The boldly patterned Norway Lemming is effective day and night, alternating periods of activity with short spells of rest. Shrubs, grasses and mainly mosses make up its diet; in winter it clears runways under the snow on the ground surface in its hunt for food. These lemmings begin to breed in spring, under the snow, and may generate as many as litters of 6 young each all through the summer.
Lemmings are fabled for their striking population explosions, which crop up about every three or four years. It is still not identified what causes these, but a fine, warm spring following two or three years of low population generally triggers an explosion that year or the next. As local population rises, lemmings are forced into surrounding areas.
Slowly more and more are driven out, down the mountains and into the valleys. Many are eaten by predators, and more lose their lives while crossing rivers and lakes. Lemmings do not knowingly commit suicide.