Energy conservation in hibernating endotherms: Why “suboptimal” temperatures are optimal

Many endotherms use facultative heterothermic responses of torpor or hibernation to conserve energy during periods of low energy availability. A common assumption when estimating winter energy budgets is that endotherms should hibernate at the ambient temperature (Ta) that minimizes torpid metabolic rate (TMR) and maximizes the duration of torpor bouts. However, previous studies of the energetic benefits of hibernation have assumed constant Ta within hibernacula. Here we use an individual-based energetic model to estimate overwinter energy expenditure of mammals hibernating at Tas that vary temporally. We show that, in accordance with the principles of Jenson's inequality, hibernators can conserve energy by selecting microclimates warmer than the single Ta value that minimizes TMR (Tmin). As temporal variation in Ta increases, endotherms should choose microclimates with mean Tas progressively warmer than Tmin. Further, as thermal conductance decreases, as it does with increasing body mass and use of social thermoregulation, the mean Ta that minimizes overwinter energy expenditure approaches, but never equals, Tmin. We suggest that the commonly held assumption of stable microclimates in hibernacula has skewed the interpretation of the optimal expression of hibernation for energy conservation. Our results contradict much of the accepted understanding of hibernation energetics and add to a growing body of literature proposing that hibernating at a Ta warmer than Tmin is optimal.