A hibernator is an animal that survives winter by entering a deep sleep called hibernation. During this time, the animal's body temperature drops dramatically, its heartbeat slows to just a few beats per minute, and it barely breathes. A hibernating groundhog's heart might beat only five times per minute instead of its normal eighty beats. Groundhogs, chipmunks, bats, and some frogs are all hibernators.

Before winter arrives, hibernators prepare by eating enormous amounts of food to build up fat reserves. A bear might gain hundreds of pounds in the fall, knowing it won't eat for months. When temperatures drop, the hibernator retreats to a den, burrow, or cave where it will sleep through the coldest months. Some hibernators wake occasionally during warm spells to stretch or even snack, while others sleep straight through until spring.

This strategy works because finding food in winter takes more energy than the animal could gain from eating it. By hibernating, animals avoid the impossible challenge of finding scarce food in freezing conditions. When spring returns and plants start growing again, hibernators emerge thin and hungry but ready to enjoy the warming weather.