Ann Gibbons (2014) suggested modern native populations of Europe largely descend from 3 distinct lineages:
(1) Western-Hunter Gatherers
(2) A derivative of the Cro-Magnon population of Europe
(3) Early European Farmers = Introduced to Europe from the Near East during the Neolithic Revolution and Ancient North Eurasians, then expanded across Indo-European regions.
ii. Sub-Saharan Africa
— Nilo-Saharan, Niger-Congo
Researchers hypothesised the Nilotic peoples derived from an earlier undifferentiated Eastern Sudanic unity by the 3rd millennium BCE. The domestication of livestock is linked to the development of the Proto-Nilotes as a population. East of the Nile, which is now called South Sudan, the original locus of the early Nilotic speakers was presumed to be situated. John Desmond Clark (1984) named the Proto-Nilotes of the 3rd millennium BCE ‘pastoralists’, and their neighbours, the Proto-Central Sudanic peoples ‘agriculturalists’.
Igor Kopytoff (1999) suggested the emergence of the Niger-Congo phylum occurred around 6,000 years ago in West or Central Africa. Its expansion relates to the expansion of Sahel agriculture in the African Neolithic period, following the desiccation of the Sahara in circa 3900 BCE. Vansina (1995) evinced the Bantu expansion spread the Bantu languages to Central, Eastern and Southern Africa, partly replacing the indigenous populations of these regions. This expansion began around 3,000 years ago, taking around 1300 years to reach South Africa.
Busby et al. (2016) suggested an admixture from ancient and recent migrations from Europe into parts of Sub-Saharan Africa. Ramsay et al. (2018) also showed that ancient Eurasians migrated into Africa, estimated the Eurasian admixture in modern Sub-Saharan Africans to range from 0% to 50%. However, this varied by region, usually higher in the Horn of Africa and parts of the Sahel zone, but lower in certain parts of Western Africa, and Southern Africa (excluding recent immigrants).
iii. Indo-Pacific
— Austronesian peoples, Austronesian expansion
William Meacham (1984) labelled the first seaborne humans to migrate from Taiwan as the “Austronesian expansion”. Those humans at the time used advanced sailing technologies, such as catamarans, outrigger boats, and crab claw shells, to build the first sea-going ships. This lead to the rapid colonisation of the Island Southeast Asia around 3000 to 1500 BCE, and then colonisation of Micronesia from the Philippines and Eastern Indonesia by 2200 to 1000 BCE.
Around 1600 to 1000 BCE, a branch of the Austronesians reached Island Melanisia, which lead to the establishment of the Lapita culture (named after the archaeological site in Lapita, New Caledonia, where their characteristic pottery was first discovered). They became the descendants of the modern Polynesians, who ventured into Remote Oceania arriving at Vanuatu, New Caledonia, and Fiji by 1200 BCE, and Samoa and Tonga by approximately 900 to 800 BCE. This outlines the furthest extent of the Lapita culture expansion. However, for the next 1,500 years, the technology to make pottery gradually became extinct (possibly due to the lack of clay deposits in the island), which was superseded by carved wooden and bamboo containers. In 1500 BCE, back-migrations from the Lapita culture merged back into Island Southeast Asia, and then into Micronesia 1300 years later. Around 700 AD, the Polynesians then voyaged further into the Pacific Ocean to colonise the Cook Islands, the Society Islands, and the Marquesas. From there, they further colonised Hawaii by 900 AD, Rapa Nui by 1000 AD, and New Zealand by 1200 AD.
Around 500 AD, Austronesians sailed across the Pacific Ocean from Borneo to Madagascar and the Comoros Islands, where they colonised. To this day, they remain the dominant ethnolinguistic group of the Indo-Pacific islands, making them the first to establish a maritime trade network reaching as far west as East Africa and the Arabian peninsula. Researchers found they assimilated early Pleistocene to early Holocene human overland migrations through Sundaland like the Papuans and the Negritos in Island Southeast Asia. Steve Lansing suggested the Austronesian expansion was the ultimate and most far-reaching Neolithic human migration event.
iv. Caribbean
The last American places settled by humans is the Caribbean. The oldest remains discovered in the Greater Antilles (Cuba and Hispaniola) are estimated to be 5500 - 6000 years old. Studies suggested these humans moved across the Yucatán Channel from Central America. Current evidence suggests that later migrants originated from South America, via the Orinoco region from 2000 BCE and onwards. Fagan (2007) theorised the descendants of these migrants include the ancestors of the Taino and Kalinago (Island Carib) peoples.
v. Arctic
— Circumpolar peoples
The earliest inhabitants of North America's central and eastern Arctic may have existed circa 2500 BCE, who are labelled the Arctic small tool tradition (AST). The AST consists of several Paleo-Eskimo cultures, including the Independence cultures and Pre-Dorset culture.
This timeline of human evolution outlines the major events in the development of the human species, Homo sapiens, and the evolution of the human’s ancestors. It includes brief explanations of some of the species, genera, and the higher ranks of taxa observed today as possible ancestors of modern humans. It combines studies from anthropology, paleontology, developmental biology, morphology, and anatomical and genetic data, however it does not aim to answer the question regarding the true origin of life. However, abiogenesis does present 1 possible line of evolutionary descent of species that eventually led to humans.
This diagram shows Haeckel’s 1879 Paleontological Tree of Vertebrates. It describes the evolutionary history of species as a “tree” with many branches arising from a single trunk. Although it is outdated, it does illustrate the principles that more complex modern reconstructions can obscure.
Timeline of Homo sapiens
Throughout history, humans have explored across large landmasses worldwide by walking before the invention of the wheel and the principles of buoyancy, hence future transport machines and contraptions. They explored in search of resources, new information about our planet’s geography and landmass, and new places to colonise and name those new towns and cities after the discoverers.
What are the most notable periods of human exploration?
a. Phoenician gallery sailings
The Phoenicians (1550 — 330 BCE) established trading routes throughout the Mediterranean Sea and Asia Minor, but where exactly those routes are unknown. The presence of tin in some Phoenician artefacts suggests that they may have traveled to Britain. According to Virgil’s Aeneid and other ancient sources, the legendary Phoenician Queen Dido from Tyre sailed to North Africa and founded the city of Carthage, the centre or capital city of the ancient Carthaginian civilisation, on the eastern side of the Lake of Tunis in modern day Tunis Governorate in Tunisia.
b. Carthaginean exploration of Western Africa
Hanno the Navigator (500 BC) was a Carthaginean navigator who explored the Western Coast of Africa. The only source of his voyage is a Greek periplus, a manuscript document listing all the ports and coastal landmarks, in order and with approximate intervening distances, that his vessel could expect to find along a shore. Modern analyses of his route suggested Hanno's expedition could have took him as far south as Gabon, whereas others suggested he reached no further than southern Morocco.
c. Greek & Roman exploration of Northern Europe and Thule
— The Greek explorer from Marseille, Pytheas (380 – c. 310 BC) was the first to circumnavigate Great Britain, explore Germany, and reach Thule (commonly thought to be the Shetland Islands or Iceland).
— The Romans, led by the first emperor of the Roman Empire, Augustus, reached and explored all the Baltic Sea.
d. Roman explorations
The Romans went on expeditions that involved crossing the Sahara desert along 5 different routes:
— Through the western Sahara, toward the Niger river and actual Timbuktu.
— Through the Tibesti mountains, toward Lake Chad and actual Nigeria.
— Through the Nile river, toward Uganda.
— Through the western coast of Africa, toward the Canary Islands and the Cape Verde Islands
— Through the Red Sea, toward actual Somalia, and probably Tanzania.
All these expeditions may have been for commercial purposes, which were supported by legionnaires (professional infantrymen of the Roman army after the Marian reforms). In 62 AD, 2 legionaries led by the emperor Nero explored the sources of the Nile river in preparation for the conquest of Ethiopia or Nubia. Jonathan Roth (2002) suggested these explorations involved camel to transport gold. Roman ships supported explorations near the African western and eastern coasts, as they were associated with the naval commerce (mainly toward the Indian Ocean). The Romans also organised several explorations into Northern Europe, and as far as China in Asia.
(I) 30 BC — 640 AD = The Romans began trading with India after they acquired Ptolemaic Egypt. This established a direct connection to the Egyptian Spice trade (established in 118 BC).
(II) 100 AD — 166 AD = This period marked the beginning of the Romano-Chinese relations. Claudius Ptolemy wrote of the Golden Chersonese (i.e. Malay Peninsula)
(III) 2nd century AD = Roman traders explored Siam, Cambodia, Sumatra, and Java.
(IV) 161 AD = An embassy from Roman Emperor Antoninus Pius or his successor Marcus Aurelius met Chinese Emperor Huan of Han at Luoyang.
(V) 226 AD = A Roman diplomat or merchant reached northern Vietnam and visited Nanjing, China and the court of Sun Quan, ruler of Eastern Wu.
e. Chinese exploration of Central Asia
During the 2nd century BC, the Han dynasty explored much of the Eastern Northern Hemisphere. Beginning in 139 BC, the Han diplomat Zhang Qian travelled west with the aim of attempting to secure an alliance with the Da Yueshi against the Xiongnu, which was unsuccessful as the Yuezhi had been evicted from Gansu by the Xiongnu in 177 BC. However, Zhang's travels discovered entire countries oblivious to the Chinese, including the remnants of the conquests of Alexander the Great (r. 336–323 BC). When Zhang returned to China in 125 BC, he reported his findings of Dayuan (Fergana), Kangju (Sogdiana), and Daxia (Bacteria, formerly the Greco-Bactrian Kingdom subjugated by the Da Yuezhi). Despite not venturing inside them, he described Dayuan and Daxia as agricultural and urban countries like China, and described Shendu (the Indus River valley of Northwestern India) and Anxi (Arsacid territories) further west.
f. Viking Age
Between about 800 and 1040 AD, the Vikings explored Europe and much of the Western Northern Hemisphere via rivers and oceans. The Norwegian Viking explorer, Erik the Red (950–1003), sailed to and settled in Greenland after being expelled from Iceland. Meanwhile, his son, the Icelandic explorer Leif Ericson (980-1020), reached Newfoundland and the nearby North American coast. He is regarded as the first European to land in North America.
This image illustrates the Viking settlements and voyages.
g. Polynesian Age
The Polynesians populated and explored the central and south Pacific for around 5,000 years, before their discovery of New Zealand in 1280. Since they were a maritime people, their invention of the outrigger canoe provided them a swift and stable platform for transporting goods and people. Studies in 2011 at Wairau Bar, New Zealand theorised the origin of boats was likely at Ruahine Island in the Society Islands. Polynesians may have used the prevailing north easterly trade winds to reach New Zealand within 3 weeks, with the Cook Islands as an intermediate stopping point.
Mathematical modelling based on DNA genome studies revealed a large number of Polynesian migrants (100–200), including women, arrived in New Zealand around 1280.
h. Chinese exploration of the Indian Ocean
The Chinese explorer, Wang Dayuan (fl. 1311—1350) travelled by ship to the Indian Ocean. Between 1328 and 1333, he visited Southeast Asia via the South China Sea and travelled as far as South Asia, before he arrived at Sri Lanka and India, as well as Australia. Then between 1334 and 1339, he visited North Africa and East Africa. Later, the Chinese admiral Zheng He (1371—1433) travelled to Arabia, East Africa, India, Indonesia and Thailand.
i. European Age of Discovery
Also known as the Age of Exploration, the Age of Discovery was regarded as one of the most pivotal periods of geographical exploration in human history, spanning between the early 15th century and 17th century. During this period, Europeans discovered and/or explored vast areas of the Americas, Africa, Asia and Oceania. They first explored Portugal and Spain, followed by England, Netherlands and France.
Explorers of Africa included Diogo Cão (c.1452 –c.1486) and Bartolomeu Dias (c. 1450–1500). The former discovered and ascended the Congo River, then reached the coasts of the present-day Angola and Namibia. The latter was the first European to reach the Cape of Good Hope and other parts of the South African coast. Explorers of routes from Europe towards Asia, the Indian Ocean, and the Pacific Ocean, include Vasco da Gama (1460–1524), Pedro Alvares Cabral (c. 1467/68–c.1520) and Diogo Dias. Vasco da Gama made the first trip from Europe to India and back by the Cape of Good Hope, and discovered the ocean route to the East. Cabral followed the path of Gama, claimed Brazil and led the first expedition connecting Europe, Africa, America, and Asia. Dias discovered the eastern coast of Madagascar and rounded the corner of Africa. Other explorers such as Diogo Fernandes Pereira and Pedro Mascarenhas (1470–1555), among others, discovered and mapped the Mascarene Islands and other archipelagos.
This map shows the outward and return voyages of the Portuguese India run in the Atlantic and Indian Oceans, with the North Atlantic Gyre (Volta do mar) picked up by Henry’s navigators, and the outward route of the South Atlantic westerlies that Bartolomeu Dias discovered in 1488, followed and explored by the expeditions of Vasco da Gama and Pedro Alvares Cabral.
António de Abreu (c.1480–c.1514) and Francisco Serrão (sometime in the 1400s — 1521) led the first direct European fleet into the Pacific Ocean (on its western edges), through the Sunda Islands, reaching the Moluccas. Andreas de Urdaneta (1498–1568) discovered the maritime route from Asia to the Americas. Jorge de Menezes (c. 1498–?) sailed across the Pacific Ocean and discovered Papua New Guinea, and García Jofre de Loaísa (1490–1526) discovered the Marshall Islands.
— Discovery of America:
This map illustrates the Transatlantic voyages of Christopher Columbus.
Christopher Columbus (1451–1506) made the initial discovery of the Americas by leading a Castilian (Spanish) expedition across the Atlantic. After this discovery, a number of further expeditions were sent out to explore the Western Hemisphere including those led by Juan Ponce de León (1474–1521), Vasco Núñez de Balboa (c. 1475–1519) and Aleixo Garcia (sometime in the 1400s—1527).
- León discovered and mapped the coast of Florida
- Balboa was the first European to view the Pacific Ocean from American shores (after crossing the Isthmus of Panama), which confirmed America being a separate continent from Asia
- Garcia explored the territories of present-day southern Brazil, Paraguay and Bolivia. He crossed the Chaco and reached the Andes (near Sucre).
- Álvar Núñez Cabeza de Vaca (1490–1558) discovered the Mississippi River and was the first European to sail the Gulf of Mexico and cross Texas.
- Jacques Cartier (1491–1557) drew the first maps of part of central and maritime Canada.
- Francisco Vázquez de Coronado (1510–1554) discovered the Grand Canyon and the Colorado River.
- Francisco de Orellano (1511–1546) was the first European to navigate the length of the Amazon River.
— Further explorations:
Ferdinand Magellan (1480–1521) was the first navigator to cross the Pacific Ocean, discovered the Strait of Magellan, the Tuamotus and Mariana Islands and nearly circumnavigated Earth for the first time across multiple voyages. In fact, the title for the first ever global circumnavigator belongs to Juan Sebastian Elcano (1476–1526).
Throughout the 16th and 17th centuries, Asia and the Pacific Ocean were explored by Andrés de Urdaneta (1498–1568), Pedro Fernandes de Quierós (1565–1614) and Álvaro de Mendaña (1542–1595).
- Urdaneta discovered the maritime route from Asia to the Americas.
- Quierós discovered the Pitcairn Islands and the Vanuatu archipelago.
- Mendaña discovered the Tuvalu archipelago, the Marquesas, the Solomon Islands, and Wake Island.
Explorers of Australia included Willem Janszoon (1570–1630), Yñigo Ortiz de Retez, Luis Váez de Torres (1565–1613) and Abel Tasman (1603–1659).
- Janszoon first recorded European landing in Australia.
- Retez discovered and reached eastern and northern New Guinea.
- Torres discovered the Torres Strait between Australia and New Zealand.
- Tasman explored North Australia, discovered Tasmania, New Zealand and Tongatapu.
Explorers of North America included Henry Hudson (1560s—1611), Samuel de Champlain (1574–1635), René-Robert Cavalier and Sieur de La Salle (1643–1687).
- Hudson explored a bay in Canada named Hudson Bay.
- Champlain explored St. Lawrence River and the Great Lakes (in Canada and northern United States).
- Cavalier and La Salle explored the Great Lakes region of the United States and Canada, and the entire length of the Mississippi River.
j. The Modern Age
After the golden age of discovery, other explorers completed the world map.
- Vitus Bering (1681–1741) explored the Bering Strait, the Bering Sea, North American coast of Alaska, , and some other northern areas of the Pacific Ocean.
- James Cook explored the east coast of Australia, the Hawaiian Islands, and circumnavigated the Antarctic continent.
This map shows the routes of Captain James Cook’s voyages. The 1st voyage is shown in red, 2nd voyage in green, and 3rd voyage in blue.
- The Lewis and Clark expedition (1804-1806), dispatched by President Thomas Jefferson, reached the newly-acquired Louisiana Purchase and found an interior aquatic route to the Pacific Ocean, examined the flora and fauna of the continent.
- The United States Exploring Expedition (1838-1842), dispatched by President Andrew Jackson, surveyed the Pacific Ocean and surrounding lands.
k. Space exploration
Humanity is continuing to follow the impulse to explore beyond Earth and the solar system in the universe. Space exploration took off in the 20th century with the invention of exo-atmospheric rockets. This allowed the first humans to the Moon in 1969, and robotic explorers to other planets such as Mars and Jupiter, and far beyond.
As of now, both of the Voyager probes have left the Solar System and entered interstellar space.
I’ll delve into the different types of exploration and names of famous explorers in another post.
What is cell migration?
This process is central in the development and maintenance of multicellular organisms. This orchestrated movement of cells in particular directions to specific locations is integral in tissue formation during embryonic development, wound healing and immune responses. Cells often migrate in response to specific external signals, whether they be chemical or mechanical. Errors in cell migration can lead to intellectual disability, vascular disease, tumour formation and metastasis. Understanding the mechanism behind cell migration is the key to developing novel therapeutic strategies for controlling invasive tumour cells, for example.
Due to the highly viscous environment, cells are required to continuously produce forces in order to move. Many less complex prokaryotic organisms (and sperm cells) use flagella or cilia to propel themselves forwards. Eukaryotic cell migration demonstrates more complexity as it consists combinations of different migration mechanisms, which involves drastic changes in cell shape driven by the cytoskeleton. Huber et al. (2013) stated 2 distinct migration scenarios called ‘crawling motion’ and ‘blebbing motility’. An example of crawling motion is demonstrated by fish epidermal keratocytes.
What do cell migration studies reveal about cell migration?
A 2013 report stated that the processes underlying mammalian cell migration are consistent with those of (non-spermatozooic) locomotion. Common observations include cytoplasmic displacement at leading edge (front) and laminar removal of dorsally-accumulated debris toward trailing edge (back). A 1970 study identified the latter feature in the cross-linkage between aggregates of a surface molecule and fluorescent antibody or bondage of small beads with the front of the cell. The amoeba Dictyostelium discoideum consistently exhibited chemotaxis in response to cyclic AMP, moved faster than cultured mammalian cells and consisted of a haploid genome that simplified the process of connecting a particular gene product with its effect on cellular behaviour.
Describe the molecular processes of cell migration
This diagram shows 2 different models for how cells move. A) Cytoskeletal model. B) Membrane Flow Model.
2 main theories for how the cell advances its front edge are suggested: the cytoskeletal model and membrane flow model.
(a) Cytoskeletal Model
A 1985 study showed rapid Actin polymerisation at the cell’s leading edge. This lead to the hypothesis that formation of actin filaments "push" the leading edge forward and is the main motile force for advancing the cell's front edge. A 2008 study found extensive and intimate interaction between the cytoskeletal elements and the cell’s plasma membrane.
— Trailing Edge = Microtubules act as “struts” that counteract the contractile forces required for trailing edge retraction during cell movement. Since the microtubules in cell’s trailing edge are dynamic, they can remodel to allow retraction. A 2010 study found suppression of the dynamics inhibit the microtubules’ ability to remodel and oppose the contractile forces. Ganguly et al. (2012) found the cells with suppressed microtubule dynamics can still extend the front edge (polarised in the direction of movement), but cannot easily retract the trailing edge. Higher drug concentrations or mutations in microtubules that depolarise the microtubules leads to restoration of cell migration and loss of directionality. This suggests microtubules act both to restrain cell movement and to establish directionality.
(A) Dynamic microtubules are necessary for tail retraction and are distributed at the rear end in a migrating cell. Green, highly dynamic microtubules; yellow, moderately dynamic microtubules and red, stable microtubules.
(B) Stable microtubules act as struts and prevent tail retraction and thereby inhibit cell migration.
(b) Membrane Flow Model
Bretscher (1983) found the front of the migrating cell is the site at which the membrane returns to the cell surface from internal membrane pools at the end of the endocytic cycle. This suggested addition of membrane at the front of the cell occurs by extension of the leading edge. Bretscher (1996) hypothesised actin filaments forming at the front help stabilise the added membrane to allow the formation of a structured extension, or lamella, rather than a bubble-like structure (or bleb) at its front. Movement of a cell requires “feet” or proteins called integrins, which attach a cell to the surface on which it is crawling) to the front. Researchers suggested integrins are endocytosed toward the rear of the cell and transported to the cell's front by exocytosis for the purpose of being reused to form new attachments to the substrate.
— Mechanistic basis of amoeboid migration
Metastatic cells and immune cells like cells such as neutrophils and macrophages demonstrate adhesion-independent migration. However, more research needs to be done to fully understand the mechanistic basis of this migration mode. In 1977, physicist E.M. Purcell hypothesised rearward surface flow provide a mechanism for microscopic objects to swim forward under conditions of low Reynolds Number fluid dynamics. In 2018, optogenetic studies observed cells that migrate in an amoeboid fashion without adhesions. Those cells exhibited plasma membrane flow towards the cell’s rear, where it can propel them by exerting tangential forces on the surrounding fluid. O’Neill et al. (2018) demonstrated polarised trafficking of membrane-containing vesicles from the cell’s rear to its front is important for maintenance of cell size. Tanaka et al. (2017) found Dictyostelium discoideum cells exhibited rearward membrane flow. Shellard et al. (2018) found evidence of the migration of supracellular clusters driven by a similar mechanism of rearward surface flow.
This diagram illustrates rearward membrane flow (red arrows) and vesicle trafficking from back to front (blue arrows) driving adhesion-independent migration.
— Collective biomechanical and molecular mechanism of cell motion
A 2011 study by the Coskuns hypothesised a novel biological model for collective biomechanical and molecular mechanism of cell motion. It stated microdomains weave the texture of cytoskeleton and their interactions mark the location for formation of new adhesion sites. Their model suggested microdomain signaling dynamics play a role in the organisation of the cytoskeleton and its interaction with substratum. Not only do Microdomains trigger and maintain active polymerisation of actin filaments, they also propagate and zigzag on the membrane to generate a highly interlinked network of curved or linear filaments oriented at a wide spectrum of angles to the cell boundary. Theories suggest microdomain interaction marks the formation of new focal adhesion sites at the cell periphery. Myosin interaction with the actin network then lead to membrane retraction/ruffling, retrograde flow, and generation of contractile forces for forward motion. Finally, continuous application of stress on the old focal adhesion sites would activate calcium-induced calpain, which cause the detachment of focal adhesions, completing the cycle.
This schematic represents collective biomechanical and molecular mechanism of cell motion.
What is polarity in migrating cells?
Migrating cells have a front and a back, giving it polarity. This prevents it from spreading i.e. moving in all directions. However it is unknown how the cell formulates this polarity at a molecular level. If a cell meanders randomly, the front gives way to become passive as another region, or regions, of the cell form(s) a new front. Chemotaxing cells demonstrated enhanced stability of its front as they migrate towards an area of a higher concentration of the stimulating chemical. At a molecular level, this polarity is demonstrated by certain molecules being restricted to particular regions of the inner cell surface. Several studies discovered the phospholipid PIP3 and activated Rac and CDC42 at the front of the cell, and Rho GTPase and PTEN toward the rear.
Actin filaments may be a major component in the locomotory mechanism, as membrane vesicles are transported along these filaments to the cell's front. Since chemotaxing cells demonstrate migratory persistence toward the target, it may be due to the enhanced stability of how the filamentous structures are arranged inside the cell, which determines its polarity. This means the arrangement of these filamentous structures inside the cell are similar to the arrangement of PIP3 and PTEN on the inner cell membrane. It is suggested the location of these structures are determined by the chemoattractant signals as these impinge on specific receptors on the cell's outer surface. However, the mechanism behind the microtubules’ role in cell migration remains unclear. Studies in 2012 argued microtubules may not be required for migration on a planar surface, however they are required for determining directionality in cell movement and ensuring efficient protrusion of the leading edge. Ganguly et al. (2012) found that the presence of microtubules retard cell movement when their dynamics are suppressed by drug treatment or by tubulin mutations.
If you have watched animal documentaries narrated by Sir David Attenborough or the movie Ice Age, you would be aware that animals travel long distances across the lands, in the air or across the seas, usually during the change of seasons. In ecology, all major animal groups migrate, including birds, mammals, fish, reptiles, amphibians, insects, and crustaceans. Triggers for animal migration include local climate, local availability of food, seasons or mating opportunities. One researcher suggested “true migration” should be defined as animal movement occurring at an annual or seasonal basis, rather than a local dispersal or irruption, or a major habitat of change as part of their life. Examples are:
— Northern Hemispheric birds migrate southwards for the winter.
— Wildebeest migrate annually for seasonal grazing.
— Young Atlantic salmon or Sea lamprey leave the river of birth after growing to a few inches in size.
Because animal migration come in many different forms in different species, there is no accepted unified definition. In 1985, J.S. Kennedy proposed his own definition by stating the “persistence and straightened movement effected by the animal’s own locomotory exertions or by its active embarkation upon a vehicle. This behaviour depends on some temporary inhibition of station keeping responses but promotes their eventual disinhibition and recurrence.”
Hugh Dingle and Alastair Drake (2007) laid out 4 concepts that encompassed migration:
1. Persistent straight movement
2. Relocation of an individual on a greater scale (both spatially and temporally) than its normal daily activities
3. Seasonal to-and-fro movement of a population between two areas
4. Movement leading to the redistribution of individuals within a population
Migration can be either:
— Obligate = Individuals must migrate
— Facultative = Individuals can "choose" to migrate or not.
Within a migratory species or even within a single population, not all individuals migrate simultaneously.
— Complete migration = 100% of all individuals migrate
— Partial migration = Some individuals migrate while others do not.
— Differential migration = The difference between migratory and non-migratory individuals is based on age or sex (for example).
In 1974, I.A. McLaren discovered many aquatic animals travel a few hundred meters up and down the water column, known as the Diel vertical migration. In 1981, Hamner & Hauri found some jellyfish migrate horizontally for a few hundred meters across a lake. In 1920, Ernest Ingersoll noted that the pressure of famine, overpopulation of a locality, or some more obscure influence can cause irregular (non-cyclical) migrations such as irruptions.
— Seasonal Migration = Movement of various species from one habitat to another during the year. Depending on seasonal fluctuations, resource availability changes, which influences migration patterns. Meanwhile, other species like the Pacific salmon migrate for reproductive purposes. Annually, they travel upstream to mate and then return to the ocean. Another seasonal driving factor of migration is temperature. During wintry seasons, many species, especially birds, migrate to warmer locations to escape the poor environmental conditions.
— Circadian Migration = This is observed in birds, which utilise circadian rhythm (CR) to regulate migration in both the fall and the spring.
This refers to the clocks of both circadian (daily) and circannual (annual) patterns being utilised by birds to determine their orientation in both time and space during their migratory journey. This type of migration benefits birds living close to the equator during winter, which allows them to monitor their brain’s auditory and spatial memory in order to remember an optimal site of migration. E. Gwinner (2018) noted that these birds have a timing mechanism that provide avians with the distance required to travel in order to reach their destination. The mammalian circadian clock is utilised to regulate the migration patterns of these birds. Boucher at al. (2016) suggests this allows the avians to determine the most appropriate time to migrate, the most suitable location to regulate their metabolism, and the most suitable environment (land or water) that provides an evolutionary advantage as they migrate.
— Tidal Migration = Organisms uses the tides to move periodically from one habitat to another, which is often facilitated by ocean current.
Organisms using this migratory method aim to search for food or mates. R. Gibson (2003) found that tides carry organisms horizontally and vertically ranging from a few millimetres to a few thousand kilometres. During daily tidal cycles, migration to and fro the intertidal zone is most commonly used by many different species such as crabs, nematodes, small fish, corals. These organisms cycle to these areas as the tides rise and fall typically about every 12 hours. Their cyclical movements associate with foraging of marine and bird species. During low tides, smaller or younger species will emerge to forage as their survival chances is higher in shallower water due to low probability of being preyed upon by predators. Whereas during high tides, larger species tend to emerge as the waters are deeper and nutrients upwell from the tidal movements. Tidal currents carry organisms along further distances than at normal swimming speeds in calmer waters, which often bring them to breeding grounds and nurseries. These breeding grounds often situate near or in intertidal zones due to the food and nutrient abundance in these areas, which makes it an ideal place for offspring to thrive.
1. Altitudinal
Altitudinal migration is defined as a short-distance animal migration from lower altitudes to higher altitudes and back.
In which regions does it occur?
i. Tropics
In the tropics, altitudinal migrations are commonly demonstrated by frugivores or nectarvores, such as tropical hummingbirds, which migrate in response to shifts in food abundance and availability. Neotropical birds, terrestrial, tropical montane species such as Baird’s tapir and white-lipped peccary are also seen undergoing altitudinal migration.
A few studies identified several tropical avian species that are altitudinal migrants such as the white-ruffed manakin, resplendent quetzal, at least 16 species of raptor, and many species of hummingbird.
A 2013 study noted several tropical bat species are altitudinal migrants, but more research is required to understand the reasons for their movements.
ii. Temperates
In montane zones such as in the Rocky Mountains, most ungulates are altitudinal migrants such as the roe deer, bighorn sheep and mountain sheep. Examples of Avian temperate species that demonstrate altitudinal migration are the mountain chickadee and the American dipper. Temperate bat species are also altitudinal migrants with females inhabiting lower elevations during reproductive periods compared to their male counterparts.
What causes it?
Since it is a short-distance migration pattern, altitudinal migration is considered easier to trace than long-distance patterns. The limited success of mark and recapture techniques used to track migratory species poses a challenge in determining the ultimate causes of altitudinal migration. Scientists have suggested hypotheses to explain why altitudinal migration may occur.
— Food abundance and nutrition
Boyle et al. (2013) hypothesised food abundance as the main motivator behind altitudinal migration. They stated that peaks in food abundance along an elevational gradient, such as the slope of a mountain, drive migration patterns as species exploit available food resources. Some frugivorous birds, such as white-ruffed manakins (Corapipo altera) were known to migrate to higher elevations to exploit peaks in fruit abundance, which coincided with the breeding season. Current evidence supported the theory of increased foraging ability over a larger area providing a competitive advantage compared to non-migrant (sedentary) species, which resulted in greater food and nutrient uptake. A 2008 study uncovered differences in diet between non-migratory and migratory species in large-scale analyses and species-pair comparisons of frugivorous tropical birds.
However, critics argued that this hypothesis failed to explain why altitudinal migrants return to lower elevations, or if it is done in response to shifting food resources. A 2010 study proposed weather-related resource availability may be a trigger for the elevational migration of some species, such as the white-ruffed manakin during storms.
— Reproduction
Male white-ruffed manakins migrate to lessen social status and mating success at leks the following breeding season. Colwell et al. (2010) found most hummingbird species at Monteverde increase altitude during the wet season in order to breed. Bildstein (2004) found most of the 16 species of neotropical raptors (including the Andean condor Vultur gryphus), known to be altitudinal migrants, breed in the high Andes and migrate to lowland areas during non-breeding seasons.
— Nest predation
The seasonal altitudinal migration of some passerine birds is found to be associated with a decreased risk of nest predation at higher altitudes. An experiment featuring 385 nests at different locations on the Atlantic slope of Costa Rica showed decreased predation at increasing altitudes, with predation highest at intermediate altitudes. This ‘nest predation’ hypothesis states altitudinal migration may have evolved among some species as a response to nest predation, in a way of lowering the risk. Alice Boyle (2008) asserted elevation of home range influences breeding time.
— Anthropogenic
Fothergill (2006) found the walia ibex are driven to higher altitudes in Ethiopian mountain ranges due to human activity impacting their native range, including war, expanding human settlement, and cultivation.
2. Tracking
In wildlife biology, conversation biology, ecology and wildlife management, animal migration tracking is used to study animals' behaviour in the wild.
This photo shows a monarch butterfly shortly after tagging at the Cape May Bird Observatory, which is one of the organisations that has a monarch identification tagging program. Researchers place plastic markers on wing of the insect with identification information. Tracking information is used to study the migration patterns of monarchs, including how far and where they fly.
What technologies are used for tracking?
In the fall of 1803, American Naturalist John James Audubon wondered whether migrating birds returned to the same place each year. Before it migrated south, he tied a string around the leg of a bird. In the following spring, he spotted the same bird with the string returning to the same location.
— Radio tracking
This is a photo of a U.S. Fish & Wildlife employee using radio telemetry to track mountain lions.
Animal tracking by radio telemetry involves 2 devices.
i. Telemetry
This involves a transmitter being attached to an animal (around the ankle, neck, wing, carapace or dorsal fin) and transmits a radio wave signal. The transmitter sends out a frequency in the VHF band as antennas in this band are small. The waves are transmitted in brief pauses to conserve battery power, around 1 every second. A specialised radio receiver called a radio direction finding (RDF) receiver, usually in a truck, ATV or airplane, detects the signal. The receiver has a unidirectional antenna (usually a Yagi antenna) that indicates the strength of the received signal, either by a metre or by the loudness of the pulses in earphones. Researchers then rotate the antenna until they achieve the strongest radio signal received, before pointing the antenna towards the animal. They use the receiver to follow the animal in order to keep track of the signal. Furthermore it homes in on animals to get the payloads they equipped back.
ii. Geolocators / “Geologgers”
This technology uses a light sensor that tracks the light-level data during regular intervals in order to determine a location based on the length of the day and the time of solar noon. It is commonly used to track small birds over long distances during migration.
iii. Passive Integrated Transponders (PIT tags)
PIT are another (albeit cost-effective) method of telemetry used to track the movements of a species. These electronic tags collect data from a specimen without recapturing and handling them. The data captured and monitored via electronic interrogation antennae includes records of the time and location of the individual. They are humane method of tracking with minimal risk of infection or mortality as there is limited contact with the specimens.
— Satellite tracking
Networks, or groups, of satellites are another technology used by scientists to track animals. Each satellite in a network detects electronic signals from a transmitter on an animal. The combination of signals from all satellites help determine the precise location of the animal, as well as tracks its movements. Satellite-received transmitters fitted to animals also provides information about the animals’ physiological characteristics (e.g. temperature and habitat use). Examples of animals being tracked via satellite transmitters are caribou, sea turtles, whales, great white sharks, seals, elephants, bald eagles, ospreys and vultures, as well as marine mammals and various species of fish. There are two main satellite tracking systems: ARGOS and GPS. Another form of satellite tracking is acoustic telemetry, which emit sound to track and monitor an animal within 3 dimensions.
— Stable isotopes
They are 1 of the intrinsic markers used for studying migration of animals. This marker, including stable isotope analysis, doesn’t require the capture, recapture and tagging of an organism. Each organism captured provides information on its actions and whereabouts based on its diet. The 2 other types of intrinsic markers are (1) contaminants, parasites, and pathogens, and (2) trace elements.
Certain geographic regions have specific stable isotope ratios that influence the chemistry of organisms foraging in those locations. This creates “isoscopes”, which provides scientists information on the locations the organisms have been feeding. The prerequisites for stable isotope analysis include:
i. The animal must have at least one light isotope of interest in specific tissues that can be sampled.
ii. The organism needs to migrate between isotopically different regions and these isotopes must be retained in the tissue in order for the differences to be measured.
Ceriani et al. (2019) stated that stable isotope analysis could determine foraging locations of terrestrial and aquatic organisms such as loggerhead sea turtles. Satellite telemetry could confirm the accuracy of the location derived from the stable isotope analysis. This allows migration studies to increase their sample sizes, as satellite telemetry is expensive and tissue, blood, and egg samples can be taken from the female turtles laying egg.
3. Coded Wire Tag (CWT)
CWT is an animal tagging device used to identify batches of fish. They consist of a length of magnetised stainless steel wire 0.25 mm in diameter and typically 1.1 mm long. The tag is marked with rows of numbers denoting specific batch or individual codes. It is usually injected into the snout of cheek of a fish so research or fisheries management can track it.
This photo shows a giant model coded wire tag with actual tags. The penny is placed next to it for scale.
How is data retrieved?
A handheld wand or tunnel type detector is used to sense the magnetised metal and detect its presence at close range. Etched on the surface of the CWT is a number code unique to either a group of fish or an individual fish. To read the code, the fish has to be killed first so the tag can be extracted and inspected under a microscope. Information about the fish such as hatching date, release date, location, species, sex, and length are recorded along with the corresponding tag code into a database. This allows codes on recovered tags to be matched to information within that database.
When was the CWT first used?
They were first used in the 1960s as an alternative to fin clipping. The first ones used coloured stripes with about 5000 different colour combinations for unique identification of different tags. In 1971, electrical discharge machining marked binary codes onto the tags with about 250,000 unique code combinations. In 1985, the sequential CWT had sequentially increasing numbers, or codes, to be cut from the same spool of wire, in order for individual or small groups of fish to be uniquely identified.
Advancements in laser technology allowed digits to be etched on the tags, which drove the the switch from binary to decimal codes. The decimal systems involved a set of numbers being marked on the CWT, which is the current standard today. Solomon (2005) estimated this system had about 1 million unique codes for batch tags, and 100,000 unique codes for individual sequential tags.
How are CWT used?
A small handheld injector is used to tag small numbers of fish, while an automatic tag injector is used for large scale tagging projects. For a juvenile fish, a tag is manually pressed into a moulded guide on its snout to correctly position it, so it can be automatically injected by the machine. These automatic injectors are integrated into a fully automated process inside of a mobile trailer called an AutoFish trailer, or autotrailer, where fish are mechanically sorted, adipose fin clipped, and tagged. Autotrailers do not require anesthetic or the fish to be removed from water. When fish are tagged, their tag code and other information such as release date and location are entered into a database. In the Pacific Northwest, the Regional Mark Information System is used.
(a) Birds
Sekercioglu (2007) estimated approximately 1,800 of the world's 10,000 known bird species migrate long distances annually in response to the changing seasons. Berthold, Bauer, & Westhead (2001) theorised many of the migrations were in a north-south direction, with species feeding and breeding in high northern latitudes in the summer, while other species move 100s of kilometres south for the winter. For example, the Arctic tern is famous its annual migration between the Northern and Southern Hemispheres, as it flies from its Arctic breeding grounds to the Antarctic and back again each year. Steve Cramp (1985) estimated this migration journey to be a distance of at least 19,000 km (12,000 mi), giving it 2 summers every year.
This diagram shows examples of long distance bird migration routes.
Movements of bird species vary depending on the food availability, habitat, or weather. Their journeys aren’t necessarily categorised as “true migration” because they are irregular (nomadism, invasions, irruptions) or unidirectional (dispersal, movement of young away from natal area). On the other hand, migratory birds are described as resident or sedentary. The most common avian migratory pattern involves flying northwards in the spring to areas with temperate or Arctic summer for breeding purposes and flying southwards to wintering grounds in warmer regions. The flying directions are reversed for birds living in the southern hemisphere. Newton (2008) identified there is less land area in the far south to support long-distance migration nonetheless. Birds migrate in search of abundant food sources such as hummingbirds, but tend to be resident if adequately fed during the winter. Since the days in the northern summer are longer, this provides additional time for breeding birds to feed their young. This helps diurnal birds produce larger clutches than related non-migratory species that remain in the tropics. Since the days in autumn are shorter, the same birds return to warmer regions where the supply of available food is quite abundant with little variability.
The advantages of migration seem to offset the high stress, physical exertion costs, and other risks. However, migration can increase the risk of being preyed upon by predators such as Eleonora’s falcon (Falco eleonorae) the and greater noctule bat. They coordinate their delayed breeding season on the Mediterranean islands with the autumn passage of southbound passerine migrants, which it feeds to its offspring. The increased numbers of migrating birds at stopover sites make them vulnerable to parasites and pathogens, which sparks the need for an alert immune response.
When a proportion of a species’ population migrates, this is depicted as a ‘partial migration’. In Australia, 44% of non-passerine birds and 32% of passerine species are partially migratory. Some species often winter at lower latitudes, and migrate at higher latitudes. These migrating birds can bypass latitudes where sedentary bird populations situate as these suitable wintering habitats have been occupied. Boland (1990) names this migratory behaviour as ‘leap-frog migration’. Peter Berthold (2001) described bird populations that 'slide' more evenly north and south without reversing order as ‘chain migration’.
Within a population, the patterns of timing and distance can differ between ages and/or sexes. For example, female chaffinches (Fringilla coelebs) in in Eastern Fennoscandia migrate first before the males during the autumn and the European tits of genera Parus and Cyanistas only migrate in their first year.
1. Flyways
They are defined as flight paths used by large bird populations migrating between their breeding grounds and their overwintering quarters. Flyways span continents and pass over oceans.
Many bird populations migrate long distances twice a year. The most common pattern is in the northerly direction during the spring for breeding in the temperate parts of the northern hemisphere or the Arctic during summer. Then in the autumn, they migrate in a southerly direction to wintering grounds in warmer regions, often on the other side of the equator. In the southern hemisphere, birds fly southwards to breed and fly northwards to overwinter. Mark Colwell (2010) stated variation in the flyway, or route taken by different bird species, with each population plotting its staging points along the route where each bird specimen feed to build up their energy reserves to prepare for the next migratory stage. There may be differences in the routes used for spring migration compared to the autumn migration as it is influenced by factors such as wind direction and the availability of food at staging points.
Flyways have curves or doglegs, which lengthens the route compared to a direct straight route. This is possibly due to avoid obstacles such as mountain ranges and oceans, and to run parallel to the barriers and follow routes along the coast or along major river valleys. Galbraith et al. (2014) found passerines often fly on a broader front across the terrain, either over or circumventing obstacles on the route, depending on their evolutionary adaptations. Vansteelant et al. (2017) found birds overcompensate for predicted winds when selecting their flyway route.
— Terrestrial birds fly over land.
— Raptors fly over thermals to provide them the required lift.
— Sea birds fly over ocean routes.
— Wetland birds fly over routes with suitable staging sites.
Deltas and coastal wetlands are abundant food sources for animals that stage, whereas inland wetlands are less predictable.
There are 5 major north/south flyways in North America and 6 covering Eurasia, Africa and Australasia.
i. Americas
— Atlantic Flyway = From Northern Canada and Greenland, along the Atlantic coast of Canada and the United States to the Caribbean Sea, and on to tropical South America.
— Mississippi Flyway = From northeastern Canada, passing over the Great Lakes, following the lower Ohio River, the Missouri and the Mississippi to the Gulf of Mexico, and on to Central and South America.
— Central Flyway = From Central Canada, crossing the Great Plains, moving southwards to the Gulf of Mexico, merging with the Mississippi Flyway.
— Pacific Flyway = From the breeding grounds in Alaska and Canada to their overwintering areas in South America, sometimes as south as Patagonia.
— Allegheny Front Flyway = Located in the central Appalachian Mountains where migratory birds travel from their northern breeding grounds to their southern wintering sites.
This diagram illustrates flyway distribution for N. American waterfowl: Atlantic, Mississippi, Central, and Pacific Flyways.
ii. Eurasia, Africa & Australasia
— East Atlantic Flyway = Starting from the northern North America, Greenland, Iceland, northern Europe and western Siberia, leading to wintering areas in western Europe and North Africa. Some birds were found to continue flying down the west coast of the continent to South Africa.
— Black Sea-Mediterranean Flyway = Starting from the northern and western Siberia, leading across Asia, the Black Sea and the Mediterranean Sea to northern Africa.
— Asian-East African Flyway = Starting from the northern breeding grounds of water birds in Siberia and leading across Asia to East Africa.
— Central Asian Flyway = Starting from the northern breeding grounds of water birds in Siberia and leading across Asia to the Indian subcontinent.
— East Asian-Australasian Flyway = Starting from the Taymyr Peninsula in Russia and Alaska, extending southwards to southeastern Asia, Australia and New Zealand. It is overlapping the West Pacific Flyway. A study found about 60 species of shorebird use this route.
— West Pacific Flyway = A link between New Zealand and Australia’s east coast, through the central Pacific Ocean and the east coast of northern Asia, including Japan and the Korean Peninsula. Then it ends up in eastern Siberia, including the Chukchi and Kamchatka Peninsulas, and Alaska. this flyway overlaps with the East Asian–Australasian Flyway.
This diagram illustrates Central Asian, East Asian-Australasian and West Pacific migratory bird flyways.
2. Reverse Migration
Also known as ‘Reverse Misorientation’, this phenomenon involves birds flying in the opposite direction than most of the same species during migration time.
The orange arrow indicates birds flying in the opposite direction of the red arrow. These birds will end up in Western Europe rather than Southeast Asia. This mechanism leads to birds such as Pallas’s warbler ending up 1000s of kilometres from the rest of the bird population.
Is reverse migration genetic or learned behaviour?
Eileen Rees (1989) suggested some large birds such as swans learn migration routes from their parents. However she found most small species, such as passerines, show signs the migratory route they take may be genetically programmed. The offspring of these species demonstrated to navigate to their wintering area. If this programming malfunctions, this leads to young birds migrating in the opposite direction from their intended destination.
As part as his vagrancy theory, James Gilroy (2003) suggested reverse migration could occur from the learning improper routes from parents or other birds. Gilroy and Lees (2003) also theorised that rare variations and defects to the genetic programs responsible for proper migration can alter the migration routes, accounting for some but not all cases of reverse migration. If some birds managed to survive after the alteration to their genetically programmed migration route, this information may be passed on their offspring. If these offspring follow this altered migration route, it provides evidence for the theory that reverse migration is genetically programmed.
Birds that adopt and continue to migrate in atypical directions are known as ‘Pseudo-vagrancy migrators’. Examples of pseudo-vagrancy migrators include yellow-breasted bunting and yellow-browed warblers.
Describe the patterns in reverse migration
— Is reverse migration only occurring in the opposite direction or in random directions?
Åkesson et al. (1996) found many species reverse migrate worldwide throughout the day and night. Despite the irregularity of the migration direction, it is evaluated to be approximately opposite to a species typical direction rather than a random direction. This phenomenon occurs in species migrating to a tropical area during the winter months, as well as temperate zone migrants, short irruptive food migrants, short distance migrant, and long distance migrants.
— Solitary reverse migration during the night
During the night, solitary birds tend to reverse migrate West rather than East. Kasper Thorup (2004) observed more occurrences of West-to-East reverse migration than either North or South reverse migration.
— Does inadequate fat stores lead to reverse migration?
Several studies suggested reverse migration occurs in bird species with low fat storage compared to higher fat storage. Smolinsky et al. (2013) found thrush songbirds that changed their normal seasonal southerly migration were lean and low on fat stores. The inland northerly path it travelled on suggested their normal stopover location had insufficient resources to replenish their fat stores. This would have forced the songbirds to reverse migrate and move inland northwards in search of more food.
D’Amico et al. (2014) found a shorebird species called red knots demonstrate reverse migration. Although no significant difference in body mass fat stores or sex were found in these reverse migrating birds, they did find reverse migration significantly decreased their hematocrit levels (% Red Blood Cells). Before large food consumption for fat storage, birds increase their hematocrit in order to have sufficient energy for their long migration flight. If the theory is confirmed, this would explain why these birds chose to travel in reverse migrate 200 km before attempting the long migration flight.
3. Drift Migration
This phenomenon involves the winds blowing the migrating birds off course mid-flight. Kasper Thorup (2003) found this often occurs in birds flying south in autumn due to a large proportion of birds being inexperienced and young, hence less capable of compensating than the adults flying north in spring.
Disorienting conditions such as mist or drizzle can contribute to drift migration, resulting in large numbers of birds arriving together in areas where they don’t usually situate. In the UK, it is called a ‘fall’, whereas in the USA, it is called a ‘fallout’.
e.g. In September, an east wind blows Scandanavian migrants such as bluethroats, wrynecks, and the continental race of robin onto the east coast of England and Scotland. This leads to temporary concentrations of these species at headlands like Spurn.
Describe nocturnal migratory behaviour
Andrew Farnsworth (2005) found that whenever birds participate in nocturnal migration, they give short, contact-type calls known as 'Nocturnal Flight Calls’. Griffiths et al. (2016) suggested they serve to maintain the composition of a migrating flock, as well as encode the gender of a migrating individual, and avoid mid-air collisions.
Nocturnal migrants that land in the morning to feed for a few days before resuming their migratory journey toward their destination are labelled as ‘passage migrants’. This helps minimise depredation, avoid overheating, and maximise feeding during the day, at the expense of sleep.
Which birds perform long-distance migration?
— Examples of landbird species involved in long flights from the north to the tropics include swallows (Hirundinidae) and birds of prey.
— Nevertheless, many species of Holarctic wildfowl and finch (Fringillidae) winter in the North Temperate Zone, especially regions with milder winters than their summer breeding grounds. For example, the pink-footed goose that migrates from Iceland to Britain and neighbouring countries, whilst the dark-eyed junco migrates from subarctic and arctic climates to the contiguous United States.
— The American goldfinch migrates from taiga to wintering grounds, which extends from the American South northwestward to Western Oregon.
— Some ducks move completely or partially into the tropics, such as the garganey, Anas querquedula.
— The European pied flycatcher (Ficedula hypoleuca) also follows this migratory trend, by breeding in Asia and Europe and wintering in Africa.
The path of long-distance migration by birds between breeding and wintering grounds isn’t always linear. Instead, the path is rather hooked or arched, with detours around geographical barriers or towards suitable stopover habitat. Most land birds take these non-linear flight paths to avoid barriers such as seas, large water bodies or high mountain ranges, a lack of stopover or feeding sites, or a lack of thermal columns (scavenged by broad-winged birds). According to evolutionary history, many migration routes are circuitous. For example, Bairlein et al. (2012) discovered the expanding breeding range of Northern wheatears (Oenanthe oenanthe) across the entire Northern Hemisphere. However, they migrate up to 14,500 km to reach ancestral wintering grounds in sub-Saharan Africa rather than establish new wintering grounds closer to breeding areas.
The barriers to long-distance land-bird migration of water birds include large areas of land without bodies of water offering feed sites. For example, Martin Green (1999) discovered brent geese (Branta bernicla) migrate from the Taymyr Peninsula to the Wadden Sea via the White Sea coast and the Baltic Sea rather than directly across the Arctic Ocean and northern Scandinavia.
i. Waders
Many species of waders or North American shorebirds, including dunlin (Calidris alphina) and western sandpiper (Calidris mauri), migrate long distances from their Arctic breeding grounds to warmer locations in the same hemisphere. However, other wader species such as semipalmated sandpiper (C. pusilla) travel longer distances to the tropics in the Southern Hemisphere.
Whether their migration was successful or not depends on the availability of certain key food resources at stopover points along the migration route. These stopover points are crucial for waders to last the distance as it requires high energy expenditure. Examples of stopover locations include the Bay of Fundy and Delaware Bay.
Gill et al. (2005) suggested the longest known non-stop flight of any avian migrant belongs to the bar-tailed godwit (Limosa lapponica), which is from Alaska to non-breeding areas in New Zealand, spanning 11,000 km. Prior to setting off on your migratory journey, they store at least 55% of their bodyweight with fat to fuel their uninterrupted voyage.
ii. Seabirds
Seabirds share similarities with waders and waterfowls in terms of migration. Black guillomot (Cepphus grylle) and some gull species are found to be sedentary. Whereas, most terns and auks move varying distances south in the northern winter, as they usually breed in the temperate northern hemisphere. The Arctic tern Sterna paradisaea holds the record for the longest distance of migration of any bird, moving from its Arctic breeding grounds to the Antarctic non-breeding areas. In 2001, Peter Pyle discovered one Arctic tern flew over 22,000 km (14,000 mi) over 3 months of fledging from the Fame Islands off the British east coast to Melbourne, Australia. This suggested many tube-nosed birds breed in the southern hemisphere and migrate north in the southern winter.
The most pelagic species, mainly in the 'tubenose' order Procellariiformes, are considered great wanderers. The albatrosses of the southern oceans are known to circle the globe as they ride the "roaring forties" outside the breeding season. They spread disperse over large areas of open ocean, but congregate at the availability of food. Among the longest-distance migrants, sooty shearwaters Puffinus griseus migrate from their breeding colony on the Falkland Islands to the North Atlantic Ocean off Norway. A 2013 report claimed some Manx shearwaters Puffinus puffinus migrate along the same pathway but in reverse. One Manx shearwater was measured to have flown 8 million km (5 million miles) during its over-50 year lifespan.
iii. Raptors (diurnal migration)
Some large broad-winged birds such as vultures, eagles, buzzards and storks use thermal columns of rising hot air over land to soar. This helps them cross large bodies of water, as these birds cannot maintain active flight for long distances. Soaring birds that aim to cross at the narrowest points face challenges flying over the Mediterranean and other seas. A large portion of giant raptors and storks are seen migrating through areas such as the Strait of Messina, Gibraltar, Falsterbo, and the Bosphorus. Other barriers, such as mountain ranges, funnel large diurnal migrants leading to bottlenecks such as the Central American migratory bottleneck and the Batumi bottleneck in the Caucasus. Maanen et al. (2001) reported funnelling of soaring birds through an area around the city of Batumi, Georgia as a result of their avoidance to fly over over the Black Sea surface and across high mountains. Gensbol (1984) estimated birds of prey such as honey buzzards lose about 10 to 20% of their weight during migration aided by thermals. This would explain their reduced foraging during migration than do smaller birds of prey with more active flight such as falcons, hawks and harriers. Panuccio et al. (2017) found bird species that did not advance their autumn migration dates over the Strait of Gibraltar correlated with declining breeding populations in Europe.
Describe short-distance migration
Many long-distance migrants are genetically programmed to respond to changing day length. Species migrating short distances require a mechanism that responds to local weather conditions. Examples including mountain and moorland breeders, such as wallcreeper (Tichodroma muraria) and white-throated dipper (Cinclus cinclus), migrate only altitudinally to escape the cold higher ground. Other species such as merlin (Falco columbarius) and Eurasian skylark (Alauda arvensis) migrate longer distances to the coast or towards the south. A 2014 book detailed species like the chaffinch are much less migratory in Britain than those of continental Europe, which migrate more than 5 km in their lives.
Short-distance passerine migrants have 2 evolutionary origins: (1) Those with long-distance migrants in the same family, such as the common chiffchaff (Phylloscopus collybita), have origins in the southern hemisphere that progressively shortened their return migration to stay in the northern hemisphere. (2) Those with no long-distance migratory relatives, such as the waxwings (Bombycilla), migrate in response to winter weather and the loss of their usual winter food, rather than enhanced breeding opportunities.
Describe irruptions and dispersal
Good breeding seasons can be followed by a food source scarcity the year after, leading to irruptions of species moving beyond their normal range. During the 19th century, there were 5 major arrivals of Bohemian waxwings (Bombycilla garrulus) into Britain, then 18 between 1937 and 2000. Cocker (2005) found in 1251, 1593, 1757, and 1791, red crossbills (Loxia curvirostra) invaded Europe in a widespread manner.
A 2013 study generalised bird migration mainly takes place in the Northern Hemisphere due to land birds living in high northern latitudes, where food sources deplete in winter, migrating to areas further south (including the Southern Hemisphere) to overwinter. In contrast, Ian Newton (2010) explained that among (pelagic) seabirds, species of the Southern Hemisphere had a higher likelihood of migrating because the southern hemisphere has a larger area of ocean and suitable islands for nesting.
Discuss the physiology and control
The response and timing of migratory control is genetically suppressed and is believed to be a primitive trait in both migratory and non-migratory bird species. Gwimmer Helm (2006) highlighted that the ability to navigate and orient themselves during migration is a complex phenomenon that may include both endogenous programs as well as learning.
i. Timing
Changes in the day length associates with the hormonal changes in birds. In 1795, Johann Friedrich Naumann described increased activity or Zugunruhe (German for migratory restlessness) as well as physiological changes such as increased fat deposition in the period prior to migration. Fusani et al. (2009) thoerised the occurrence of Zugunruhe in cage-raised birds with no environmental cues (e.g. shortening of day and falling temperature) is due to circannual endogenous programs responsible for controlling bird migrations. A 1999 study suggested caged birds demonstrated a preferential flight direction that corresponded with the migratory direction they would take in nature. This lead to changes in their preferential direction around the same time their wild conspecifics changed course. Other studied described the phenomenon of male bird species returning earlier to the breeding sites than their females, especially in polygynous species with sexual dimorphism, as ‘protandry’.
ii. Orientation and navigation
Many birds are known to navigate on a variety of senses such as a sun compass, magnetoreception and olfactory perception. Ketterson and Nolan Jr. (1990) proposed long-distance migrants dispersed in their youth and developed bonds with potential breeding sites and favourite wintering sites. These bonds developed into high fidelity for those sites by visiting those wintering sites annually. This is debate on the mechanisms behind the birds’ capability to navigate during migrations. Researchers argued endogenous programming and response to environmental cues lack substance in covering all scenarios of long-distance migration. Understanding the cognitive ability of the birds behind their habitat recognition and mental map formation is the key to explain the success of long-distance migration. Thorup et al. (2003) found older individuals of day migrating raptors such as ospreys and honey buzzards demonstrated efficiency in making adjustments in response to wind drift. Migratory birds use an innate evolutionary biological sense for navigation, as well as 2 electromagnetic tools to locate their destinations. 1 tool is innate, while the other tool relies on experience. On its first migration, young birds are theorised to fly in the correct direction using magnetoreception, but lack the sense of distance required to fly towards their destination. Researchers proposed a radical pair mechanism whereby chemical reactions in special photopigments sensitive to short wavelengths were affected by the field. Although this mechanism works during the daylight hours, it doesn’t require the position of the sun in any way. Because at that stage, birds were in a position of a Boy Scout where they had a compass but no map, until they grew accustomed to the journey and utilised its capabilities efficiently. These birds gained experience after learning various landmarks and created a mental map using magnetites in the trigeminal system, which indicates the strength of the magnetic field. Wiltschko et al. (2006) suggested variable magnetic field strengths at different altitudes maximised accuracy of the interpretation of the radical pair mechanism, which helped indicate its approach to their destination after a long migration between the northern and southern regions. A few studies identified a neural connection between the eye and "Cluster N”, a region of the forebrain that is active during migrational orientation, which suggested birds can visually perceive the magnetic field of the earth.
iii. Vagrancy
Vagrancy is a phenomenon whereby individual animals lose their way and appear outside their normal range. In the case of birds, they fly past their destinations (i.e. breeding areas) as in the "spring overshoot” and end up further north than intended. Because of their location, certain areas are known as watchpoints for birds such as Point Pelee National Park in Canada, and Spurn in England. Thorup (2004) stated ‘reverse migration’ can lead to certain bird species 1000s of kilometres out of range due to failed genetic programs. A 2003 study wind can blow many birds undergoing drift migration off course towards coastal sites. ‘Abmigration’ is another phenomenon defined as individual species from one region joining another species of similar taxonomy from a different breeding region in the common winter grounds and then migrating back along with the new population. A 2005 study found abmigration is common among some waterfowl, which shift from one flyway to another.
iv. Migration and conditioning
A few studies on Canada geese (Branta canadensis) found a migration route can be taught to a flock of birds using re-introduction schemes. For example, a microlight aircraft teaching safe migration routes to reintroduced whooping cranes (Grus americana).
What are the adaptations?
To meet the demands of migration, birds lower their metabolism to conserve energy by storing fat and controlling sleep, which requires physiological adaptations. In addition, birds moult their feathers suffering from wear-and-tear, which varies in terms of timing between species as they migrate to their winter grounds or breeding grounds. Weber (2009) indicated behavioural changes such as flying in flocks can help reduce the energy required in migration or reduce the risk of predation.
What are the evolutionary and ecological factors?
Pulido (2007) suggested the lability of bird migration may have developed independently in many avian lineages. There is debate between authors regarding any genetic change required for migratory behaviour to develop in a sedentary species. A 2003 study argued the genetic framework for migratory behaviour exists in nearly all avian lineages. A 2006 study associated this framework associated with the rapid appearance of migratory behaviour after the most recent glacial maximum.
Theoretical analyses evaluated that detours increase flight distance by up to 20%, which require aerodynamic adaptations. This means birds loading themselves with food to cross a long barrier would fly less efficiently. The migration of continental populations of Swainson’s thrush Catharus ustulatus usually fly east across North America before turning south via Florida to reach northern South America. Alerstam (2001) theorised this route formed at a consequence of a range expansion that occurred about 10,000 years ago. Detours may also be caused by differential wind conditions, predation risk, or other factors.
— Climate Change
Studies have associated large scale climatic changes with a variety of effects including timing changes in migration, breeding, and population declines. Watson and Davis (2017) found many species expanded their range in response to climate change.
Describe the ecological effects
Bird migration encourages the movement of other species including ectoparasites such as ticks and lice. This, in turn, carries micro-organisms including those of concern to human health. Rappole & Hubálek (2006) found the import of pet and domestic birds poses a significant risk to the global spread of avian influenza. A 2000 study found viruses maintained in birds without lethal effects, such as the West Nile virus, can spread through migrating birds. Birds may also play a role in the dispersal of propagules of plants and plankton. Studies found some predators such as
greater noctule bats prey on migrating birds such as nocturnal passerines and waders.
(b) Fish
Most fish species are relatively limited in their movements, which means they remain in single geographical area and migrate short distances during the winter in order to spawn, or to feed.
Classify the different types of fish migration
— Anadromous = These fish migrate from the sea up into fresh water to spawn. Anadromous comes the Greek words ἀνά ana, "up" and δρόμος dromos, “course”. Examples include salmon, striped bass, and the sea lamprey.
— Catadromous = These fish migrate from fresh water down and into the sea to spawn. Catadromous comes the Greek words κατά kata, "down" and δρόμος dromos, “course”. An example of which is the eel.
— Diadromous, Amphidromous, Potamodromous, Oceanodromous = Diadromous fish migrate between the sea and fresh water. Amphidromous fish migrate in similar fashion to diadromous fish but not for breeding purposes. Potamodromous fish migrate occur wholly within fresh water, while oceanodromous fish live and migrate wholly in the sea.
What are forage fish?
Schools of forage fish (Prey fish / Bait fish) migrate between their spawning, feeding and nursery grounds via a migratory triangle. e.g. A stock of herrings spawn in southern Norway, feed in Iceland, and nurse in northern Norway. Since forage fish are unable to distinguish their own offspring during feeding, wider triangular journeys are considered important.
This diagram illustrates the main spawning grounds and larval drift routes taken by Icelandic capelin. Green shaded areas and arrows show capelin migrating to feeding grounds, blue shaded areas and arrows show capelin back migrating, and the red shaded areas and arrows indicate the breeding grounds.
Capelin are a forage fish of the smelt family found in the Atlantic and Arctic oceans. During the summer, they graze on dense swarms of plankton at the edge of the ice shelf, as well as krill and other crustaceans. Large schools of capelin then move inshore to spawn and migrate in spring and summer to feed in plankton rich areas between Iceland, Greenland, and Jan Meyen. Ocean currents can affect the migration patterns between these areas. Between September and November, capelin then back-migrate before migrate to spawning areas north of Iceland around December to January.
What are the highly migratory fish species?
Some of the best-known anadromous fishes are the Pacific salmon species, such as Chinook (king), coho (silver), chum (dog), pink (humpback), sockeye (red) salmon, which hatch in small freshwater streams. From there, they migrate to the sea for maturity purposes, living there for 2 to 6 years. Mature salmon then return to the same streams where they first hatched to spawn. Other examples of anadromous fishes are sea trout, three-spined stickleback, sea lamprey, and shad.
— Several Pacific salmon species such as the Chinook, coho and steelhead introduced to the US Great Lakes have evolved to be potamodromous, meaning they migrate between their natal waters to fresh water feeding grounds.
— The larvae of the catadromous fish freshwater eels of genus Anguilla drift from spawning grounds in the Sargasso sea before entering freshwater river and streams as glass eels or elvers.
— Found in Lake Nicaragua of Central America and the Zambezi River of Africa (both of which are freshwater habitats) is a euryhaline specimen called the bull shark. It migrates to the ocean from both of these habitats with Lake Nicaragua bull sharks known to migrate to the Atlantic Ocean and Zambezi bull sharks known to migrate to the Indian Ocean.
1. Diel Vertical
Also known as diurnal vertical migration (DVM), it is a pattern of movement used by organisms living in the ocean and in lakes. Organisms, such as copepods, migrate to the epipelagic zone at night and return to the mesopelagic zone of biomass. Crustaceans (copecods), molluscs (squid), and ray-finned fishes (trout) are found to demonstrate DVM. Stimuli known to cause DVM include changes in light intensity and biological clocks. Cisewski et al. (2010) suggested the purpose of DVM is to access food and avoid predators.
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| This echogram captures the duel vertical migration of a small silvery light fish (Maurolicus muelleri) layer impacted by schools of predators (white arrow) at 100m depth at about 12:00 UTC. Note how the depth distribution of prey changes substantially after contact with the predator (Godø et al. 2014).https://www.researchgate.net/figure/Echogram-capturing-the-diel-vertical-migration-of-a-small-silvery-lightfish-Maurolicus_fig3_264581185 |
What are the different types of vertical migration?
i. Diel
= Organisms migrate through different depths in the water column on a daily basis. A 2012 report stated that migration usually occurs between shallow surface waters of the epipelegic zone and deeper mesopelagic zone of the ocean or hypolimnion zone of lakes. Diel vertical migration can be categorised into 3 types:
— Nocturnal Migration = This involves organisms ascending to the surface around dusk, remaining there overnight, then diving back to the depths again around dawn.
— Reverse Migration = This involves organisms ascending to the surface at sunrise, remaining high in the water column during the daytime, then descending at sunset.
— Twilight Migration = This involves 2 separate migrations in a single 24-hour period. The first ascent occurs at dusk, followed by a descent at midnight, often referred to as the “midnight sink”. The second ascent to the surface and descent to the depths occurs at sunrise.
ii. Seasonal
= Depending on the season, organisms are found at various depths. Seasonal changes to the environment can influence the migration patterns of these organisms. Regular diel vertical migration usually occurs in species of foraminifera throughout the year in the polar regions. Manno & Pavlov (2014) found foraminifera species remain at the surface during the midnight sun, as there weren’t any differential light cues, to feed upon the abundant phytoplankton, or to facilitate photosynthesis by their symbionts.
iii. Ontogenetic
= Kobari & Tsutomu (2012) observed organisms spend different stages of their life cycle at different depths. This leads to profound differences in migration patterns of adult female copepods, like Eurytemora affinis. They are known to stay at depth with only a small upward movement at night, whilst the rest of its life stages migrate over 10 metres. Other copepods, like Acartia spp. demonstrate an increasing amplitude of their DVM as their life stages progress. Holliland et al. (2012) suggested the increasing body size of the copepods and the associated risk of visual predators, such as fish, contributes to their enlargement, which in turn, increases their noticeability.
What are the different factors of vertical migration?
Vertical migration is influenced by both endogenous and exogenous factors.
(A) Endogenous Factors
= They originate from the organism itself.
i. Endogenous rhythm
= Organisms have biological clocks that give them a sense of time, which allow them to anticipate environmental changes and cycles. This allows them to physiologically and behaviourally respond to the expected change. Studies on the copepod species, Calanus finmarchicus, uncovered evidence of circadian rhythms playing a role in controlling DVM, metabolism and gene expression. Hafker et al. (2017) found these copepods exhibited these daily rhythms of vertical migration even in darkness, after their capture from n actively migrating wild population. Enright & Hammer (1967) at the Scripps Institution of Oceanography kept organisms in column tanks with light/dark cycles. After a few days, the light intensity decreased to a constant low level and the organisms managed to display DVM. This finding suggested that some type of internal response was causing the migration.
ii. Clock gene expression
= Many organisms, including the copepod C. finmarchicus, maintain their biological clock through genetic means. Hafker et al. (2017) found the expression of these genes increased following dawn and dusk at times of greatest DVM observed in the copepods. This indicated these genes play a molecular role in vertical migration.
iii. Body size
= Bull trout were found to express daily and seasonal vertical migrations with smaller individuals often staying at deeper layers than the larger individuals. Gutowsky et al. (2013) implied this phenomenon is associated with predation risk, depending on the individuals’ size.
(B) Exogenous Factors
= They originate from the environment and act on the organism.
i. Light
= It is the most common and critical cue for vertical migration in organisms. A 1996 study demonstrated organisms tended to limit their migration during a full moonlit night, and restarted their migration during an eclipse.
ii. Temperature
= Some fish species tend to migrate to warmer surface waters to aid its digestion. Changes in water surface temperatures are known to influence swimming behaviour of copepods. Some zooplankton may be inclined to pass through thermocline in order to migrate to the water’s surface, Joop Ringelberg (2010) found the marine copepod, Calanus finmarchicus, migrated through gradients with temperature differences of 6 °C over George's Bank. However, they remained below the gradient in the North Sea.
iii. Salinity
= Organisms that are stenohaline or unequipped to handle regulating their osmotic pressure are forced to seek out more suitable waters due to changes in salinity. Margaret Barnes (1993) observed areas affected by tidal cycles accompanied by salinity changes such as estuaries contained some species of zooplankton vertically migrating. Other areas such as the Arctic contain a layer of freshwater from melting ice, which organisms aren’t able to cross.
iv. Pressure
= Many species of zooplankton reacted to increased pressure with positive phototaxis, a negative geotaxis, and/or a kinetic response resulting in ascent in the water column. Margaret Barnes (1993) found decreases in pressure promoted zooplankton to passively sink or actively swim downward to descend in the water column.
v. Predator Kairomones
= A 2000 study suggested chemical cues released by predators called kairomones are responsible for its prey to perform vertical migration for the purposes of avoidance. When a fish was introduced into habitat of diel vertical migrating zooplankton, this affected the distribution patterns seen in their migration. A 2010 study on Daphnia and its fish predator Lebistus reticulatus found the Daphnia remained below the thermocline where the fish predator wasn’t present. This showed the effects of kairomones on Daphnia DVM.
vi. Tidal Patterns
A study on a species of small shrimp, called Acetes sibogae, observed a majority of them tended to ascend in the water column during flood tides compared to ebb tides experiences at the mouth of an estuary. Barnes’ theory suggested factors associated with the tides can trigger migration rather than the movement of the water itself, like the salinity or minute pressure changes.
Why does vertical migration occur?
Many hypotheses were proposed to explain why organisms would vertically migrate.
i. Avoiding Predators
= Zooplankton often demonstrate DVM due to the presence of light-dependent fish predators. Any given body of water is regarded as a risk gradient making the riskiest for residency at the surface layer during the day compared to deep water. A 2012 study found this leads to zooplankton residing in variable depths during the day, leading to varied longevity. Zooplankton migrating to deep waters during the day would help them avoid predators, and then migrating to the surface at night to feed.
ii. Metabolic advantages
= When organisms feed in the warm surface waters at night and reside in the cooler deep waters during the day, they are able to conserve energy. Meanwhile, organisms that digest their food at warmer temperatures would migrate to surface waters at night after feeding on the bottom in cold water during the day.
iii. Dispersal and Transport
= This involves organisms using deep and shallow currents to find food patches or to maintain a geographical location.
iv. Avoid UV damage
= Small organisms, such as microbes, can suffer from UV damage during daylight whenever they are situated too close to the water’s surface as sunlight penetrates into the water column. Therefore, they avoid approaching the water’s surface during the day time.
v. Water Transparency
= A 2015 report argued that “water transparency is the ultimate variable that determines the exogenous factor (or combination of factors) that causes DVM behaviour in a given environment”, better known as the “Transparency Regulator Hypothesis”. Since less transparent waters had more fish and available food, DVM was mainly driven by fish. Tiberti & Iacobuzio (2012) found that more transparent waters contained less fish and deeper waters associated with higher food quality, as UV light travels further through the water, therefore it functions as the main driver of DVM in such cases.
What are some unusual events?
Anomalies in stimuli and cues used to initiate DVM can change the pattern drastically.
An example anomaly is the midnight sun in the Arctic. When this occurs, it induces changes to planktonic life that would normally perform DVM with a 24-hour night and day cycle. During the Arctic summers, the Earth's north pole directs towards the sun, lengthening the days and increasing the amount of continuous day light for more than 24 hours at high altitudes. Manno & Pavlov (2014) found this leads to cessation of the DVM patterns of species of foraminifera, forcing them to remain at the surface in favour of feeding on the phytoplankton such as Neogloboquadrina pachyderma. For these species of phytoplankton that contain symbionts, like Turborotalita quinqueloba, they remain in sunlight to aid photosynthesis.
During solar eclipse events, the sunlight intensity suddenly decreases, which replicates the typical lighting experienced at night time that stimulate the planktonic organisms to migrate. This leads some copepod species, such as Calanus finmarchicus, to migrate and pool near the surface. This creates a classic diurnal migration pattern but on a much shorter time scale.
How important is the biological pump?
A 2002 study described how the biological pump uses plant photosynthesis to convert CO2 and inorganic nutrients into particulate organic matter in the euphotic zone and transference to the deeper ocean. DVM is crucial in maintaining this process’ efficiency in the ocean. The deep ocean receives most of its nutrients from the higher water column when they sink downwards as marine snow, which consists of dead or dying animals and microbes, faecal matter, sand and other inorganic material.
Steinberg et al. (2002) found organisms migrate up to the surface (0 - 100 metres) at night to feed and then migrate down to deep waters (800 - 1000 metres) during the day to defecate large sinking faecal pellets.
2. Lessepsian
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| This diagram shows the Lessepsian Province of the Mediterranean featuring several selected species (Goldschmid & Madl 2001). |
Also known as the Erythrean invasion, Lessepsian migration involves marine animals migrating across the Suez Canal, usually from the Red Sea to the Mediterranean Sea and vice versa.
In 1869, the Suez Canal opened to allow more direct shipping trade from Europe to India and the Far East. It is 162.5 km (101.0 mi) long, 10–15 m (33–49 ft) deep and between 200 and 300 m (660 and 980 ft) wide. Because the Red Sea’s surface is slightly more elevated than the Eastern Mediterranean, the canal serves as a tidal strait for the Red Sea to pour into the Mediterranean Sea. A group of natural hypersaline lakes that form part of the canal is called the Bitter Lakes. For decades, they were an obstacle for the migration of Red Sea species into the Mediterranean. Since the opening of the Suez Canal, the salinity of the Bitter Lakes gradually equalised with that of the Red Sea, which eliminated the barrier to migration. Since the Red Sea is an extension of the Indian Ocean, it contains more saline and less nutrients than the Mediterranean. Since the Mediterranean is an extension of the Atlantic Ocean, Red Sea species have higher tolerance of harsh environments in the Eastern Mediterranean compared to Atlantic species. Accordingly, more species migrate from the Red Sea into the Mediterranean than in the opposite direction.
In the 1960s, the construction of Aswan High Dam across the Nile River restricted the inflow of fresh water and nutrient-rich silt from the Nile into the eastern Mediterranean. This lead to ocean conditions in the eastern Mediterranean mimicking that of Red Sea, which increases the impact of migrations and facilitates the occurrence of new migrations.
Daniel Golani compared the structure and ecology of the Red Sea with that of the Mediterranean Sea. He found the Red Sea is an tropical marine environment that contained species sharing similarities with those with the eastern Indo-Pacific region, whereas the Mediterranean Sea is a temperate environment with reduced productivity. This meant the construction of the Suez Canal provided a pathway for Red Sea species to migrate into the Eastern Mediterranean, leading to substantial zoogeographic and ecological consequences. Galil et al. (2015) estimated 100s of Red Sea a and Indo-Pacific species colonising and establishing themselves in the Eastern Mediterranean system, causing biogeographic changes comparable to that of continental drift.
So far, about 300 species native to the Red Sea have been identified in the Mediterranean Sea, with many others still yet to be identified. In the late 20th and early 21st centuries, the Egyptian governments announced plans to deepen and widen the canal. This sparked concerns from marine biologists, who fear this project would increase the migration of Red Sea species into the Mediterranean and facilitate the crossing of the canal for additional species.
Describe the ecological impacts
a. Out-competition of natives
— Native Argyrosomus regius Vs. Invasive Scomberomorus commerson
The Argyrosomus regius species is indigenous to the Eastern Mediterranean and Atlantic, one of the most common commercial fish in Israel. As this species disappeared from local catches, the narrow-barred Spanish mackerel Scomberomorus commerson, a known Lessepsian migrant, ballooned in population. Galil et al. (2010) concluded this phenomenon is an example of an invasive migrant outcompeting a native species and occupying its niche, based on similar life histories and diets.
— Native Melicertus kerathurus Vs. Invasive prawns
8 species of invasive prawns from the Erythraean Sea have been recorded in the Eastern Mediterranean, which were highly prized in Lezantine fisheries. They composed most of the catches off the Mediterranean coast of Egypt, approximately 6% of total Egyptian landings. This ultimately lead to the reductions of a native penaeid prawn, Melicertus kerathurus, which supported a commercial Israeli fishery throughout the 1950s. Galil (2007) discovered that out-competition and overrunning of its habitat by prawn migrants lead to the disappearance of the native species, resulting in detrimental impacts on the commercial fishery.
b. Parasitic Invaders
The invasion of new Red Sea species into the Mediterranean also facilitated the invasion of their associated parasites, e.g. copepod Eudactylera aspera, which was found on a spinner shark, Carcharhinus Brevipinna, taken off the coast of Tunisia. Parasites originating from the Red Sea used related native Mediterranean fish species as alternative hosts, e.g. the copepod Nipergasilus Bora was known to parasitise the grey mullets Mugil cephalus and Liza carinata in the Red Sea. Both species have been recorded as Lessepsian migrants, and subsequently observed parasitising the native Mediterranean mullets Chelon aurata and Chelon labrosus.
On the contrary, Indo-Pacific swimming crab Charybdis longicollis was found in the Mediterranean in the mid-1950s, overtaking the silty and sandy substrates off the coast of Israel, making up to 70% of the total biomass in the habitat. Until 1992, a few of these crabs were infected with the parasite Heterosaccus dollfusi, which is a barnacle that desexes its host. By 1995, the parasite had spread to southern Turkey and 77% of the crabs collected in Haifa Bay were infected. This rapid increase and high infection rate lead to the increased high population density of the host and the year-round reproduction of the parasite. Remarkably, Bella Galil (2000) found the Mediterranean native swimming crab Liocarcinus vernalis recovered from this parasite.
c. Species displacements
In the 1930s, the goldband goatfish, Upeneus moluccensis, was first detected in the Eastern Mediterranean. Following the warm winter of 1954–1955, this population of goatfish increased to 83% of the Israeli catch. Considering this replaced the native red mullet, this also affected the Egyptian fishery, being 3% of their total landings. Galil (2007) asserted the higher water temperatures during the warm winter decreased the survival rates of red mullet juveniles, which allowed the goatfish population to expand into the opened niche. This forced the displacement of native mullet into deeper, cooler waters, where Lessepsian migrants consist of only 20% of the catch. Whereas, in shallower, warmer waters, the goatfish population takes up about 87% of the catch. This data suggests the Lessepsian migrants haven’t fully adapted to the more temperate waters in the deeper areas of the basin, nevertheless established dominant populations in habitats similar to their home habitats. Bos & Ogwang (2018) reported the fusilier population of Caesio varilineata in the eastern Mediterranean Sea, leading to the prediction of their development in a similar fashion.
d. Food web phase shift
In 1924, the marbled spinefoot (Siganus rivulatus) and dusky spinefoot (Siganus luridus), both indigenous Red Sea rabbitfish, were first identified off the coast of Mandate Palestine. Within a few decades, schools of these herbivorous fish settled in a range of habitats forming abundant populations such as “rocky outcropping… with algal cover”. Galil (2007) found they comprised 80% of the abundant herbivorous fish schoolings in the shallow coastal sites of Lebanon. This lead to the development of marked phase shifts within the food web on multiple levels. Due to the high influx of herbivorous species in a small period of time, the food web had been normalised, which increased the rate of algae consumption. This served as a major prey item for large predators. This, in turn, lead to the proliferation of the settlement and colonisation of a non-indigenous species of mussel from the Indo-Pacific. Since the mussel species has a thicker shell than that of the native mussel, this altered the predation patterns as they were difficult to consume. This phenomenon impacted the fisheries because it out-competed the native fish of high commercial value, such as the seabream Boop boops.
What is Anti-Lessepsian migration?
Species that migrated from the Mediterranean to the Red Sea for colonisation purposes are referred to as anti-Lessepsian migrants, as they swam against the predominant southward flow of the canal. Examples include the sea star Sphaerodiscus placenta, found in specialised habitats such as the lagoon of El Bilaiyim, about 180 km (110 mi) south of the southern entrance to the Suez Canal. This location contains more saline than the surrounding waters of the Gulf of Suez.
In 1961, the sea slug Biuve fulvipunctata, first identified in the Mediterranean, was seen in the Red Sea in 2005 by Malaquias et al. (2017), possibly due to Anti-Lessepsian migration. Moreover, Abd-Elnaby (2009) identified 6 species of polychaete worms in the southern Suez Canal as anti-Lessepsian migrants. Several studies have also identified these fish species as anti-Lessepsian migrants:
— Peacock blenny (Salaria pavo)
— Solea aegyptiaca
— Mediterranean moray (Muraena helena)
— Rock goby (Gobius paganellus)
— Argyrosomus regius
— Comber Serranus cabrilla
— European seabass Dicentrarchus labrax
— Spotted seabass Dicentrarchus punctatus
What factors facilitate Lessepsian migrant colonisation and expansion?
a. Anthropogenic stressors: Aswan Dam
The construction of the Aswan Dam across the Nile River in Aswan, Egypt posed as an anthropogenic factor to the Lessepsian migration. Prior to construction, the Nile River impacted on the marine environment of the Eastern Mediterranean, as it discharged large volumes of nutrient-rich water. This lead to an abundance of phytoplankton in the delta, benefiting the surrounding sea in terms of productivity, and large schools of sardines, therefore a highly lucrative commercial fishery. After the Aswan Dam was completed in 1964, productivity diminished, leading to a cessation of nutrients in the Mediterranean. This decreases in fish populations, namely sardines, resulting in the collapse of the sardine fisheries. Hence, the Egyptian purse-seine fishing industry takes only about 10% of the pre-dam catch, due to the dispersion of the Red Sea invasive species.
b. Natural stressors: Climate Change
Climate change and warming of seawater temperature would make it easier for the thermophilic Lessepsian migrants to reproduce, grow, and survive, giving them an advantage over native temperate Mediterranean taxa. Both global warming and the influx of Lessepsian migrants seemed to influence the teetering fisheries by displacing commercially important native species. This lead to a phase shift in coastal ecosystems and changing seascape patterns. Moreover, deepening of the warm surface layer killed organisms that were unable to tolerate high temperatures.
Studies suggested climate change lead to the decline of natural barriers that existed to prevent many Red Sea natives from migrating to the Mediterranean. Global warming has increased water temperatures and salinity in the Eastern Mediterranean, which decreased the hydrological barrier between the two seas. This phenomenon favoured the migrants from the tropic Indo-Pacific because they had a warm-water affinity, which lead to the decreases in temperate Eastern Mediterranean native populations. The Bitter Lakes had a natural salinity barrier within the Suez Canal that blocked many species from migrating due to their higher deposits of salt. As these lakes freshened, the natural salinity barriers weakened, which allowed a higher mitigation of invasive species.
Any other examples of Lessepsian migration?
a. North America
Sea lamprey reached Lake Ontario from the Atlantic Ocean through shipping canals and was observed in Lake Ontario in the 1830s, before Niagara Falls halted their migration. As the Welland Canal deepened in 1919, this allowed the sea lamprey to bypass Niagara Falls and reach the Great Lakes by 1938.
A species of shad from the Western Atlantic called the alewife (Alosa pseudoharengus) also invaded the Great Lakes via the Welland Canal to bypass Niagara Falls. Then they colonised the Great Lakes and became abundant in Lake Huron and Lake Michigan, peaking by the 1950s and 1980s.
b. Europe
Freyhof & Kottelat (2008) reported the white-eye bream (Ballerus sapa) invaded the Vistula River after migrating along the Dnieper-Bug Canal in Belarus, which links the Vistula drainage basin with that of the Dnieper River.
c. Panama
6 species of Atlantic fish were recorded using the Panama Canal to migrate from the Atlantic Ocean to the Pacific Ocean, and vice versa. The Atlantic fish included Lupinoblennius dispar, Hypleurochilus aequipinnus, Barbulifer ceuthoecus, Oostethus lineatus, Lophogobius cyprinoides and Omobranchus punctatus, and the Pacific fish included Gnathanodon speciosus. McCosker and Dawson (1975) found the Gatun Lake’s freshwater environment acts as a barrier to the interchange of marine species.
You can check out the list of Lessepsian migrants by clicking on the link below.
3. Salmon run
It is defined as the time when salmon that migrated from the ocean swim to the upper reaches of rivers where they spawn on gravel beds.
Most salmon are anadromous, meaning they are “running upward” and grow up mostly in salty water, mainly oceans. After maturation, they migrate or "run up" freshwater rivers to spawn, a phenomenon called ‘the salmon run’. Anadromous salmon spend their ocean phase in either the Atlantic or the Pacific Oceans in the Northern Hemisphere, but not thrive in warm water. The Atlantic salmon, the only known species in the Atlantic, is known to run up rivers on both sides of the Atlantic Ocean. In the Pacific Ocean, 7 different species of salmon inhabit the Pacific, which are collectively known as Pacific Ocean. They are named Chinook, Chum, Coho, Pink, Sockeye, Masu and Biwa salmon. Chinook, Chum, Coho, Pink and Sockeye run up rivers on both sides of the Pacific, whereas Masu and Biwa run up on the Asian side of the Pacific. Walrond (2010) found Chinook salmon successfully established themselves in the Southern Hemisphere in the early 19th century, distant from their native range in New Zealand rivers.
- Assuming it survives the full course of its natural life, the life cycle of an anadromous salmon begins and ends in a gravel bed in the upper reaches of a stream or river. This is where the salmon spawning grounds (or salmon nurseries) are situated, where salmon deposit their eggs in the gravel for safety purposes. The eggs hatch after 2 to 6 months into larvae called ‘sac fry’ or ‘alevin’.
- The alevin consists of a sac that contains the remainder of the yolk, which is hidden in the gravel while the young salmon feed on the yolk. After the yolk is depleted, the salmon then swim away from the gravel in search of plankton to feed on. At this point, the baby salmon have matured to become ‘fry’. At the end of the summer, the fry develop into juvenile fish called ‘parr’.
- Parr then feed on small invertebrates, camouflaging themselves in a pattern of spots and vertical bars, which remains for up to 3 years. When parr lose their camouflage patterns, they undergo a process of physiological changes that allow them to survive the shift from freshwater to saltwater as part of their preparation for migration. At this point, the parr salmon become ‘smolt’ salmon.
- Smolt then wander in the brackish waters of the river estuary to allow its body chemistry to adjust their osmoregulation that could cope with the higher salt levels in the ocean. In order to visually confuse and avoid predators, smolt develop silvery scales on its body surface. In the late spring, they grow to about 15 - 20 cm long, which is sufficient for them to swim out of the rivers and into the sea. There they become ‘post-smolt salmon’.
- Post-smolt then school with other post-smolt, before they all set off to find deep-sea feeding grounds. Studies found they then spend up to 4 years as adult ocean salmon developing their full swimming ability and reproductive capacity.
This image shows the differences between the adult ocean phase and spawning phase pink salmon (male).
This photo shows the sac fry remaining in the gravel habitat of their redd (nest) until their yolk sac, or "lunch box" is depleted.
This photo shows the young salmon emerging from the gravel habitat as parr to feed after the nutrients provided from the yolk sac.
How do salmon return from the ocean?
Moyle found most surviving salmon return to the same natal rivers where they were spawned years after wandering long distances in the ocean. Then most salmon swim up the rivers until they reach the very spawning ground that was their original birthplace. Lohmann et al. (2008) proposed salmon used geomagnetic and chemical cues to help guide them back to their birthplace. This suggested the fish use the Earth’s magnetic field to orient itself in the ocean, which helps them navigate back to the estuary of its natal stream.
Trevanius (1822) first speculated that odours provided salmon homing cues perceived by their strong sense of smell. Hasler (1951) hypothesised salmon detect chemical cues unique to their natal stream through their olfactory system as part of a mechanism to home onto the entrance of the stream once they were in the vicinity of the estuary or entrance to its birth river. In 1978, Hasler and his students demonstrated that salmon used their sense of smell to locate their home rivers with precision. Furthermore, they demonstrated the knowledge of the river’s odour is consolidated in salmon as they transition into smolts, prior to their migration out to sea. Moreover, homecoming salmon have recognised characteristic smells in tributary streams as they swim upriver and demonstrated sensitivity to characteristic pheromones released by juvenile conspecifics.
Leggett (1977) identified certain mechanisms that are worth researching, such as sun compass for navigation, orientation to various possible gradients, such as temperature, salinity or chemicals gradients, or geomagnetic or geoelectric fields. Ogura (1995) observed migrating salmon maintaining direction during cloudy times and night time, as well as swimming in deep waters. Rommel and McCleave (1973) found Atlantic salmon have conditioned cardiac responses to electric fields with strengths similar to those found in oceans. It’s suggested this sensitivity allowed a migrating fish to align itself upstream or downstream in an ocean current without fixed references. Walker et al. (1998) discovered iron, in the form of a single domain magnetite, inside the skulls of sockeye salmon, which is evidence for magnetoception.
Studies on tagged fish found they don't find their natal rivers, but instead swam up other, usually nearby streams or rivers. Quinn (1984) suggested dynamic equilibrium between homing and straying is genetically controlled. Natural selection tended to favour the descendants that home accurately, assuming the spawning grounds have a uniform quality. But if the spawning grounds have a variable quality, then natural selection favoured a mixture of the descendants that both strayed and homed accurately.
Fish typically swim by contraction of its longitudinal red muscle and obliquely oriented white muscles. Red muscles are required for sustained activity, such as ocean migrations, whereas white muscles are required for short bursts of activity, such as bursts of speed or jumping. As the salmon nears the end of its ocean migration and enters the estuary of its natal river, its energy metabolism faces 2 major challenges. It has to (1) supply energy suitable for swimming the river rapids, and (2) supply the sperm and eggs required for the reproduction process. Miller et al. (2009) found evidence of 2 key metabolic changes being triggered upon salmon encountering the decreases in salinity and increases in olfactory stimulation as they leave the natal river into the estuary. They discovered a physiological switch between recruitment of red muscles for swimming and recruitment of white muscles for increases in the sperm and egg load. Moreover, pheromones at the spawning grounds triggered a 2nd switch event to further enhance reproductive loading.
As they prepare for the spawning event ahead, the salmon undergo radical morphological changes such as losing the silvery blue to darken their colour. Since salmon are sexually dimorphic, some males develop canine-like teeth and their jaws develop a pronounced curve or hook (kype), while other males develop develop large humps.
What are the obstacles to the salmon run?
Salmon require high swimming and leaping abilities to battle the rapids and other obstacles the river may present, which demand full sexual development to ensure a successful spawn at the end of the run. They expend their energy into the physical rigours of the journey and the important morphological transformations before they proceed with the spawning events. Jeffries et al. (2011) estimated Chinook and sockeye salmon from central Idaho travelled 900 miles (1,400 km) and climbed about 7,000 feet (2,100 m) before they are ready to spawn. Beach (1984) estimated salmon jump as high as 3.65 metres in order to negotiate waterfalls and rapids. The height achieved is dependent on the position of the standing wave or hydraulic jump at the base of the fall, and the water’s depth.
Fish ladders, or fishways, are specifically designed to help salmon and other fish to bypass dams and other man-made obstruction, so they can continue on to their spawning grounds further upriver.
This photo is a fish ladder to allow salmon to negotiate a weir.
Skilled predators such as bears, bald eagles, and fishermen await the salmon during the run. Solitary animals such as grizzly bears congregate by streams and rivers when the salmon spawn. Additional predators such as Harbor seals, California sea lions, and Stellar sea lions pose a significant threat to salmon populations. Klinka & Reimchen (2009) found black bears fish salmon at night time, despite usually operating at day time. Reimchen (2009) hypothesised this behaviour is partly due to avoid competition with the more powerful brown bears, and partly due to higher salmon spawns at night. Furthermore, salmon are more evasive and attuned to visual clues at day time, whereas they are more focused on their spawning activities at night. This generates the acoustic clues bears listen for. A 2009 study found the white-coated Kermode bear did not have a greater advantage over the black bear species in terms of catching salmon at night time, instead having more success at day time.
Studies in 2011 found otters also hunt for salmon, however their odours and faecal matter gives olfactory cues to surviving salmon to avoid those waters.
Describe the salmon spawning
The term ‘prespawn mortality’ is defined as fish arriving successfully at the spawning grounds, and then die without spawning. Study estimates of prespawn mortality rates ranged from 3% to 90%. The risk factors of prespawn mortality include high temperatures, high river discharge rates, and parasites and diseases. However, Jeffries et al. (2011) highlighted the lack of reliable indicators to predict whether an individual arriving at a spawning area will in fact survive to spawn.
The term for the eggs of a female salmon is called ‘roe’, which she lays in a spawning nest in a riffle with gravel as its streambed, called a ‘redd'. The term for a relatively shallow length of stream where the water is turbulent and flows faster is called a ‘riffle’. To build her redd, she uses her tail (caudal tail) to create a low-pressure zone, which lifts gravel to be swept downstream and excavates a shallow depression. Susan McGrath estimates the redd may contain up to 5,000 eggs, each about the size of a pea, covering 30 square feet (2.8 m2). A 2011 report stated 1 or more males will approach the female in her redd, depositing his sperm, or milt, over her eggs. The female then disturbs the gravel at the upstream edge of the depression to cover the eggs before moving on to make another redd. Studies in 2011 estimated as many as 7 redds are made by the female before her supply of eggs is exhausted.
Before they spawn, male pink salmon and some sockeye salmon develop pronounced humps, which may have evolved due to conferring species advantages. They help the salmon lessen the likelihood of spawning in the shallow water at margins of the streambed, which have the tendency to dry out during low water flows or freeze in winter. Furthermore, riffles contain multiple spawns of salmon, which make it attractive for predators (such as bears) to catch the more visually prominent humped males, as their humps project above the surface of the water. Groot and Margolis (1991) suggested this would provide a protective buffer for the females.
This photo shows spawning salmon building redds on a riffle.
The most dominant male salmon rush at and chase intruders to defend their redds. They use their canine-like teeth to butt and bite intruders, and their kypes to clamp around the base of the tail (caudal peduncle) of an opponent.
The longer salmon remain in fresh water, their condition deteriorates. This explains why most deteriorate rapidly and die after spawning. Studies suggested this programmed senescence is characterised by immunosuppression and organ deterioration. An example of a semelparous animal that spawns once in their lifetime is the Pacific salmon, which is known to live for years in the ocean before they swim to the freshwater stream of its birth, spawn, and then die.
4. Sardine run
Between May and June, billions of sardines, specifically the Southern African pilchard Sardinops sagax spawn in the cool waters of the Agulhas Bank and migrate north along the east coast of South Africa, where a feeding frenzy occurs, to Mozambique. From there, it leaves the coastline and swims east into the Indian Ocean.
This NASA map of the Agulhas Current shows the levels or primary production during 2009. This measures the amount of food available for the spawning sardines.
What are the causes?
Ecologists currently don’t know what causes the sardine runs. Various hypotheses, even contradictory ones, were suggested to explain why and how the run occurs.
Fréon et al. (2010) interpreted that the sardine run is a seasonal reproductive migration of a genetically distinct subpopulation of sardine moving along the coast from the eastern Agulhas Bank to the coast of KwaZulu-Natal.
What are the oceanographic influences?
Barange & Hampton (1997) found sardines preferred to be waters of temperatures between 14 and 20 °C. This temperature range is measured along the South African south east coast every southern winter. However, water temperatures along the KwaZulu-Natal coast are warmer than 20 °C. Scientists suggested other factors influenced the movement of sardine along the KwaZulu-Natal coastline, such as predation pressure.
i. Oceanographic regions of the KwaZulu-Natal coast
O’Donoghue et al. (2010) found the KwaZulu-Natal coast includes varied oceanographic regions, each influenced by distinct environmental forces.
— Warm Agulhas Current dominate the continental shelf waters of the KwaZulu-Natal Mid to Lower South coasts, which flows toward the south west. It has a mean water temperature of 23 °C and a mean current speed of more than 1 m/s within 5km of the coast.
— The main stream of the Agulhas Current flows just offshore of the continental shelf break. This suggests that conditions are normally unsuitable for sardines along that part of the coast.
— Local winds have little effect on the currents.
— Sardine move closer to shore as they travel northwards along the coast, but more research is required to understand the environmental and biological conditions behind it.
— When warm Agulhas Current water flows onto the shelf and the resulting inshore current direction is from south to north, this forms a persistent cyclonic gyre known as the Durban Eddy. This coastal section is considered a transition from the wind-dominated section of the continental shelf to the north, to the Agulhas Current dominated section of shelf to the south.
— Since the North Coast section of continental shelf is about more than 25 km wider than that of the south coast, it leads to the Agulhas Current flowing farther offshore and variable current conditions over the shelf. A 2000 study found longshore north-easterly or south-westerly winds precede currents of similar direction by roughly 18 hours. Furthermore, sea temperature is often lower and nutrients is more abundant than along the South Coast.
— The North Coast provided a suitable habitat for sardines for reasons unknown.
ii. Oceanographic variables and sardine presence
What are the oceanographic predictors of sardine presence?
Favourable:
— Decreasing sea surface temperature
— Calm current conditions
— Light north-westerly land breeze
— Stable atmospheric conditions
Unfavourable:
— Increasing sea surface temperatures
— Moderate north to south currents
— Large swells
— Turbid water.
North-easterly and north-westerly winds and north to south currents are found to cool nearshore sea surface temperatures. On the other hand, south-easterly winds and increasing air temperatures increase nearshore sea surface temperature.
(c) Insects
Insects known to migrate seasonally include species of dragonflies, beetles, butterflies, and moths. The distance varies with species and involves large numbers of individuals. Occasionally, insects migrate unidirectionally and the subsequent generation migrates in the opposite direction.
All insects move ranging from a few centimetres (e.g. sucking insects and wingless aphids) and thousands of kilometres (e.g. locusts, butterflies and dragonflies).
Describe the general patterns
Butterflies migrate within a boundary layer, with a specific upper limit above the ground. Air speeds in this region are relatively slower than the average flight speeds of insects. In 1974, Taylor identified the 'boundary-layer' migrants as larger day-flying insects.
Many migratory species contain 2 polymorphic forms i.e. a migratory phase and a resident phase. The migratory phases are indicated by well-developed and long wings as observed in aphids and grasshoppers. Denno (1994) found migratory locusts, however, have distinct long and short winged forms.
In 1962, Southwood suggested that insects living in habitats with seasonal changes in resource availability learn to adapt for migration. Other researchers suggested that species living in isolated islands of suitable habitats developed strategies for successful migration. It is known that migration is influenced by parasites and infection, shortening the affected species’ lifespans. The effect created is called ‘cullling’ whereby migrating animals are less likely to complete the migration. Bartel et al. (2011) explained that culling resulted in populations with reduced parasite loads.
Discuss the orientation
Every migratory pathway consists of well-defined destinations, and approaching them requires both navigation and orientation. In 1996, Srygley, Oliveira, and Dudley identified that insects performed corrections to their flight path in response to crosswinds by sensing its windspeed and direction. Insects flying mainly in daytime use the sun as a reference for orientation, which requires compensation for the sun’s continuous movement. Experiments on butterflies flying in the dark in order to shift their internal clocks lead to changes in flight direction, which led to the proposal that insects have endogenous time-compensation mechanisms.
In 1993, Hyatt found most insects demonstrated capability in sensing polarised light even when the sun is obscured by clouds. However more research is required to understand these mechanisms deeply.
In 1982, Jones & McFadden suggested migratory butterflies are sensitive to the Earth's magnetic field based on the presence of magnetic particles. In 1999, Perez, Taylor and Jander observed that magnets altered the direction of migratory flight of monarch butterflies. However, the directions of monarch butterflies’ flight did not differ significantly compared to that of controls.
I. Lepidoptera
(See below)
II. Orthoptera
They are short-horned grasshoppers that occasionally swarm during long flights. However, their irregular migrations due to resource availability may not fulfil the agreed definition of insect migration. In 1957, Williams discovered some populations of species such as locusts (Schistocerca gregaria) regularly migrated due to seasonal changes across parts of Africa.
III. Odonata
Many dragonflies species such as Libellula, Sympetrum and Pantala are known to mass migrate. Studies in 2010 found Pantala flavescans migrated between India and Africa assisted by winds, which is regarded as the longest ocean crossings among insects.
IV. Coleoptera
Ladybird beetles such as Adalia bipunctata, Coccinella undecimpunctata and Hippodamia convergens were observed to swarm in certain locations, which were inferred to be searching for hibernation sites.
1. Butterfly migration
Populations of the Lepidoptera family (butterflies and moths) migrate long distances to and from areas seasonally across all continents except Antarctica, including from or within subtropical or tropical areas. They migrate to avoid undesirable circumstances, including weather, food shortage, or over-population.
Migration in Lepidoptera is defined as a regular, predictable movement of a population from one place to another, determined by the seasons. This process occurs in 2 of 3 modes of migration identified by Johnson in 1969. Firstly (Johnson’s 1st), within their short lifespan, the Lepidoptera move in one direction and never return. Scoble (1995) found the pierid butterfly (Ascia monuste) breeds in Florida but occasionally migrates along the coast up to 160 km in order to find more suitable breeding areas. Secondly (Johnson’s 3rd), migration occurs at a location of hibernation or aestivation where Lepidoptera undergo diapause and the same generation survives to return. e.g. Nymphalid monarch butterfly (Danuas plexippus).
i. Adventive
Species recorded in unexpected locations are labelled as ‘adventive’, meaning they are not regarded as migratory. This is due to the fact that their own strength is inadequate in leaving their habitat. Examples include species importing as egg or caterpillar alongside host plants or individuals reared by a collector before escaping. e.g. Galleria mellonella is found worldwide as it is reared as food for captive birds and reptiles. Chrysodeixis chalcites and Helicoverpa armigera are found to reach western Europe solely.
ii. Seasonal migration
Lepidoptera species often migrate between areas in the summer and winter season or the dry and wet season. However, it’s difficult to determine whether partially migratory species migrate during seasonal changes. They can live in certain regions of their habitat, as well as reach areas where permanent residence cannot be established. Only the season dictates whether the species can inhabit this area or not.
Describe the flight behaviour of Lepidoptera
In general, migratory Lepidoptera are exceptional flying organisms, even in strong headwinds. Species likeVanessa atalanta are found to manage headwinds by flying low and develop a goal-oriented approach. Gibo (1981) and Mikkola (2003) found these species migrate up to 2 km, maintaining high altitude. Since higher altitudes corresponds with lower temperatures, species flying in the daytime rely on the external environment for thermoregulation.
For species that migrate across continents, they rely on heavy winds of up to 10 m/s to traverse enormous distances, displacing between 300 and 400 km in one day. e.g. The painted lady (Vanessa cardui) migrates from Africa to Spain, assisted by tail winds. Migratory species rated as ‘good flyers’ do not fit the description of ‘robust flyers’. e.g. The small diamondback moth migrates about 3000 km, flying at an altitude of 100+ m.
This diagram illustrates the migratory pathways flown by Vanessa cardui between North Africa and Europe.
— How do they navigate?
There are 3 methods of navigation Lepidoptera use for migration.
i. Landscape
Lepidoptera use coastal lines, mountains, and even roads to orient themselves. Blab et al. (1989) observed that Lepidoptera demonstrate more accuracy in terms of flight direction if the coastal landscape is visible to them.
ii. Celestial navigation
It’s well known that butterflies use the sun as a point of reference for navigation. Since polarisation of the sun’s light changes with the angle of the rays, butterflies uses polarised light to navigate in cloudy weather. Scott, J. (1992) found that diamondback moths fly in a straight trajectory, independent of the angle of the sun’s rays. Merlin et al. (2009) found the antennae associates with the location of the circadian clock which helps butterflies localise in any one direction during flight. On the other hand, butterflies migrating at night time use the moon and stars as points of reference for navigation.
iii. Earth’s magnetic field
Baker’s 1997 study on the heart-and-dart moth revealed the Earth’s magnetic field is important to them for navigation. A 2012 study on the silver Y moth found they correct its course upon changes in wind direction, preferably flying with wind directions favouring their course. This demonstrated silver Y moths had a sense of direction. A study by Srygley et al. (2006) on Aphrissa statira found Earth’s magnetic field determined its navigational capacity in setting their favoured direction of flight in migration.
What areas can migratory butterflies and moths be found?
Since Lepidoptera migrate within or from the Tropics and Subtropics, they can found in all continents even as North as the Spitsbergen, above the Arctic Circle.
— Prior to the monsoon season, over 250 species of Lepidoptera are known to migrate in the Indian subcontinent.
— On Madagascar, Chrysiridia rhipheus migrate between populations of 4 plant species of the genus Omphalea, the host plant of the species. Smith (1991) found the 3 western Omphalea species live in dry coniferous forests, whereas the eastern species is found in the rainforest where it is green all year round.
— Rhodometra sacraria is usually found in Africa, large parts of Asia and southern Europe. Occasionally they migrate north, reaching central and northern Europe.
— Vanessa cardui is found worldwide except South America. Although their habitat is in subtropical steppe areas, they fly at altitudes of up to 3,000 meters above sea level. Constantí Stefanescu (2001) found its cousin Vanessa atalanta is also migratory.
— Uraniidae is found in South and Central America, which tend to migrate to the south and east. Smith (1983) suggested the migration peaks are caused by increases in toxicity after a large consumption of a toxic plant and vice versa after a small consumption of the same plant.
— Euploea core, Euploea sylvester, Tirumala septentrionis and Tirumala limniace migrate between the Western Ghats and Eastern Ghats in India, spanning between 350 and 400 km, which occurs twice annually. Push factors for their migrations include heavy monsoons, reproductive diapause, and a lack of host plants, nectar and alkaloidal resource availability.
— Dingle et al. (1994) found the Australian painted lady periodically migrates down the coast of Australia, occasionally to New Zealand. Common (1954) also found the bogong moth follows a similar migratory pattern to and fro the Australian Alps.
— Merckx, T. & Hans Van Dyck (2002) found the gatekeeper butterfly (Pyronia tithonus) migrates between both southern and eastern Britain, and northern Britain, despite their preference for warmer climate.
— Several studies have found the clouded yellow (Colias croneus) migrates between Europe and Siberia to Northern Africa and Southern Asia.
— McNeill (2011) and Showers (1997) have discovered the true armyworm moth (Mythimna unipuncta) and the black cutworm moth (Agrotis ipsilon) migrate north from North America in the spring to avoid the warm climate, and subsequently migrate south in the fall to avoid the cold climate. The south bound migration impacts on the reproductive systems in both males and females, whereas the north bound migration increases its calling behaviour hence promoting mating.
— Anagrapha falcifera (a celery looper) were found to migrate across the United States.
— Macroglossum stellatarum normally spawns in the subtropical part of the Palearctic ecozone year round.
In this map, the green areas indicate where Macroglossum stellatarum can survive year round. The blue areas indicate its presence in the summer (i.e. Scandanavia and Iceland), whereas the yellow areas indicate its presence in the winter (i.e. Africa and Indian subcontinent).
What are the causes of migration and evolution of Lepidoptera?
Butterflies and moths migrate to escape from potentially harmful situations such as shortage of proper food plants, unfavourable climate e.g. cold or heavy downpours or overpopulation.
The migration of Lepidoptera is a sign of evolutionary development because it allows the species to survive the process of natural selection. However, migration can be either advantageous or disadvantageous for individual specimen. Individuals that are infected are weaker, unable to spread their wings to fly, unable to enclose, which lead to shorter lifespans, with parasite levels varying in populations.
In recent times, climate change such as global warming has caused a spike in migrations of Lepidoptera towards north-western European countries like the Netherlands, Belgium and the United Kingdom.
Example: Monarch Butterfly migration
The best-known example of lepidopteran migration is the monarch butterfly.
This map shows the timeline of migratory pathways of the Monarch Butterfly. (1) March; (2) April; (3) End of April; (4) April - June; (5) June – August; (6) September – November.
Expound the migrations
i. Southwards
By the end of October, Monarch butterfly populations migrate from the eastern section of the Rocky Mountains to the sanctuaries of the Mariposa Monarca Biosphere Reserve within the Trans-Mexican Volcanic Belt pine-oak forests in the Mexican states of Michoaćan and México. Taylor et al. (2002) found monarchs migrate to other locations such as Cuba and Florida in the fall. The known flyways are as follows:
(A) 2 through North America.
(B) 1 in the Central States leading to the Mexican overwintering areas.
(C) A smaller one along the eastern North American seaboard.
Since there is less likelihood of recovering Monarchs in Mexico, McCord & Davis (2010) suggested they’re migrating along the eastern seaboard to locations besides Mexico or a higher rate of mortality relative to inland migratory Monarchs.
Chapman (1998) reported that Monarch butterflies respond to various cues that promote the fall season, southern migration, such as angle of sunlight relative to its position, the senescence of larval host plants, the decreasing day period and temperature drop. Around August, they migrate from the northernmost summer range, and detect the nectar of fall flower composites lying on the migration path to navigate. However, research is still ongoing to validate this hypothesis.
It’s known the eastern population of Monarch butterflies migrate from southern Canada and the Midwest United States just directly south toward Mexico. Monarchs from the Northwest migrate southwestwards, whereas monarchs transplanted from the midwest to the east coast tend to migrate southwards before reorienting their journey in a southwestwards direction.
— Monarch diapause
In most individual adult butterflies, diapause is a period that begins with its southern migration and remains active. Oberhauser (2009) found that diapause marks the moment when butterflies accumulate and store lipids, proteins and carbohydrates, which is measured to be as high as 34%. Brower et al. (1995) found evidence that Monarchs migrating to Mexico accumulate more lipids than those migrating to California. A 1979 study noted that these fats and lipids decrease water levels in order to provide energy reserves and prevent desiccation. They also maintain the insect throughout diapause by providing fuel for development following the termination of diapause. Kostal (2006) noted diapause occurs genetically well in advance of environmental stress. Chapman (1989) added that diapause leads to the cessation of high-metabolic activities including reduced oxygen use.
The organ responsible for storing fat in monarchs substantially enlarges during migration and overwintering circumstances compared to the summer generations. Tissue samples are measured to have elevated levels of free lipids in the haemolymph.
There is little evidence to support the notion that females in diapause ovulate and make mature eggs, which suggests diapause represses mating unless the monarch is overwintering. This phenomenon may have lead to increased survivability of winter populations and maintenance of fat reserves to promote spring northward migration. Kostal found one Monarch population stayed in diapause until the middle to the end of January, before they came out of diapause by the beginning of February.
Diapause has a series of distinct phases. The first phase is the inhibition of juvenile hormone production caused by the decreased day period and lowered temperatures. This leads to the repression of gonadal activity development, mating behaviours, and egg-laying. Denlinger (1986) discovered the emergence of new behaviours such as the development of social nectaring groups and late afternoon formation of night-time clusters or roosts. A 1992 study found the reduction of the loss of water caused by roosting, which suggested decreased surface area to volume ratios lead to reductions in evaporative water.
ii. Northwards
A 2014 report stated that migration in the northwards direction occurs mainly in spring. Pyle (1981) found that these migrations allow female monarchs to lay eggs for the next generation. Northward migration from Florida occurs from the middle of March until the middle of May, with the initial migration wave marking the offspring of monarchs overwintering in Florida and along the northern Gulf Coast.
The 1st generation migrate from the overwintering sites to as far north as Texas and Oklahoma. However, the 2nd, 3rd and 4th generations return to their northern breeding locations in the United States and Canada in the spring.
The southern migration often coincides with the cessation of diapause, the beginning of breeding activity and the movement north. Whereas, the western population disperses in a westerly and northwesterly direction, which shifts the roosting sites and the monarchs to lower elevations.
The northern migration is affected by rising temperatures and increasing day lengths, which force mated females to leave the overwintering sites before the males. In addition, monarchs migrating north don’t form roosts.
iii. Roosting and Overwintering
During the migration, the eastern and western populations tend to group together during the migration and then at the overwintering sites. Studies found these roosts form along the migration routes, with its locations indicating the possible flyways. Urquhart (1960) observed roosting behaviour in south-migrating butterflies in Mexico and Michoacan, and documented 1500 monarchs roosting at Lighthouse Point, Florida. Pyle (2010) observed Californian monarchs roosting in a wide variety of locations: Fremont, Natural Bridges Beach, golf courses, suburban areas. This suggested Californian roosts occur in inland areas and on non-native tree species compared to Mexican roosts.
Overwintering sites in California, Northwestern Mexico, Arizona, the Gulf Coast, central Mexico and Florida share the same habitat characteristics. They consist of moderating climatic conditions (thermally stable and frost free), which are relatively humid. This allows access to drinking water and provides trees for roosting and avoiding predators. A 1997 study estimated more than 200 overwintering sites in California. McCord & Davis (2010) observed overwintering sites in coastal South Carolina along with ovipositing females. Monarchs were also observed as far north as Lago Mar, Virginia Beach, Virginia on the US East Coast.
Californian overwintering sites exist in developed areas that has less tree density than a typical forest. They have a uniform vegetation population of either Monterey pine or eucalyptus trees. Brower (1977) indicated the dynamic nature of overwintering sites after observing tagged monarchs in different roosts throughout the winter.
iv. Range and characteristics
Pyle (2014) found the western population of migrating monarchs overwinters in coastal sites in central and southern California, United States, notably in Pacific Grove, Santa Cruz, and Grover Beach, as well as Baja, California's central valley, and the Sierra Nevada foothills.
Zhan et al. (2014) highlighted the coexistence of migrating and non-migrating monarch populations in many areas. Taylor et al. (2002) observed some monarchs reside in Florida for about a year, while other monarchs migrate to Florida and Gulf coast areas, and can often continue to breed and survive the winter. Studies concluded the Floridian monarch population are butterflies that do not migrate north in the spring.
Asclepias curassavica is an introduced annual ornamental that provides larval food if native species are unavailable. Since this plant poses a risk to monarchs from the spread of the parasite, OE, it is not recommended for planting.
Year-round breeding of resident monarch populations exist in the Caribbean, and in Mexico as far south as the Yucatán peninsula, but they don’t migrate over most of their global range.
Pyle (2014) demonstrated that the eastern and western populations of tagged monarchs were not entirely separate by capturing Arizona butterflies at overwintering sites in both California and Michoacan, Mexico.
Monarchs migrating during the fall don’t share similar characteristics with monarchs migrating northwards. In fact, the the northern-migrating monarchs are at least 4 generations removed from overwintering sites. Oberhause (2004) estimated the eastern population migrates up to 4830 miles (7,778 km) to overwintering sites in Mexico.
Davis (2009) found migrating monarchs tended to have darker orange and larger wings than they do during the breeding phase in the summer. In addition, Davis et al. (2012) identified the darkness of the orange colour in monarch wings to be a reliable indicator of the monarch’s migratory capability. Alitzer & Davis (2009) found monarchs migrating southwards were larger in size and weight. Studies on stable isotopes found larger monarchs tended to migrate longer distances. Satterfield and Davis (2014) indicated early migrants tended to be more robust and healthier, whereas late migrants weren’t as robust and healthy, which made them unsuitable for migration. Furthermore, early-migrating monarchs were redder and had larger, and more elongated wings, and larger bodies, than those at the tail end of the migration.
v. Sex ratios
Normally during the breeding season, the ratio of males and females is about 1:1, but the ratio skews towards males during the migration. This skewed ratio persists during the overwintering period as well, possibly due to the same cohort advancing from migration to the overwintering sites. Davis & Rendon-Salinas (2010) found the skewed sex ratio has grown more pronounced in recent years, which indicated the declining female monarch population over the past 30 years. A 2015 study evaluated similar skewed ratios from its observations of roosting or migrating monarchs, where fewer than 30% of the monarchs are females.
Describe the migratory theory mechanisms
Researchers have proposed many migratory mechanisms in an attempt to explain monarch migration.
(I) Time-Compensated Sun Compass Theory
i. What is the Time-Compensated Sun Compass?
Researchers observed that butterflies travel during the day and use a circadian clock according to the sun’s position in the sky to orient themselves. Mouritsen & Frost (2002) explained this clock mechanism compensates for time, where each butterfly entrains to the light-dark cycle of its surroundings. This allows them to interpret the changing light patterns throughout the day. A 2016 study asserted that more research is required to understand the underlying mechanisms behind the interpretation of the orientation and timing cues, which lead to the migratory patterns of the monarchs.
The monarch’s compound eyes detect light hitting the retina, which registers its azimuthal angle. Labhart et al. (2009) found the a specialised feature of the compound eye called the ‘dorsal rim area’ detects light polarisation, which is important for navigation. These cues are then transmitted to the central complex of the brain for interpretation, where neurons integrate the azimuthal location of the sun and the e-vector angle (angle of polarised skylight). This information is then processed and further integrated with other locational and orientational cues before producing migratory behaviour. Reppert et al. (2016) asserted that a neural structure called the ‘central complex’ underlies the monarch's sun compass, which may play a role in spatial learning, memory and awareness. However, further research is required in order to model the neuronal network and fully understand how spatial cues are modelled and stored in the brain.
Studies found the butterfly's antennae associates with the circadian clock underlying the migratory patterns. Merlin, Gegear & Reppert (2009) found various genes and proteins involved in circadian rhythms contributed to the antennae exhibiting their own circadian fluctuations, even when they are removed from the butterfly and studied in vitro. A 2016 study concluded that the antennae associated with the proper functioning of the time-compensated sun compass and possessed their own circadian clocks that function independently of the butterfly’s brain.
ii. Describe the molecular basis of circadian navigation
Like Drosophila and mammals, the core mechanism of the monarch circadian clock is described as a transcriptional-translational auto-regulatory negative feedback loop that drives rhythms in the mRNA and protein levels of core circadian clock components. Studies found the monarch mechanism diverges from other clock mechanisms in terms of elemental function, which share certain aspects from the circadian clocks of both Drosophila and mammals. This mechanism involves 2 proteins cryptochrome proteins, CRY1 and CRY2, each with differing functions. CRY1 functions as a blue light photoreceptor, which resembles the Drosophila CRY1. On the other hand, CRY2 functions as a major repressor in the feedback loop, which resembles the mammalian CRY.
In the monarch clock mechanism, CRY1 provides the clock with a means to entrain the butterfly to light-dark cycles. The proteins CLOCK (CLK) and CYCLE (CYC) function as transcription factors that drive the transcription of the period (per), timeless (tim) and cry2 genes. The PER, TIM, and CRY2 proteins form complexes in the cytoplasm, before they translocate back into the nucleus, which allow CRY2 to repress transcription.
In addition to the core feedback loop, a second modulatory feedback loop similar to the Drosophila’s second feedback loop includes genes encoding the orthologs of VRILLE and PDP1, which regulate CLK transcription in Drosophila.
iii. Bi-directionality of sun compass
Monarchs are found to use their time-compensated sun compass, during both the the southern migration in the fall and the northern remigration in the spring. Guerra & Reppert (2013) demonstrated cold temperatures during overwintering in the coniferous forests of Mexico requires a change in directionality in order to re-orient the monarchs. Furthermore, they argued that changes in sun compass direction occur independently of the changes in the photoperiod during the winter months. Nevertheless, the photoperiod changes impacts on the timing of the northern remigration in the spring. They also found exposure to colder environments while overwintering is required to continue the monarch’s migration cycle.
It’s known that the time-compensated sun compass uses the same substrates for both the northern remigration of monarchs in the spring and fall. However the mechanistic differences in these substrates that lead to a switch in the directionality of the compass is poorly understood. There’s a theory that RNA-sequencing differences found between the fall and spring butterflies is associated with the mechanism responsible for the recalibration, which would utilise a temperature sensor to start the switch. However, more research is required to understand this.
(II) Genetic Memory Theory
Sue Halpern (2002) proposed that the monarch’s ability to find overwintering sites in California and Mexico is an inherited trait. A 2014 study from the University of Minnesota suggested the butterflies follow streams and recognise landmarks with an inherited map. Mouritsen (2014) disputed this theory of an inherited map with evidence for the contrary.
(III) Landscape Theory
Brower et al. (1995) noted that terrain such as mountains, rivers, lakes and oceans all play a role in migration of monarchs. Large populations of migrating monarchs roost at obstacles that usually impede their pathway in a south / southwesterly direction. Researchers theorised these monarchs roost to wait for ideal weather conditions that will aid them in crossing these landforms, such as clearing rain, temperature, tailwinds, and sunlight. A 2009 report stated other geographic features such as the Appalachian Mountains and the Sierra Madres in Mexico 'funnel' the migration, orienting it to the S/SW.
(IV) Columbus Hypothesis
This theory discusses the number of butterflies engaging in mass movements to expand their range or relieve pressure on their habitat. It suggests the eastern population lacks such an such an extensive range and did not migrate. During the colonial period in America, deforestation of the Northeast lead to mass migration of monarchs. This suggests there are residents of subtropical and tropical areas before moving north to breed on the increased numbers of larval host plants that replaced the deforested areas. Nonetheless, monarch populations in other countries such as Australia don’t migrate such long distances, which suggests the migratory behaviour of monarch populations in the eastern hemisphere developed after other populations of monarchs had become established in other regions.
(V) Other theories
Brad Plumer (2014) hypothesised monarchs leave behind certain chemical marks on trees to orient themselves on their return trip the following winter. Halpern (2002) argued against mass migration and insisted monarch adults in Canada and the upper Midwest received an environmental trigger (such as change in photoperiod or seasonal cold snap) and ceased egg laying during the fall. Jet streams moving south out of Canada carry high and low pressure cells across extreme southern Canada and later across the US. This is when monarchs require thermal air to rise during clearing conditions to help them carry toward the South out of the region in which they were reared. If the thermals help monarchs rise to sufficient altitudes, he north winds can carry them as far as Mexico.
(d) Mammals
Some mammals exhibit extraordinary migrations, such as the caribou migration, which is the longest known terrestrial migration on Earth. It’s measured to reach as much as 4868 km/year in North America. Joly et al. (2019) found 1 grey wolf covered a total cumulative annual distance (TCAD) of 7247 km. Mammals known to mass migrate include the wildebeest, gazelles and zebras. Millions of these large organisms participate in the Serengeti ‘great migration’, which is an annual circular pattern of movement. A 2009 literature search by Grand Harris listed more than 20 species known to engage, or previously engage, in mass migrations.
Below is the list of mammals known or used to mass migrate:
i. Africa:
— Hartebeest
— Springbok
— Black wildebeest
— Blue wildebeest
— Blesbok
— Tiang
— Burchell’s zebra
— Quagga (extinct)
— Thompson’s gazelle
— White-eared kob
— Grant’s gazelle
— Scimitar-horned oryx
— Giant eland
ii. North America:
— Pronghorn
— Mule Deer
— Bison
— Wapiti
iii. North America & Eurasia:
— Reindeer / Caribou
iv. Eurasia
— Siberian roe deer
— Chiru
— Mongolian gazelle
— Saiga
Of these migrations, those of the springbok, black wildebeest, blesbok, scimitar-horned oryx, and kulan have ceased.
In 1981, Lockyer and Brown found cetaceans such as whales, dolphins and porpoises migrate long distance.
A 2014 study found some bat species, such as the Mexican free-tailed bat, migrate long distances, spanning between Oregon and New Mexico.
Russel et al. (2006) found some reptiles and amphibians migrate, while a 2014 study noted the Christmas Island red crab moves en masse each year by the million.
(e) Homing
In biology, it is defined as the ‘inherent ability of an animal to navigate towards an original location through unfamiliar areas. The location can be their home territory, or a breeding spot.
Animals use their homing abilities to find their way back to home through a migratory pathway. They normally use them in reference to returning to a breeding spot viewed years prior, e.g. salmon. Furthermore, they home to return to familiar territory after displacing a considerable distance, e.g. red-bellied newt.
Animals use their homing abilities as part of true navigation. When they’re in familiar locations, they recognise landmarks such as roads, rivers, or mountains whilst flying e.g. homing pigeons, or islands and other landmarks whilst swimming e.g. sea turtles.
Other animals use magnetoreception (magnetic orientation) based on the Earth’s magnetic field to find their way home. This is often combined with other methods, such as a sun compass, as in migrations of birds and turtles. Other examples include lobsters, which live underwater, and mole rats, which home through their burrows.
- Displaced marbled newts use celestial orientation i.e. using the stars for navigation, for homing purposes.
- A few studies found several salamanders, such as the red-bellied newts, and salmon use olfaction (smell) for homing.
- Animals with limited intelligence, such as molluscs and limpets, use their topographic memory of the contours surrounding the destination to navigate their way back home.
1. Natal
This homing process involves some adult animals returning to their birthplace for reproduction purposes. Aquatic animals are known to demonstrate this process.
a. Sea turtles
Loggerhead sea turtles demonstrate 2 types of homing: (1) During the early stages of life, they first head out to sea with the tides and currents carrying them, requiring little swimming effort. Studies showed how these animals homed to feeding grounds near their natal birthplace.
Bowen (2004) found differences in the mitochondrial DNA of turtles of a specific natal beach, which distinguishes them from turtles of other nesting areas. Once turtles reach sexual maturity in the Atlantic Ocean, the females swim back to her natal beach to lay her eggs, covering more than 9000 mile round trip.
b. Salmon
In the North Pacific, salmon migrate from the ocean to freshwater habitats, and back again to complete its life cycle. They spend about 4 - 5 years in the ocean to reach sexual maturity, leading them to return to the same streams they were born in to spawn. Several hypotheses were proposed to explain the salmon’s capability.
First hypothesis suggests the use of both chemical and geomagnetic cues to help salmon return to their birthplace, such as the Earth’s magnetic field. Lohmann (2006) believed chemical cues unique to the salmon’s natal stream would help it locate where the river dumps into the sea.
Another hypothesis suggested that the salmon’s strong olfactory sense allows it to retain an odour imprint of their natal stream during their downstream migration. This odour memory would help salmon return to the same stream years later. Researchers suggested young salmon release a pheromone during their migration downstream, which their olfactory system detects. This helps them return to the same stream years later.
c. Bluefin tuna
This fish spawns on both the east and west shores of the Atlantic Ocean. Every bluefin tuna hatch leaves a chemical imprint in its otoliths based on the water’s chemical properties. Bluefin tuna born in other ocean regions will show different chemical imprints. Rooker et al. (2006) found 95.8% of the tuna yearlings returned from the Mediterranean Sea to spawn in their natal region, and 99.3% returned from the Gulf of Mexico.
d. Atlantic puffins
They situate in the sea during winter, and return to their place of birth during summer. Breeding sites are typically inhospitable cliff-tops and uninhabited islands. Kress & Nettleship (1988) showed that birds removed as chicks and removed elsewhere demonstrated fidelity to their point of liberation rather than to their birthplace.
What are the navigational tools?
— Geomagnetic imprinting
A study from the University of North Carolina suggested the "geomagnetic imprinting hypothesis” to explain how animals accomplish natal homing. It stated that these animals imprint on the unique magnetic field that exists in their natal area and then use this information to return years later. In animal behaviour, the term “imprinting” refers to a special type of learning. Although definitions vary, the important aspects of the process always includes (according to Gunther Zupanc (2010):
(1) Learning occurs during a particular, critical period, usually early in an animal’s life.
(2) Effects lasting a long time.
(3) Effects aren’t easily modified.
Since the earth's magnetic field isn’t consistent throughout the globe, different geographic areas will exert different different magnetic field strengths. Lohmann et al. (2007) found sea turtles with well-developed magnetic senses are able to detect both Earth’s magnetic field strength (intensity) and the inclination angle (the angle of intersection between the magnetic field lines and the earth’s surface).
— Chemical cues and olfactory imprinting
Dittman and Quinn (2006) found pacific salmon imprint on the chemical signature of their home river. This helps the salmon locate their home river once they reach the coast from the open sea. Lohmann et al. (2008) suggested salmon equipped 2 different navigational systems that they used sequentially to migrate from the open sea to their spawning grounds. The first system uses the earth’s magnetic field to navigate across the open ocean to lure it close to their home river. Once within range of their home river, the second system is activated to detect olfactory (chemical) signals to locate their spawning area.
How does thermal pollution affect natal homing?
Thermal pollution (i.e. degradation of water quality through changes in ambient water temperature) is known to impact natal homing capability of Chum salmon. They preferably swim in water temperatures around 10 degrees Celsius. If water temperatures increases above 10 degrees Celsius due to thermal pollution, Chum salmon dive deeper for thermoregulation. This decreases the time these fish spend in the surface water column, which in turn, decreases their chances of approaching natal river since the chemical cue for natal homing is concentrated on surface water.
How did natal homing evolve?
At a beach in eastern Mexico, Kemp's ridley turtles nest has been studied and recorded by Lohmann and co.. Over a decade prior, a navigational error from the inclination angle led the turtles only within an average of 23 kilometres (14 mi) from their natal region. Scientists concluded geomagnetic imprinting help marine animals navigate close to their birthplace and then subsequently rely on chemical cues of the tributaries and rivers to direct them back to their birthplace.
Marine animals return to their birthplace as it is regarded as the safest place to lay their eggs. Reasons for its safety include lack of predators, correct temperature and climate, and suitable type of sand for turtles.
Lohmann (2006) implied the few animals failing to return to their natal region stray to other places to reproduce. This would increase the number of locations of reproduction, ultimately increase the species' survival chances.
There is still much more research to be conducted by scientists to fully understand how these animals can travel such great distances to reproduce
2. Philopatry
It is defined as the organism tending to stay in or habitually return to a particular area. The term derives from the Greek ‘home-loving’.
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| This diagram depicts philopatric events in the wild orange clownfish population of Kimbe Island within and between microhabitats. 5 representative families were selected to present the variation in size and depth of family trees. (A) Families spanning from 2 to 5 generations and (B) a family of 4 generations. The arrows represent links between parents and offspring. The direction of the arrow indicates the geographical origin and settlement of dispersal events. Dots refer to anemone locations (black and white dots correspond, respectively, to H. magnifica and S. gigantea). Numbers 0 to 4 identify the corresponding generation in the pedigree. A dot with 2 numbers indicates the presence of related fish from different generations in the same anemone. White corresponds to the lands, and shades of grey refer to the water. Shallow water 0 - 2 m (light grey) and lagoons 2 - 15 m (dark grey) (Salles et al. 2016). |
a. Breeding-site philopatry
1 type of philopatry is called ‘breeding-site philopatry’ or ‘breeding-site fidelity’. It is defined as an individual, pair, or colony returning to the same location to breed, year after year. When sedentary animals reuse a breeding site due to territorial competition outside their home range, this provides advantages in terms of survival. This allows animals, such as megapodes, to construct nests or associated courtship areas. For example, a megapode called the Australian malleefowl, Leipoa ocellata, is known to construct a large mound of vegetation and soil or sand to lay their eggs in. After reusing the same mound for years, megapodes then abandon it when it becomes damages beyond repair, or becomes disturbed. A 1993 study explained that nest fidelity is crucial for successful reproduction, as malleefowls spend 5 or 6 months per year tending to a mound. A 2014 study found colonial seabirds required multi-scale information for nest fidelity. Such information includes the breeding success of the focal breeding pair, the average breeding success of the rest of the colony, and the interaction of these two scales.
A 1994 study found birds, dispersing as fledglings, take advantage of exceptional navigational skills to return to a previous site. This is documented as ‘breeding fidelity’. While philopatric individuals exhibit learning behaviour, they don’t return to a location after an unsuccessful breeding attempt. Individuals risking themselves while searching for a more favourable breeding site have a higher fitness than individuals persisting with an unfavourable site. Tryjanowski et al. (2007) noted philopatry is not homogenous within a species, as it occurs at a higher likelihood in isolated breeding habitats. Weatherhead & Boak (1986) added that non-migratory populations demonstrated higher likelihood of philopatric behaviours than migratory populations.
Moore and Ali (1984) found there is no sexual bias in philopatric tendencies demonstrated by species that exhibit lifelong monogamous pair bonds, even outside of breeding season. However, Paul Greenwood (1980) evaluated a higher rate of breeding-site philopatry in males than females among birds, and vice versa among mammals. To explain this bias among polygynous species, Greenwood proposed his hypothesis regarding male birds investing effort in resource protection such as territory against other males. He explained that over consecutive seasons, a philopatric male will have higher fitness than a non-philopatric male. Meanwhile, females are free to disperse, and assess their male partners. Anne Pusey (1987) argued mammals’ predominant mating system is one of matrilineal social organisation.
b. Natal Philopatry
Commonly referred to as ‘the return to the area the animal was born in, or to animals remaining in their natal territory’, natal philopatry is a form of breeding-site philopatry. There is still debate over the evolutionary causes. Frederick & Ogden (1997) found natal philopatry is demonstrated by females as it decreases competition for mating and increases the rate of reproduction, increasing the survival rate of its offspring. Moreover, it is responsible for a kin-structured population, which are more genetically related than that between individuals in a species. Shitikov et al. (2012) explains this leads to inbreeding and increases the rate of natural and sexual selection within a population.
What are the evolutionary causes of philopatry?
The exact causes for the evolution of natal philopatry are currently unknown. Nonetheless, 2 major hypotheses have been proposed. In 1982, Shields suggested the optimal-inbreeding hypothesis, in which philopatry ensured inbreeding. His argument was inbreeding had evolutionary advantages as philopatry lead to the concentration of related individuals in their birth areas, reducing genetic diversity. Otherwise, inbreeding would not be as prevalent, hence it would be a detriment to evolution. He claimed the main benefit of inbreeding is the protection of a local gene complex that finely adapted to the local environment. Weatherhead and Forbes (1984) added that inbreeding reduced the cost of meiosis and recombination events. This meant maladaption of non-philopatric individuals over multi-generational time, fixing philopatry within a species. Outbreeding depression provides evidence for Shields’ hypothesis, as it involves reduced fitness caused by random mating. Michael Lynch (1991) explained how outbreeding depression is caused by the breakdown of coadapted gene complexes as a result of combining alleles that don’t cross well with those from a different subpopulation. Note that outbreeding depression becomes increasingly detrimental the longer (temporally) that subpopulations have been separated, meaning the hypothesis may fail to provide an initial mechanism for the evolution of natal philopatry.
The second hypothesis proposed was that natal philopatry evolved as a way to minimise the costs dispersal among offspring. A 1994 review of records of natal philopatry among passerine birds found migrant species demonstrated significantly less site fidelity than sedentary birds. Either way, migratory species still pay off the cost of dispersal. Michael Lynch (1991) defined ‘inbreeding depression’ as a phenomenon whereby deleterious alleles become fixed more easily within an inbreeding population. Moore and Ali (1994) implied the high cost of inbreeding depression is greater than the cost of outbreeding depression.
Other hypotheses, one of which proposed by Hasler, Scholz and Horrall (1976) whom regarded philopatry as a method of ensuring sexual interaction in breeding areas amongst migratory species. The other suggested philopatry increased the chances of breeding success. Weatherhead and Forbes argued that strict habitat requirements allow certain species to be more familiar with the sites they return to, meaning they have higher success rate in either defending it, or locating potential mates. Baker (1978) asserted that the hypothesis doesn’t justify whether philopatry is caused by an innate behaviour in each individual, or to learning. Nevertheless, most species demonstrated higher site fidelity with age.
What are the consequences of philopatry?
— Speciation
One major outcome of multi-generation natal philopatry is genetic divergence and, eventually, speciation. A lack of genetic exchange would have lead to genetic drift within geographically and reproductively isolated populations, particularly those living on islands. Mobile island-breeding animals may experience difficulties in finding new breeding locations. Genetic drift can occur on shorter timescales than is observable in mainland species, combined with a small populations.
Marchant & Higgins (1990) discovered high levels of natal philopatry and subsequent genetic drift between populations in animals that mainly situated in the sea before returning to land, such as the island-nesting albatross. This explained why many species of albatross delayed breeding until between 6 and 16 years of age. Van Ryzin & Fisher (1976) found more than 99% of Laysan albatross (Phoebastria immutabilis) returned to the same nest they left in consecutive years. They suggested such site-specificity can lead to speciation. Abbott & Double (2003) detected genetic differences in the shy albatross (Thalassarche cauta) microsatellites between 3 breeding colonies located off the coast of Tasmania. However, those differences are insufficient in identification of populations as distinct species, which means divergence continues without outbreeding.
Van Bekkum et al. (2005) suggested not all isolated populations demonstrated evidence of genetic drift. 2 hypotheses were proposed to explain genetic homogeneity, suggesting natal philopatry is not absolute within a species. The first hypothesis implies founder effects may account for the lack of divergence, which may explain how individuals carry the genes of their source population, leading to the beginning of a new population. However, a significant amount of time must elapse for sufficient divergence to occur. Burg & Croxali (2001) failed to find a significant difference in mitochondrial DNA microsatellites between colonies of black-browed albatross (T. melanophrys) on the Falkland Islands and Campbell Island. Phalan et al. (2016) observed how white-capped albatross (T. [cauta] steadi) made attempts to build nests on a south Atlantic Island, where it was recorded for the first time. This demonstrated roaming sub-adult birds extending their range of habitation. The second hypothesis proposed by Van Bekkum et al. (2005) stated that sufficient gene exchange is required to prevent divergence. However, distance between populations doesn’t appear to be a determining factor in divergence because of the albatross’ dispersal capabilities. A 2010 study found an increased association between family members and philopatric animals at birthsites. This also occurs in situations where cooperative breeding increases inclusive fitness, which incurs evolutionary benefits to families that exhibit such behaviours. ‘Inclusive fitness’ is defined as ‘the sum of all direct and indirect fitness’, while ‘direct fitness’ fitness is defined as ‘the amount of fitness gained through producing offspring’. A 2012 study defined ‘indirect fitness’ as ‘the amount of fitness gained through aiding related individuals offspring’.
— Cooperative breeding
Non-migratory philopatric species that evolve to breed cooperatively is known as ‘cooperative breeding’. A form of cooperative breeding called ‘kin selection’ is defined by researchers as the care provided by individual offspring to further offspring produced by their relatives.
Lukas & Dutton-Brock (2012) defined ‘cooperative breeding’ as ‘a hierarchical social system characterised by a dominant breeding pair surrounded by subordinate helpers’. Nevertheless, the system does exert both costs and benefits on the dominant breeding pair and their helpers. Cooperative breeding does skew the reproductive success of all sexually mature adults towards 1 mating pair. Gerlach (2002) suggested this result stated the reproductive fitness of the group is maintained by a certain individual breeding members and helpers have little to no reproductive fitness. This means breeders increase their reproductive fitness, while helpers increase their inclusive fitness.
Like speciation, cooperative breeding is a self-reinforcing process for a species. If the fitness benefits increases the inclusive fitness of a family, this would fix their hereditary trait in the population. Over time, this leads to the evolution of obligate cooperative breeding, as demonstrated by the Australian mudnesters and Australo-Papuan babblers. Cockburn (2006) stated that obligate cooperative breeding requires natally philopatric offspring to help raise offspring.
(f) Sea Turtle migration
Sea turtles of the superfamily Chelonioidea are known to migrate long distances. When sea turtle hatchlings emerge from underground nests, they crawl across the beach towards the sea. This offshore heading is maintained until they approach the open sea. A 2005 study estimated the separation distance between the feeding and nesting sites of adult sea turtles to be about 100s or 1000s of kilometres.
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| Sea turtle migration scene from Finding Nemo |
How do hatchlings migrate?
Hatchlings aim to move efficiently away from the beach and shallow coastal waters in order to minimise the time vulnerable to predators. If nesting sites are affected by artificial lighting, this navigational mechanism is handicapped as hatchlings are unable to distinguish between artificial lights and the moonlit sea. Therefore, this is an ‘evolutionary trap’ to using moonlight as a cue for navigation. Meanwhile, loggerhead and green turtle hatchlings use the orbital movement of waves as a cue to swim perpendicular to the waves crests. Since wave crests closer to the shore run parallel to the beach, hatchlings are guided to swim offshore. In order to maintain an offshore direction and therefore head towards the open sea, hatchlings detect the Earth’s magnetic field for navigational means.
Goodenough et al. (2010) implied turtles have a magnetic compass mechanism to help them head in a given direction without reference to landmarks. Lohmanns’ (1996) study demonstrated loggerheads use the magnetic field to stay within the gyre. They found loggerheads oriented themselves in a direction that kept them within the gyre relative to the fields characteristic of a region at the edge of the gyre. It is suggested these behavioural responses were inherited rather than learned since the hatchlings tested were captured before reaching the ocean. Lohmann’s 2008 study asserted that adult turtles learned of the magnetic field and used it for navigation in a learned rather than innate way.
How do juveniles migrate?
Juvenile turtles are known to reside in coastal feeding grounds. Adult turtles are classified into 3 categories according to their movements.
— Leatherbacks and olive ridley turtles tend to roam widely and unpredictably before returning to specific breeding sites. Alok Jha (2011) used satellites to track leatherbacks and found them staying within relatively food-rich areas of the ocean during their migration.
— Kemp’s ridley, loggerheads and flatback turtles migrate between breeding areas and a series of coastal foraging areas.
— Green sea turtles and hawksbill sea turtles tend to migrate between fixed foraging and nesting sites.
Both olive and Kemp’s ridley turtles tend to nest in large aggregations, known as an arribada.
In 1873, Charles Darwin stated that a compass mechanism inadequately explains the adult’s precision of migration across featureless and dynamic oceans. Referring to the migration of green sea turtles from the coast of Brazil to Ascension Island, a journey of 2200 km to an island only 20 km in diameter, he said:
"Even if we grant animals a sense of the points of the compass ... how can we account for [green sea turtles] finding their way to that speck of land in the midst of the great Atlantic Ocean.”
Lohmann et al. (2008) asserted that an error in heading of only a few degrees would lead a turtle to miss the island by almost 100 km, which demonstrates the imperfection of animal compass analogues. Furthermore, since there is a lack of position-fix, he highlighted a compass mechanism is unable to correct current displacement. A 2007 study by Luschi et al. uncovered evidence of green turtles’ sensitivity to magnetic cues. After the turtles’ exposure to magnetic fields oriented north and south relative to a capture site (i.e. displaced in geomagnetic but not geographical space), they oriented themselves in a direction back towards the capture site. This suggested green turtles used the earth's magnetic field to acquire positional information. Hays et al. (2003) argued that turtles use wind-borne cues emanating from the goal to home in on their target. Mott (2010) asserted that a ‘sun compass’ is responsible for the orientation of juvenile green turtles.
What methods do turtles use for migration?
So far, it’s unknown what navigational skills turtles use for migration. Researchers have proposed a number of hypotheses including astronomical cues and and the Earth's magnetic fields.
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