NARRATIVE STUDENT REVIEW Changes to Metabolism and Cell Physiology that Enable Mammalian Hibernation

Abstract Heterothermy is a widespread, adaptive strat-egy used by many species of bird and mammal to conserve energy during periods of energetic deficit, the expression of which varies greatly depending on the species and envi-ronment. A temporary, reversible reduction in metabolic rate and body temperature (i.e., torpor) is an adaptive re-sponse used by many species of birds and mammals to conserve energy during periods of resource scarcity. Long-term employment of torpor (i.e., hibernation) is a season-ally expressed phenotype, the genetic and regulated path-ways of which can be found throughout all mammal lineages, including hibernators and nonhibernators alike. In mammals, adaptations that allow for hibernation can be classified as those involved in preparation for hibernation, metabolic reduction, continued cellular function and pro-tection, and arousal. Key physiological changes involve seasonal regulation of metabolic hormones, a shift to largely using endogenous fuel sources (i.e., increased lipolysis), global down regulation of protein transcription by posttranslational modification and microRNA save for the increased production of a small number of protective proteins, shifts in membrane composition, and thermoge-nesis by brown adipose tissue. There is some evidence of cold acclimations in nonhibernators, such as during fetal development, but responses are limited and cursory, and eventually cellular damage occurs. Therefore, it appears that a complete suite of adaptations to metabolism, vital physiological functions, and thermogenic mechanisms is required for the successful expression of the hibernation phenotype