migration of animals

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migration of animals,

movements of animals in large numbers from one place to another. In modern usage the term is usually restricted to regular, periodic movements of populations away from and back to their place of origin. A single round trip may take the entire lifetime of an individual, as with the Pacific salmonsalmon
, member of the Salmonidae, a family of marine fish that spawn in freshwater, including the salmons, the trouts, and the chars (subfamily Salmoninae), the whitefish and the ciscoes (subfamily Coregoninae), and the grayling (subfamily Thymallinae).
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; or an individual may make the same trip repeatedly, as with many of the migratory birds and mammals. The animals may travel in groups along well-defined routes; or individuals may travel separately, congregating for breeding and then spreading out over a wide feeding area, as do some of the sealsseal,
carnivorous aquatic mammal with front and hind feet modified as flippers, or fin-feet. The name seal is sometimes applied broadly to any of the fin-footed mammals, or pinnipeds, including the walrus, the eared seals (sea lion and fur seal), and the true seals, also called
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Types of Migration

Seasonal migrations occur in many species of insects, birds, marine mammals, and large herbivorous mammals. These migrations often provide the animals with more favorable conditions of temperature, food, or water. Many birds and a few batsbat,
winged mammal of the order Chiroptera, which includes 900–1,000 species classified in about 200 genera and 17 families. Bats range in size from a wingspread of over 5 ft (150 cm) to a wingspread of less than 2 in. (5 cm).
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 of cold and temperate regions migrate to warmer areas during the winter. Herbivores of cold regions, such as wapitiwapiti
, large North American deer, Cervus canadensis, closely related to the Old World red deer. It is commonly called elk in America although the name elk is used in Europe to refer to the moose.
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 (elk), cariboucaribou,
name in North America for the genus (Rangifer) of deer from which the Old World reindeer was originally domesticated. Caribou are found in arctic and subarctic regions. They are the only deer in which both sexes have antlers.
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, and moosemoose,
largest member of the deer family, genus Alces, found in the northern parts of Eurasia and North America. The Eurasian species, A. alces, is known in Europe as the elk, a name which in North America is applied to another large deer, the wapiti.
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, have summer and winter ranges; many herbivores of warm regions, such as the African antelopesantelope,
name applied to any of a large number of hoofed, ruminant mammals of the cattle family (Bovidae), which also includes the bison, buffalo, sheep, and goats. Found in Africa and Eurasia, they range in size from pygmy antelopes, 12 in.
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, migrate seasonally to avoid drought. These migrations may involve a change of latitude, of altitude, or both.

In many cases the chief function of seasonal migration is to provide a suitable place for reproduction, which may not be the place most suitable for the feeding and other daily activities of adults. Hundreds of thousands of gnus (wildebeests) of E Africa take part in annual migrations to calving grounds. Many fishes migrate to spawning grounds, and in some cases this involves a change from saltwater to freshwater (e.g., salmon) or vice versa (e.g., freshwater eelseel,
common name for any fish in the order Anguilliformes, and characterized by a long snakelike body covered with minute scales embedded in the skin. Eels lack the hind pair of fins, adapting them for wriggling in the mud and through the crevices of reefs and rocky shores.
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). Sea turtlessea turtle,
name for several species of large marine turtles found in tropical and subtropical oceans. These turtles are modified for life in the ocean by having flipperlike forelimbs without toes and lightweight shells. Their heads are too large to be withdrawn into the shell.
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, seals, and many sea birds come ashore to breed, and most amphibiansamphibian,
in zoology, cold-blooded vertebrate animal of the class Amphibia. There are three living orders of amphibians: the frogs and toads (order Anura, or Salientia), the salamanders and newts (order Urodela, or Caudata), and the caecilians, or limbless amphibians (order
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 gather near water at the breeding season. Fur sealsfur seal,
fin-footed marine mammal of the eared seal family (Otaridae), highly valued for its fur. Like the closely related sea lion, the nine species of fur seals are distinguished from the true seal by external ears and the ability to turn their hind flippers forward for
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 and many whaleswhale,
aquatic mammal of the order Cetacea, found in all oceans of the world. Members of this order vary greatly in size and include the largest animals that have ever lived. Cetaceans never leave the water, even to give birth.
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 make ocean voyages of thousands of miles to their breeding grounds, the former coming ashore on islands. Such migration is seriously affected by the increasing rate of destruction of natural habitats.

The term emigration refers to irregular movements out of an area, with no return. When such emigration is the result of sudden, explosive population increase, it is called an irruption. Irruptions are common among small rodents, notably lemmingslemming,
name for several species of mouselike rodents related to the voles. All live in arctic or northern regions, inhabiting tundra or open meadows. They frequently nest in underground burrows, particularly in winter, although they do not hibernate.
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, and various species of birds and insects. The mass movements of the so-called migratory locusts of N Africa (Locusta) and North America (Melanoplus) are actually irruptions; however, the N African desert locust (Schistocerca) makes true migrations between its winter and summer breeding grounds.

Another type of one-way travel is the regular dispersal of the young of most species. The simplest type of regular migration is the diurnal movement of some marine microorganisms from one depth to another in response to light changes. Certain marine invertebrates, such as the palolo worm (see AnnelidaAnnelida
[Lat., anellus=a ring], phylum of soft-bodied, bilaterally symmetrical (see symmetry, biological), segmented animals, known as the segmented, or annelid, worms.
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), have a monthly migration pattern influenced by the phases of the moon.

Initiation of Migration

Various factors determine the initiation of migration. In some cases external pressures—temperature, drought, food shortage—alone may cause the animals to seek better conditions. For example, most of the mule deer of Yellowstone Park, Wyo., migrate between summer and winter pastures, but those living near hot springs, where grazing is available all year, do not. In many species migration is initiated by a combination of physiological and external stimuli. In birds the migratory instinct is related to the cycle of enlargement of the reproductive organs in spring and their reduction in fall. Experiments have shown that variation in day length is the chief external stimulus for this cycle: light received by the eye affects production of a hormone by the anterior pituitary gland, which stimulates growth of the reproductive organs.

Orientation and Navigation

Much work has been done on orientation and navigation in migrating animals, although the subject is still not well understood. Studies of salmon indicate that they depend on the olfactory sense to locate and return to their stream of origin. Herbivorous mammals often follow well-established trails and probably also use their sense of smell. Bats, whales, and seals use echolocation to navigate in the dark or underwater; in addition, some whales appear to take visual bearings on objects on the shore in their migrations.

Migratory birds are believed to use the stars, sun, and geographic features as guides. The probability that stellar navigation is used has been strengthened by experiments in planetariums indicating that birds navigate at least in part by the stars. Night-migrating birds are sometimes disoriented in prolonged heavy fog. Day-flying birds navigate by the sun and also make some use of geographic features, particularly of shorelines. Many birds also have the ability to orient themselves to the earth's magnetic fields and this appears to contribute to their ability to navigate, but the mechanisms by which this happens are not well understood. Most migratory birds travel within broad north-south air routes known as flyways. There are four major flyways in North America, called the Pacific, central, Mississippi, and Atlantic flyways. The space within the flyway used by a particular group of birds is called a corridor. Bird migration is not always in a north-south direction. Many European birds migrate in an east-west direction, wintering in the more temperate British Isles, and many mountain-dwelling birds descend to lower altitudes in winter. The breeding grounds of a bird species are regarded as its home territory. Some migratory birds winter only a few hundred miles from their breeding grounds, while others migrate between the cold or temperate zones of the two hemispheres. The longest journey is made by the arctic terntern,
common name for a sea bird of the Old and New Worlds, smaller than the related gull. Because of their graceful flight and their long pointed wings and forked tails, some terns are called sea swallows. They plunge headlong into the water to catch small fish.
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, which alternates between the Arctic and the Antarctic.

The monarch butterfly has a north-south migration pattern that resembles that of many birds. One monarch population that inhabits northeastern and midwestern North America averages c.12 mph (19 kph) as it heads for the winter to Mexico's Sierra Madre mountains. Monarchs start the return trip in the spring, but they breed along the way and then die; the new generation completes the journey. Studies suggest that monarchs are able to use both the sun and the direction of the earth's magnetic field to navigate. The painted lady butterfly, which is found widely across the world, also migrates great distances, both in North America and in Europe and Africa, where the butterfly's migration takes it from as far as Scandinavia in the summer to south of the Sahara in the winter.

Tools for Studying Migration

The movements of migrating animals are often studied by tagging individuals. Bird banding has been carried on extensively since the 1920s; more recently there has been tagging of fishes, butterflies, and marine mammals. Use is now made of radar, sonar, and radio for following migrations, particularly those of marine animals. Radio transmitters attached to whales or seals emit signals that can be picked up by weather satellites at regular intervals.


See R. R. Baher, The Evolutionary Ecology of Animal Migration (1978); D. J. Aidley, Animal Migration (1981).

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