Mammalian Hibernation: Biochemical Adaptation and Gene Expression

The core body temperature (Tb) of most mammals is maintained remarkably constant (typically 36 to 388C, depending on the species), regulated by a thermostat in the hypothalamus of the brain and with heating and cooling achieved, as needed, by a variety of physiological and biochemical mechanisms. All mammals, including humans, are endotherms (generating internal heat by metabolic reactions) and most are also good homeotherms (maintaining a highly constant core Tb), and this supports a variety of mammalian successes, including high-speed locomotion, range extension into cold environments, and advanced brain functions. However, endothermy is costly and the metabolic rate of mammals is typically 4 to 7 times higher than that of comparably sized reptiles. This must be supported by equally higher rates of fuel consumption, supplied by foraging or, if food supply is limiting, by food caches or body adipose reserves. When winter approaches and environmental temperature falls, the metabolic rate, Tb, and food needs of an ectothermic (cold-blooded) organism decline along with it. However, a mammal in the same situation loses body heat faster at colder temperatures and, hence, needs a higher metabolic rate and a greater fuel consumption to support the increased thermogenesis needed to maintain a constant core Tb. Many mammals, even some very small ones such as shrews and some mice, can meet this challenge and remain active throughout the winter by spending as much time as possible in sheltered environments (e.g., under the snowpack and/or in insulated nests), increasing their body insulation (thicker fur, more body fat), and assembling adequate fuel supplies of forage, food caches, and/or body fat. For others, the combination of cold temperatures and lack of food availability in winter makes survival as a homeotherm impossible. The problem is particularly acute for animals such as insectivorous bats or grazing herbivores (e.g., ground squirrels, marmots) that have little or no access to edible food in the winter. The solution to this problem is hibernation. For 6 to 9 months of the year, many small mammals abandon one of the defining characteristics of mammalian life (homeothermy) and allow their Tb to fall, tracking environmental temperature. By doing so, hibernators gain tremendous energy savings. For example, it has been calculated for ground squirrels that winter hibernation saves 88% of the energy that would otherwise be needed to maintain a euthermic Tb of 378C over the winter. The present chapter explores metabolic regulation as it applies to mammalian hibernation. The field of hibernation research is a huge one that examines the phenomenon at ecological, physiological, and biochemical levels and also includes a huge body of applied research that seeks to use the lessons learned from hibernation in hypothermic medicine, organ preservation (see Chapter 19), and understanding complex brain functions. These subjects fill books of their own so the treatment here is selective and focuses on recent advances in understanding the principles of metabolic regulation as they apply to hibernation.