of Little Brown Myotis

Abstract Hibernation permits endotherms to survive prolonged periods of cold temperatures and reduced food supply through a combination of energy storage, microhabitat selection, and metabolic reduction. For many mammals, predictable thermal relationships define both the length of the hibernation period and the level of energy expenditure during hibernation, facilitating the ability to make precise predictions about the energetic consequences of microhabitat and latitudinal temperature gradients. Here we develop a quantitative model predicting the effect of ambient temperature on the hibernation energetics of little brown myotis (Myotis lucifugus) and compare predictions of the model with the observed distribution of bats across thermal gradients within caves and across the North American continent. Our model predicts pronounced effects of ambient temperature on total winter energy requirements and a relatively narrow combination of hibernaculum temperatures and lengths of winter that permit successful hibernation. Empirical distributional patterns of M. lucifugus correspond closely to these predictions, suggesting that temperature effects on hibernation energetics severely constrain the distribution of bats within hibernacula at the northern limit of their range. INTRODUCTION Endothermic organisms generally maintain a constant, elevated body temperature (Tb) independent of ambient temperature (Ta), but their activity and energy use remain highly temperature-dependent (Speakman, 2000). Small endotherms in temperate and arctic zones face a dual problem in winter when increased thermoregulatory requirements coincide with reduced food availability. The adaptations that permit persistence during these periods generally involve some form of avoidance of the most extreme energetic stresses (King and Murphy, 1985). Migration (Gwinner, 1990)