Hibernation: Endotherms

The main function of hibernation and daily torpor in mammals and birds is to conserve energy and thus survive during adverse environmental conditions or periods of food shortage no matter if they live in the arctic or the tropics... Endothermic mammals and birds differ from ectothermic organisms primarily in their ability to regulate body temperature by high internal heat production via combustion of fuels. Because the surface area/volume ratio of animals increases with decreasing size, many small endotherms must produce an enormous amount of heat to compensate for heat loss during cold exposure. Obviously, prolonged periods of such high metabolic heat production can only be sustained by high food intake and, during adverse environmental conditions and/or food shortages, costs for thermoregulation may be prohibitively high.