LEMMING

any of several small rodents belonging to the family Cricetidae (order Rodentia) and 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 (wood, or red-backed, lemmings); and Synaptomys (bog lemmings). Lemmings are short-legged, with small ears and long, soft fur. They are 10 to 18 cm (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 Russia and Canada have dark back and face stripes, except in winter, when their fur is completely white. Lemmings feed on roots, shoots, and grasses 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. Their population explosions occur about three or four years apart in the genus Lemmus and are not completely understood. Factors influencing their population explosions and migrations 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. After several years of optimal breeding conditions and low rates of mortality from predation, the lemmings in an area may move in detectable waves away from centres of denser lemming population. The emigrating lemmings begin to move in greater numbers, at first erratically and under cover of darkness and later in bold groups that may travel in daylight. The movements of the Norway lemming (Lemmus lemmus) are the most dramatic, as many of the migrants may die by drowning in the sea. 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.