Sunday, October 11, 2009

Kinds of Lemmings

Lemming, any of several small rodents, family Cricetidae (order Rodentia), found primarily in north temperate and polar regions of North America and Eurasia. Lemmings are placed in four genera: Dicrostonyx (collared, or Arctic, lemmings); Lemmus (“true” lemmings); Myopus (the wood, or red-backed, lemming); and Synaptomys (bog lemmings). Lemmings are short legged, with small ears and long, soft fur. They are 10 to 18 centimetres (4 to 7 inches) long, including the stump of a tail, and are grayish- or reddish-brown above, paler below. The wood lemming (Myopus schisticolor) of the Old World has a reddish back. The collared lemmings of the Arctic regions of the U.S.S.R. and Canada, have dark back and face stripes, except in winter, when their fur is completely white. Lemmings feed on roots, shoots, grasses, and other plant material, and live in burrows or rock crevices. They breed from spring to fall, the female producing up to nine young after a gestation period of 20 to 22 days.

Lemmings are noted for the regular fluctuations of their populations and for their periodic migrations. The population “explosions” (occurring about three or four years apart in the genus Lemenus) are not completely understood.

Factors influencing population change include the following: natural increase in numbers after the last migration and its subsequent population decline; reduction in predators resulting from the decline in lemmings, their prey, after a migration; and optimal breeding conditions for lemmings.

The migrations of lemmings tend to occur in spring and in fall. The movements of the Norway lemming (Lemmas lemmas) are the most dramatic, as many of the migrants may end by drowning in the sea. The causes of lemming migrations are unknown, but the major factor seems to be related to seasonal changes in habitat. The animals tend to follow paths and roadways established by people or animals, and they apparently move outward in all directions from a central area. Lemmings hesitate to enter water and generally try to avoid swimming across rivers and other bodies of water, seeking land crossings whenever possible. They do not, as is popularly supposed, plunge into the sea in a deliberate, suicidal death march. Ivan Prostorovski


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