Lemming , mouse like arctic rodent characterized by a small, short body that is about 13 cm (about 5 in) long, with a very short tail. Lemmings are tan above and light gray, mixed with tan, below. The head is round, with small ears concealed by fur and with a stubby, hairy snout. The legs are short. The animals live in extensive burrows near the water, feed on vegetation, and build nests out of hair, grass, moss, and lichen. The female produces several broods a year, each of which contains about five young. The species occurring in North America is the brown, or true, lemming; the blackfooted lemming is a variety of the same species. The Norway lemming appears in the cultivated fields of Norway and Sweden during the course of the periodic mass migrations for which is it famous. When overpopulation of Norway lemmings leads to a scarcity of food and overcrowding of habitat, many thousands of the animals migrate in search of food. The migrators swim lakes and rivers, cross mountains, and eat all vegetation in their path. Eventually, some reach the sea; attempting to swim it as if it were a river, they are drowned. This phenomenon of mass migration and drowning is relatively infrequent, although population fluctuations occur every three or four years. Other lemming species do not migrate in this way .