These tundra mice inhabit the vast treeless areas of northern Canada and Alaska. These large populations recur every three to four years, and because many other animals depend on lemmings for food, their impact is a dominant feature of animal life on the tundra. When full grown they are about 5 to 6 inches long and weigh about two or three ounces.
Lemmings are active all year.
Instead of hibernating in winter, they burrow under the snow, takinq advantage of the insulation. When the air temperature is minus 60 degrees Fahrenheit, the temperature under a 24-inch layer of snow may be as high as 20 degrees. Lemmings' winter nests, constructed of finely shredded grass, resemble hollow balls of hay, and when the snow melts in spring they become very conspicuous. During the summer lemmings build their nests underground in tunnel systems which they dig in the thawing soil. The collared lemming has also been called the varying lemming because, unique among rodents, it changes color to adapt to winter's white environment.