Mammals of the colder latitudes prepare for winter in a number of ways. Some, including whales, elk and caribou, migrate to wintering areas where food will be more available or where conditions will favor their survival. Others, such as chipmunks, pikas, tree squirrels and voles, store seeds, nuts or dried vegetation to get them through the winter. Many mammals remain active, putting on fat and a dense coat of fur through the autumn; while some den up during periods of intense cold or heavy snow, they will continue to forage and hunt through the winter months. Finally, there are the true hibernators: woodchucks, marmots, ground squirrels, some bears and most bats; they will spend the winter in caves or dens, surviving on a dense layer of brown fat.
Hibernation is a complex and risky adaptation to cold weather, used primarily by mammals of the Temperate Zone. Tropical mammals have no need to hibernate and few Arctic mammals attempt the feat (Arctic winters are too long and too severe to permit their survival). True hibernators eat voraciously during the late summer and autumn, putting on a layer of brown fat that will fuel their metabolism through the winter; at the same time, they gradually decrease their activity, helping to insure that their calorie storage will be sufficient.
Shortening daylight, periods of cold weather and diminishing food supplies are thought to trigger hibernation, which generally occurs after several false starts. Distinct from sleep, hibernation is an active process in which reduced respiration and heart rate precede a fall in core body temperature. At some point, the hibernating mammal enters a coma-like state and its metabolic rate falls to a level just adequate to support cellular metabolism; these mammals have an innate ability to tolerate hypothermia, hypoxemia and lactic acidosis. Should their energy supply be insufficient to fuel arousal in the spring, the animal will die.