hibernation [Lat.,= wintering], practice, among certain animals, of spending part of the cold season in a more or less dormant state, apparently as protection from cold when normal body temperature cannot be maintained and food is scarce. Hibernating animals are able to store enough food in their bodies to carry them over until food is again obtainable. They do not grow during hibernation, and all body activities are reduced to a minimum: there may be as few as one or two heartbeats a minute. Cold-blooded animals (e.g., insects, reptiles, amphibians, and fish) must hibernate if they live in environments where the temperature—and hence their own body temperature—drops below freezing. Some insects pass their larval stage in a state of hibernation; in such cases hibernation is closely associated with the reproductive cycle (see larva; pupa). However, most warm-blooded animals, i.e., birds and mammals, can survive freezing environments because their metabolism controls their body temperatures. Many hibernating animals seek insulation from excessive cold; bears and bats retire to caves, and frogs and fish bury themselves in pond bottoms below the frost line. Analogous to hibernation is aestivation, a dormant period of escape from heat and drought. Other methods of avoiding excessively high or low temperatures and destructive increases or decreases in the water supply are encystment and ensuing dormancy, e.g., in plant seeds and bacteria, and migration. Some animals, such as rabbits, raccoons, and squirrels, store food against scarcity and spend cold periods asleep in their burrows, though they may emerge on warm days.

State of greatly slowed metabolism and low body temperature in winter in certain animals. True hibernators include many cold-blooded animals and a few mammals (e.g., bats, hedgehogs) that go into a near-dead state with a near-freezing body temperature and very slow breathing and heart rate. Mammals such as bears that sleep in dens with only slightly lowered body temperature wake easily and are not considered true hibernators. Most hibernators build up a reserve of body fat or store food ahead of time. They may wake and eat several times during the winter. Cold-blooded animals must hibernate where the weather drops below freezing. Hibernation's warm-weather equivalent is estivation.

Learn more about hibernation with a free trial on Britannica.com.