Daily Heterothermy by Temperate Bats Using Natural Roosts

Abstract Periods of heterothermy, or torpor, can save mammals and birds substantial amounts of energy. Temperate-zone insect-eating bats often use torpor to accommodate the thermoregulatory challenges associated with their large surface area to volume ratio and dependence on food that fluctuates within and between seasons. Torpor associated with hibernation has been well studied, but heterothermy during summer has received less attention, especially in free-ranging bats. Roosting ecology almost certainly influences patterns of daily heterothermy, but, to date, the majority of studies have focused only on individuals roosting in buildings, as opposed to those roosting in naturally occurring sites (e.g., tree cavities, rock crevices). In this chapter I review a number of predictions about torpor use and roost selection by bats as suggested by the costs and benefits of torpor in different circumstances. I evaluate these predictions with examples from the literature on bats in buildings and natural roosts, and with new data on bats roosting in natural sites. I also suggest avenues for future research addressing this important aspect of the physiological ecology of bats. INTRODUCTION Endothermic animals rely on behavioral, physiological, and morphological adaptations to reduce thermoregulatory costs (Pough et al., 1996). These adaptations are especially important for animals in temperate regions where food resources fluctuate within and between seasons (McNab, 1982). A number of birds and many mammals reduce thermoregulatory costs by using periods of heterothermy, or torpor (Geiser, 1996; Wang, 1989), defined as a state of inactivity and reduced responsiveness to stimuli during which metabolic rate (MR) and body temperature (Tb) fall below normothermic levels, and from which animals can actively arouse in the absence of an exogenous heat source (Barclay et al., 2001; Bligh and Johnson, 1973; Geiser, 1998). Heterothermy in temperate insect-eating bats is common (Audet and Fenton, 1988; Brigham, 1987; Geiser and Ruf, 1995;