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 very low levels, with body temperature and breathing rates lowered, gradually using the body fat reserves that were stored during the active warmer months. Some hibernating animals stir as often as once a week; Other animals sleep through the entire season.
Both land-dwelling and aquatic mammals hibernate. Animals that hibernate include mice, batss, ground squirrels, terrapins, snakes, frogs, and newts. 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 the water.
One animal that some consider to be a hibernator but is not a true hibernator is the bear. The reason for that is because while heart rates slow, the bear's body temperature remains relatively stable and they can be easily aroused. Other non hibernators that are thought to be a hibernator are: badgers, raccoons, and opposums.
Before entering hibernation most species eat prodigious amounts of food and store energy in large 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.