Field evidence for a proximate role of food shortage in the regulation of hibernation and daily torpor: a review

Hibernation and daily torpor (heterothermy) have long been assumed to be adaptive responses to seasonal energy shortage. Laboratory studies have demonstrated that food shortage alone can trigger the use of heterothermy. However, their potential to predict heterothermic responses in the wild is limited, and few field studies demonstrate the dependence of heterothermy on food availability under natural conditions. Thus, the view of heterothermy as an energy saving strategy to compensate for food shortage largely remains an untested hypothesis. In this paper, we review published evidence on the proximate role of food availability in heterothermy regulation by endotherms, and emphasize alternative hypotheses that remain to be tested. Most studies have relied on correlative evidence. Manipulations of food availability, that demonstrate the proximate role of food availability, have been conducted in only five free-ranging heterotherms. Several other metabolic constraints covary with food availability and can confound its effect. Shortage in water availability, the nutritional composition of food, or subsequent conversion of food in fat storage all could be actual proximate drivers of heterothermy regulation, rather than food shortage. Social interactions, competition for food and predation also likely modulate the relative strength of food shortage between individuals. The ecological relevance of the dependence of heterothermy on food availability remains to be assessed in field experiments that account for the confounding effects of covarying environmental and internal factors.