The boldly patterned Norway lemming is active day and night, alternating periods of activity with short spells of rest. Grasses, shrubs and particularly mosses make up its diet; in winter it clears runways under the snow on the ground surface in its search for food. These lemmings start to breed in spring, under the snow, and may produce as many as eight litters of 6 young each throughout the summer.

Lemmings are fabled for their dramatic population explosions, which occur approximately every three or four years. It is still not known what causes these, but a fine, warm spring following two or three years of low population usually triggers an explosion that year or the next. As local populations swell, lemmings are forced into surrounding areas. Gradually more and more are driven out, down the mountains and into the valleys. Many are eaten by predators, and more lose their lives crossing rivers and lakes. Lemmings do not deliberately commit suicide.