The animals that most famously migrate are birds, but the phenomenon of migration is actually widespread both among vertebrates and invertebrates and presents a lot of variability. It generally occurs en masse, but can also occur singly; it may require intergenerational changes; it can occur every day or every year, lasting weeks or months; it is regulated in different ways according to the organism. How is migration defined in ethology and what characterizes it? In this lesson we will analyze migratory behavior by following Tinbergen’s four questions and see which animals implement it.
What is migration?
Migration is a phenomenon that is not easy to define due to the extreme variability with which it occurs and the different levels of analysis ( primarily , the ecological and the individual) to which it can be subjected. In summary, it can be defined as a periodic movement of a population (or a part of it) between specific areas in which there are different favorable environmental conditions depending on the period. It is a very important phenomenon for population regulation.
The migratory behavior is characterized by:
- Persistence . Migration always occurs in a similar way. In fact, it is generally synchronized with environmental changes and occurs from one specific area, where there is a decline in resources, to another.
- Directionality . There are precise, generally linear, paths followed by the individual during migration. On the basis of the direction of displacement, migrations are classified into: barimetric or vertical (if they occur at different sea depths, as for copepods); latitudinal (if the animals migrate to different latitudes); and longitudinal (if the animals are brought to different altitudes, as for the robin, Erithacus rubecula ).
- Deactivation of some behavioral responses triggered by stimuli that would “distract” from migration. For example, some migratory animals do not feed on the way, such as migratory aphids who ignore the bean leaves which normally constitute a strong stimulus to feed.
- Display of peculiar behaviors before, during and after the trip.
- Establishment of specific energy reserves for migration. These long journeys, in fact, are very expensive: sometimes the animals run out of energy, dying, before reaching their destination. For this reason it is essential that animals prepare themselves for travel with some adaptations, for example by feeding more than usual (hyperphagia) and training to exercise the muscles.
Read also: Record-breaking migratory animals
Function: the evolutionary meanings of migrations
If migrations have such high costs, how has migratory behavior evolved? The evolutionary benefits clearly outweigh the costs. Migration is typically an adaptive response to seasonal or geographic changes that result in resource variations . The animal, migrating, moves to an environment that allows it to survive with greater probability. This is due to factors that vary according to the species.
The main biological meanings of migration are two:
- Greater availability of resources . For example, North America in the summer sees the appearance of numerous populations of insects: these constitute abundant food for birds that migrate from the south. Another example can be observed in Africa, with the great migration of wildebeest, zebras and gazelles ( see figure 1 ) . By the millions, these animals migrate north each year in the dry season due to decreased water availability. It is in search of water sources that these species move, only to return south during the rainy season that supplies the African basins.
- Presence of more favorable climates . Many animals migrate to optimal habitats for the completion of the life cycle , as in the case of sea turtles , or to avoid seasonal changes in the area of origin that would compromise their survival. This is the case with migratory birds that overwinter. Another example is that of monarch butterflies ( Danaus plexippus ): migrating from North America to central Mexico they avoid the night frosts that characterize the North American winter. In fact, these insects find an ideal microclimate, not too cold and sufficiently humid not to dry them up, in the forests of sacred fir trees ( Abies religiosa) at 3000 meters above sea level.
Other possible advantages of migration are, for example, the avoidance of predators , as can be observed in the niche (daily) migrations of zooplankton, and the avoidance of intraspecific competition . This is the case, for example, in the case of species that have subpopulations that migrate separately, such as the ground bug ( Lygaeus equestris ) and in the case of some partial migrants , i.e. species that do not migrate in mass (in which case we speak of complete migration ).
A social significance of migration occurs in particular in differential migrants , where the fraction of the population that migrates belongs to a single gender or a specific age group. An example is the blackbird ( Turdus merula ): the young usually migrate, while the older members tend to be sedentary. This probably happens for social reasons: in case of competition for the same area of residence, in winter, the subordinates avoid the competition preferring to migrate and return in the spring, to occupy any free territories due to the death of the remaining blackbirds.
Warning: a same species can migrate with multiple different purposes. For example, herrings move between mating, feeding and wintering areas.
Figure 1. Large wildebeest migration through Serengeti National Park (Tanzania). Photo shared according to CC0 attribution .
Physiological mechanisms that allow migration
During migration, animals undergo a series of physioanatomical changes that improve the chances of survival during migration, for example by inhibiting behaviors such as feeding, maturation and reproduction. Another example of adaptation to migration is the increase in size of some organs , such as the heart in some migratory birds, and the shrinkage of others, such as those used for digestion. This results in an optimization of the weight in favor of that useful for locomotion, so that these changes are favored by natural selection.
These physiological and morphological modifications can be triggered by endogenous factors, independent of the environment, or by exogenous factors, dependent on environmental elements.
Exogenous factors and migrations
According to the Zugschwelle hypothesis (migration stimulus threshold) the animals would have limits of environmental conditions (such as temperature variation, brightness, humidity, food availability), beyond which they would have an impulse to migrate. For example, migratory seals start their journey as a result of shrinking food resources.
In general, the factor that triggers the migration is the photoperiod , the alternation of the hours of light with those of darkness. In vertebrates there is a photosensitive gland, the epiphysis or pineal gland , which depending on the photoperiod increases the production of gonadotropic hormones, involved in the control of certain behaviors and morphological aspects of animals. The photoperiod, for example, triggers hyperphagia in birds, caribou, whales and monarch butterflies.
In some animals the stimulus to migrate derives from the variation in temperature . This is particularly evident in animals where migratory periods are anticipated as a result of global warming. This phenomenon is a problem for migratory animals because there is a mismatch in the optimal periods of reproduction, feeding and wintering. For example, in the black nurse ( Ficedula hypoleuca ) the reproductive period is anticipating the optimal period for obtaining food resources. Predators of migrating animals can also be at risk, as their activities are often synchronized with those of their own prey.
Endogenous factors and migrations
In other animals, the stimulus to migration is endogenously controlled , as has been observed in experiments. Individuals of the big dog ( Phylloscopus trophylus ) were kept in the laboratory, under constant conditions of light and temperature. In correspondence with the migration period of their conspecifics in nature, they too manifested the Zugunruhe , the restlessness associated with the need to migrate. These birds, therefore, exhibited migratory behavior as a consequence of their circannual rhythms .
In many migratory insects, hormone production changes greatly depending on the period of the life cycle. A change in concentration in juvenile hormone (hormone found in insects) is related to the transition from a reproductive to migratory vital stage. In this phase the animals show behavioral and morphological changes; for example, the monarch butterflies, in their southward migration, do not have the sexual organs, which will develop in spring.
The physiology of orientation
Orientation modes can also be innate or learned. Migratory animals orient themselves in the most varied ways and generally take advantage of different modes of orientation at the same time. They can exploit cognitive maps, landmarks , stars, polarization of sunlight, earth’s magnetic field (as we saw in the lesson on animal orientation ).
Some use their sense of smell, such as red salmon ( Oncorhyncus nerka ). These animals are anadromous: they are born in a river, move towards the seas where they spend most of their life and then return to the original watercourse to reproduce and die. During the upriver of rivers, twelve of their genes are expressed, giving them a far superior capacity to discern odors. This allows them to recognize the smell of the river they were born in and to which they are headed. As soon as they reach their destination, the expression of these genes is reduced, and consequently the olfactory capacity of the fish.
Read also: The migration of plants and the colonization of emerged lands
Reduce the costs of migrations
Migratory behavior has very high costs, especially in animals that make long migrations. In fact, great energy is spent on travel, so much so that many animals exhaust them and lose their lives during the journey. Before migration, therefore, many species undergo physiological and morphological changes, such as alteration of metabolism, wing lengthening in migratory winged insects and increased hydrodynamics of migratory fish.
Some animals also exhibit behaviors aimed at reducing the disadvantages of migration, training the muscles and accumulating energy resources . Before the migratory period, many birds make short flights or flap their wings when stationary, resulting in hypertrophy of the muscles involved in the flight. Hyperphagia is typical of long-distance migrants. Some passerines (such as the sandpiper , Calidris canutus , and the godwit, Limosa lapponica ) feed up to double in weight; this affects the efficiency of the movement, exposing the animals to predation, but it still has an evolutionary advantage. Some species switch to a different diet than the rest of the year, such as woodcocks (Sylvia borin ) who begin to feed on fruits, as insectivores they were: as a result they spend less energy on nourishment.
The passerines undergo a revolution in their habits: in view of the migration they pass from being diurnal to being nocturnal. They tend to travel at night to further reduce costs: the heat of the sun, in fact, would feed the temperature of these small animals, resulting in energy consumption. On the contrary, often the big birds (such as eagles, cranes, geese and storks) move during the day to take advantage of the current upward , air flows upwards which allow to make passive part of the displacements carried out in flight, with a consequent energy saving. Likewise, many marine animals take advantage of ocean currents to move around. Among these are the leatherback turtle ( Dermochelys coriacea), manta rays ( Manta birostris ) and whale sharks ( Rhincodon typus ).
Figure 2. Flight in “V” formation. Photo shared according to CC0 attribution.
Another adaptation that reduces travel costs is flight in V-formation (see figure 2), which is adopted by many large birds. A research carried out on a group of common pelicans ( Pelecanus onocrotalus ) in fact revealed that this flight training involves a 10-14% less effort than that carried out by individuals flying alone. In fact, birds benefit from the air currents created by their mates in front of them, with a significant reduction in heart rate and respiratory rate.
Another adaptation that would reduce migration costs would be to change the route , choosing the shortest one. This choice, however, is not always the most advantageous. For example, the red-eyed vireo ( Vireo olivaceus) in autumn they migrate from the western United States to the Amazon following two different routes: a shorter one, flying over the waters of the Gulf of Mexico, and a longer one, which follows the Central American coast. A 1996 study by Sandberg and Moore showed that the choice of route was related to the accumulated mass: vireos with more than 5 grams of body fat flew over the sea while others preferred the coastal route. In fact, making such long flights on the water is a danger to survival if sufficient energy has not been accumulated, as the animal could fall and drown.
Ontogenesis of migratory behavior
How does the ability to migrate within an individual develop? Migratory behavior is often genetically determined , in fact many species can pass from being sedentary to being migratory and vice versa in a few generations. For example, in the blackcap, Sylvia atricapilla , it was possible to select the migratory behavior in a few generations through genetic selection experiments. It is estimated that the passage from a sedentary behavior to a migratory one and vice versa takes about 25 generations, which for a small passerine means about 40 years.
Migration is often not fully genetically controlled. In fact, it can be observed that the juveniles of some species, if moved from the migratory route, are not able to find the correct direction, unlike the adults. In these species the young are accompanied by their parents in their first migrations, with a consequent learning of the migratory route . Furthermore, in some species, young individuals during migrations carry out exploratory behaviors (in a mechanism called exploration-refinement ), which allow learningof further possible paths. These behaviors allow adaptability, particularly adaptive in species that have a relatively long average life span and therefore may have to face variations in their migrations.
In some species, migratory behavior is a conditional tactic , meaning it is only implemented under certain conditions. In this case the species shares the same genotype which is expressed in different behavioral phenotypes according to the stimuli that trigger different physiological responses. An example is that of the blackbird ( Turdus merula ).
The ontogenesis of orientation skills
Scientists have not yet discovered how some orientation skills work, such as geomagnetic, which unites butterflies, salamanders, bats, sea turtles, cetaceans and sharks. However, we know that often the ability to orientate is innate, as is particularly evident in species without parental care (such as the Caretta caretta ) and in some species that have subpopulations that migrate to different areas. An example is that of the bats of the subspecies Tadarida brasiliensis mexicana : the hybrids between two subpopulations are not able to migrate to any of the parental mating sites, because the migratory path is not learned but is dictated by the genes.
Phylogeny of migratory behavior
How has this behavior evolved? Migratory animals probably derive from sedentary ancestors in which the ability to move periodically, following environmental conditions more favorable for reproduction and / or nourishment, has evolved gradually , by stages. This may have occurred with the shift from erratic movements to short-range to long-range migrations.
Figure 3. Phylogenetic tree of thrushes of the genus Catharus in which the character of migratory and sedentary behavior is highlighted. The image, reworked by Alcock 2017, is built on the basis of the similarity between species based on two genes.
This hypothesis is supported by some observations, such as the one concerning the catharus genus of thrushes . These birds are divided into twelve species, of which seven are sedentary and the other five migratory. According to the most sparing hypothesis, the migratory behavior of the latter has evolved three times, independently, starting from the sedentary one ( see figure 3 ).
The migratory behavior would have evolved about 2.5 million years ago , favored by natural selection because it allowed to avoid disadvantageous areas. In particular, it is thought that the first migratory behaviors occurred in areas with a tropical climate.
The importance of migratory behavior
Migratory behavior has a strong impact on food webs as well as a great economic and social importance: for example, the study of insect migration can help in agriculture and fish migration is very important for the purposes of trade in fish. Research in the ethological field of this behavior is therefore very important, as well as for the conservation of species, also for strictly “anthropocentric” purposes.
