Caribou and lemmings are migratory animals. Caribou live in herds in the cold northern regions, feeding on lichen. Each year, when snows bury their food supply, they migrate south to warmer regions. The caribou return when the snows thaw.
Lemmings are small rodents that live in similar regions to the caribou. After a year of abundant food, they breed in such enormous numbers that their food supply runs out. The lemmings then migrate towards the coast in search of more food. Many drown while crossing rivers, but Arctic foxes and predatory birds eat most of them.
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