Lemming

Lemmings are small rodents. They live in or near the Arctic, in tundra biomes.
Life.
Lemmings and voles are the rats and mice of the Arctic. Lemmings have very short tails. They live underground in summer. Because the top soil freezes in the winter, they cannot burrow underground then, so they live under the snow during the colder months. They eat plants and roots. If they find plenty of food, they will have extra large families that year.
In a year with plenty of food, millions of lemmings will be roaming the tundra. The owls and foxes have more food than they can eat. They will have extra babies in they years when there are so many lemmings. However, the tundra does not have enough food for millions of lemmings. They rush across the tundra looking for food, and are eaten by predators or starve to death.
Many lemmings leave to find a new home, when they run out of food and space. Soon, millions of lemmings decide to follow the crowd. They go across the Arctic as a huge mass. Foxes and owls kill many of them. Some drown when trying to swim across rivers. It is often said that they fall off cliffs or even commit mass suicide, but that is an urban legend created by a Disney documentary ("White Wilderness", 1958) in which someone actually pushed the lemmings over the edge.
The lemmings that are left have enough food and space to survive. Then they start to breed and multiply again.