the movement of animals to different locations. It occurs as a result of changes in the animals’ habitats or is associated with their developmental cycles. The first type of migration may be regular (seasonal, daily) or irregular (during droughts, fires, floods). The second type ensures distribution of a species and may occur in the larval stage (in such sessile animals as ascidians, corals, and sponges) or during sexual maturation (in most animals). Regular migrations follow more or less definite routes. Irregular migrations and settling in new areas are not directed and are often chaotic. Migrations may be horizontal (on land or water) or vertical (in mountains, soil, open sea, plant cover); they are active or passive. Migration is studied by various marking techniques, such as animal tagging and bird banding; marking provides vital information for commercial fishermen and hunters and is valuable in controlling such pests as the migratory locusts and rodents.
Among mammals the most extensive migrations are those of whales, seals, and walrus. Many species of whales migrate annually in the Pacific and Atlantic oceans from polar regions to subtropical and tropical regions, and back again. Harp seals spend the summer at the edges of ice floes in the Arctic Ocean and in the fall migrate to the White Sea. Northern fur seals whelp and molt in the summer off the Komandorskie Islands and Tiulenii Island; for the winter the females migrate to the shores of Japan. Wild reindeer in Eastern Siberia migrate for the winter from the tundra to the forest tundra and to the northern taiga. Some species of bats make seasonal migrations extending 1,500 km or greater.
Vertical seasonal and daily migrations are characteristic of mountain goats and sheep. They are determined by the depth of the snow cover and the difficulties of movement and obtaining food. These vertical migrations are also determined by the location of pastures and resting places. When there are great numbers of squirrels, arctic foxes, or lemmings, mass migrations, or irruptions, occur, in which thousands of individuals move en masse in the same direction, overcoming considerable water obstacles along the way. As a result of one such migration, squirrels settled in Kamchatka in the 1920’s.
The seasonal migrations of birds have been studied most extensively. The range of seasonal movements of amphibians is limited. Many species of acaudal and caudate amphibians concentrate in bodies of water during the reproductive period, moving to dry land up to a distance of 5 km. Among reptiles the most extensive migrations are made by sea turtles. For example, the green turtle, which lives off the shores of Brazil, migrates to isolated Ascension Island to deposit its eggs, covering a distance of about 2,000 km. Passive migrations, owing to ocean currents, are evident among sea snakes. Annual migrations of several kilometers to wintering places are characteristic of many species of snakes.
The active, or directional, migrations of fishes (spawning, feeding, and wintering) are among the most characteristic features of their life cycle. Migrations for spawning are particularly evident in migratory fishes that feed in the sea but enter rivers for reproduction. Such anadromous migrations are primarily characteristic of fishes inhabiting the northern hemisphere, for example, the Clupeidae, Salmonidae, and Acipenseridae. A few fishes (common eel, some species of Cottidae, tropical species of Siluridae) are catadromous, feeding in freshwaters and migrating to the sea for spawning. Many marine fishes approach the shores for spawning (flounder, common lumpfish); others descend to the depths (halibut, ocean perch). Freshwater fishes also make short spawning migrations. Feeding migrations are movements of fishes to good feeding grounds. Migratory fishes travel for fattening to the open sea (Atlantic salmon, Far Eastern trout) or to brackish estuarine waters (whitefish, true smelt, some Cyprinidae and Acipenseridae). Pelagic marine fishes of temperate latitudes (herring, sardine, mackerel) migrate to regions in northern latitudes where there is much plankton. Benthopelagic fishes (cod, haddock, halibut) migrate in the summer from spawning grounds near the Lofoten Islands and the Kola Peninsula to eastern parts of the Barents Sea, which arc rich in benthic organisms. Winter migrations are characteristic of many marine and diadromous fishes, primarily of temperate and high latitudes: plaice and other benthic fishes migrate to deep, warmer waters; anchovies, after feeding in the shallow Sea of Azov, migrate to the deeper Black Sea; bream, catfish, and some true sturgeons in southern rivers of the USSR concentrate in wintering pits at the mouths of rivers.
Passive migrations include the transport of roe, larvae, and sometimes even adult individuals by sea or river currents. Thus, the larvae of the common eel are carried by the Gulf Stream from their spawning grounds in the Sargasso Sea to the shores of Europe and North America. The young of migratory fishes float down rivers in a basically passive manner.
The migrations of insects usually coincide with their distribution. For example, the Asiatic and desert locusts, migrate hundreds and sometimes thousands of kilometers from their hatching grounds. Short seasonal migrations of insects from one biotope to another are also known, with return to former environments occurring in the same or in another generation. For example, Eurygaster integiceps and the lady bug migrate to the mountains, shelterbelts, and other places during the summer and autumn; they return in the spring to the place where their cycle of development occurs. The seasonal migrations of some species of aphids are associated with a change in the food supply (plants). Daily migrations of insects are limited by the boundaries of a single biotope—in soil, plant cover, water, or air; the movements of parasites occur in or on the body of the host. Such daily migrations are characteristic of grasshoppers, beetle larvae in the soil (depending on moisture), and the larvae of warble flies in the body of an animal.
Migrations are characteristic of many aquatic invertebrates. A great majority of plankton animals make seasonal and daily vertical migrations, sometimes traveling several hundred meters. After sunset they usually rise to the surface layers, where they feed intensively on algae and phytoplankton; in the morning they descend to greater depths. Horizontal migrations are also characteristic of many aquatic invertebrates. The king crab travels distances up to 185 km; squids travel up to 8,000 km over the course of four months.
An indispensable condition of animal migration is navigation, that is, the ability to determine the direction of movement. The mechanisms of navigation are diverse. Some animals use constantly directed winds, such as tradewinds or monsoons (migrations of swarms of locusts), or currents (larvae of eels) to successfully reach locations favorable for reproduction. Arctic foxes and other mammals are guided in their migrations by wind-borne odors. With active navigation, fishes, reptiles (sea turtles), birds, and mammals can use certain indicators (changing them on different parts of the journey): celestial bearings (celestial navigation) and visual landmarks on the earth’s surface (outlines of shores, mountain ridges, river valleys, and other visually perceived features of the earth’s surface).
The perception of the “native landscape,” whose features usually are memorized in the early stages of an animal’s independent life (imprinting), enables birds migrating for the first time to reach their wintering places and return to their native lands on their own. This same familiarity with the features of the native landscape ensures the homing instinct, that is, the ability to return to the nest even from an unfamiliar place. Many other features of the environment (including geochemical and acoustic ones) and magnetic fields may serve as orientation indicators. Celestial navigation is probably used by birds, marine mammals, sea turtles, and some fishes. The orientation of migrating schools of fishes may be based on the earth’s magnetic field. The chemism of sea currents serves as an orientation indicator for migrating whales, while the odor of river water is used by salmon during migration to their spawning grounds. In selecting indicators that determine direction of motion, all sensory systems are used, and the information is correlated and integrated by the central nervous system. Undoubtedly great, but not yet completely understood, is the significance of genetically fixed features of behavior which execute the program of migration encoded in the genotype.
The herd (flock, shoal) life of animals is particularly important in migration. It helps defend the animals from predators. It also enables animals to correct each other’s behavior and to use the most experienced individuals as leaders, thus increasing the reliability of bionavigation.
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N. P. NAUMOV