During hibernation, an animal lowers its metabolic rate--the rate at which an animal uses energy and stops generating the heat necessary to keep its body temperature above that of the environment.
As body activities slow, the animal becomes less and less capable of coordinated movement, gradually slipping into a state of dormancy, or torpor. If, however, the animal's body temperature slips below a certain range, the animal will generate heat to boost body temperature to a safe range. Animals may hibernate for several months, but they do not remain completely inactive during this time. Hibernation typically occurs in bouts, or episodes, lasting from a few days to a few weeks depending upon the animal, body size, outside temperature, and time of year. These bouts of inactivity are interspersed with brief periods of activity, when the animals increase their body temperature to a normal level.
Although hibernation helps animals survive adverse environmental conditions, hibernating animals can still freeze to death, and their lack of mobility and coordination makes them vulnerable to predators. To help protect themselves, many animals hibernate in protected areas, such as caves or underground burrows. These sites often remain several degrees above freezing even when the outside temperature is far colder. Animals usually choose sites that are inaccessible to predators.
Hibernation is primarily associated with animals that live in cold climates, but scientists have learned that hibernation occurs in many animals found from the Arctic to the tropics. Many of these animals are mammals. Mammals are endotherms--warm-blooded animals that generate heat internally and maintain a high, relatively constant body temperature regardless of the surrounding environment.
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