Hibernation is a state of regulated hypothermia, lasting several days or weeks, that allows animals to conserve energy during the winter. During hibernation animals slow their metabolism to a very low level, with body temperature and breathing rates lowered, gradually using up the body fat reserves stored during the warmer months. Some hibernating animals stir as often as once a week; others sleep throughout the season.

Both land-dwelling and aquatic mammals hibernate. Animals that hibernate include mice, bats, ground squirrels, terrapins, snakes, frogs, and newts. Although Pliny thought that swallows hibernated, and even a keen observer like the Rev. Gilbert White (The Natural History of Selborne) agreed, birds typically do not hibernate, instead using torpor, but a rare bird known as the Poorwill does hibernate. Aquatic animals can hibernate either in or out of water. Red-eared Terrapins hibernate in water, burying themselves in the mud at the bottom of a pond. Newts are capable of hibernation on land or in water.

One animal that some consider to be a hibernator but is not a true hibernator is the bear. While its heart rate is slow, the bear's body temperature remains relatively stable and it can be easily aroused. Other non-hibernators that are commonly assumed to be a hibernators include badgers, raccoons, and opposums.

Before entering hibernation most species eat a large amount of food and store energy in fat deposits in order to survive the winter. Some species of mammals hibernate while gestating young, which are born shortly after the mother stops hibernating.

For a couple of generations during the 20th century it was thought that basking sharks settled to the floor of the North Sea and hibernated. Tracking devices installed on 20 basking sharks in 2002 dispelled this hypothesis.

Until recently no primate, and no tropical mammal, was known to hibernate. However, animal physiologist Kathrin Dausmann of Philipps University of Marburg, Germany and coworkers present evidence in the 24 June 2004 edition of Nature that the Madagascan fat-tailed dwarf lemur hibernates in tree holes for seven months of the year. This is interesting because Madagascan winter temperatures sometimes rise to over 30 °C (86 °F), so hibernation is not exclusively an adaptation to low ambient temperatures. The hibernation of this lemur is strongly dependent on the thermal behaviour of its tree hole: if the hole is poorly insulated, the lemur's body temperature fluctuates widely, passively following the ambient temperature; if well insulated, the body temperature stays fairly constant and the animal undergoes regular spells of arousal. Dausmann found that hypometabolism in hibernating animals is not necessarily coupled to a low body temperature.

Noise and vibration from snowmobiles, all-terrain vehicles and the like is said to sometimes awaken hibernating animals, who may suffer severely or die as a result of premature awakening in times of food shortage.

See also

• Estivation - a state of dormancy similar to hibernation, except it is used in the summer
• Torpor - regulated hypothermia for less than a day, often used by birds
• Hibernation Inducement Trigger