A sleeplike state in which some animals pass the winter months as a way of surviving food scarcity and cold weather. Various physiological changes occur, such as lowering of the body temperature and slowing of the pulse rate and other vital processes, and the animal lives on its reserve of body fat. Hibernating mammals may cool to near freezing and have metabolic rates as much as 20 times lower than normal. They may also briefly undergo short-lived periods of arousal before returning to deep hibernation. Animals that hibernate include bats, hedgehogs, and many fish, amphibians, and reptiles. See also dormancy. Compare aestivation.