migration of animals definition of migration of animals in the Free Online Encyclopedia.

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 salmon salmon (săm`ən), member of the Salmonidae, a family of marine fish that spawn in freshwater, including the salmons, the trouts, and
..... Click the link for more information. ; 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 seals seal, 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
..... Click the link for more information. .

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 bats bat, 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).
..... Click the link for more information. of cold and temperate regions migrate to warmer areas during the winter. Herbivores of cold regions, such as wapiti wapiti (wŏp`ĭtē), large North American deer, Cervus canadensis, closely related to the Old World red deer.
..... Click the link for more information. (elk), caribou caribou, 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.
..... Click the link for more information. , and moose moose, 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 .
..... Click the link for more information. , have summer and winter ranges; many herbivores of warm regions, such as the African antelopes antelope, name applied to a large number of hoofed, ruminant mammals of the cattle family (Bovidae), which also includes the sheep and goats. The North American pronghorn is sometimes called an antelope, but belongs to a separate, related family (Antilocapridae).
..... Click the link for more information. , 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 eels eel, common name for any fish of the 10 families constituting the order Anguilliformes, and characterized by a long snakelike body covered with minute scales embedded in the skin.
..... Click the link for more information. ). Sea turtles sea 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.
..... Click the link for more information. , seals, and many sea birds come ashore to breed, and most amphibians amphibian, 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
..... Click the link for more information. gather near water at the breeding season. Fur seals fur 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
..... Click the link for more information. and many whales whale, 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.
..... Click the link for more information. 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 lemmings lemming, 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.
..... Click the link for more information. , 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 Annelida Annelida (ənĕl`ĭdə) [Lat.
..... Click the link for more information. ), 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. It has long been proposed that birds perceive the direction of the earth's magnetic field and use it for navigation, but experimental evidence for that hypothesis is inconclusive. 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 tern tern, 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.
..... Click the link for more information. , 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.

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.

Bibliography

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