Many small and medium-sized mammals in north-temperate regions solve the problem of winter scarcity of food and low temperature by entering a prolonged and controlled state of dormancy. True hibernators, such as ground squirrels, groundhogs, and mice, prepare for hibernation by building up large amount of body fat. Some, such as the groundhog, also lays in stores of food in their burrow. Entry into hibernation is gradual. After a series of “test-drops” during which the temperature drops a few degree and then returns to normal, the animal cools to within a degree or less of the surrounding temperature. Metabolism is greatly reduced. In the ground squirrel, for example, the respiratory rate drops from a normal of 200 per minute to 4 to 5 per minute, and the heart rate from 150 to 5. This means that the body slows down In most hibernators, the body temperature is closely checked by internal systems. If the body temperature drops dangerously close to the freezing point, the animal will awaken. The hibernators also awaken at irregular intervals to eat and eliminate wastes and then return to sleep.

Some mammals such as bears, badgers, raccoons, and opossums enter a state of prolonged sleep in winter with little or no drop of body temperature. This is not true hibernation. Bears of the northern forest may sleep for several months. Their heart rate may drop from 40 to 10 beats per minutes, but their body temperature remains normal and the bear is awakened if sufficiently disturbed.Mammals are not the only hibernators. There are several other animals such as the toad and frog that survive winter by hibernating.