Hydration state of bats may explain frequency of arousals from torpor

Abstract

Bats hibernate to cope with unfavorable conditions, such as low ambient temperature (Ta) and low food availability. However, hibernation is frequently interrupted by arousals that are believed to be responsible for more than 85% of the animal's winter energy expenditure. To date, arousal in bats has been associated with either variation in body temperature (Tb), in Ta, or in both. Considering either Tb or Ta to be the cue to arousal has led to a single testable prediction, i.e. that torpor bout length (TBL) is negatively correlated with Ta and Tb. It is now known that Ta and Tb are both correlated with TBL, but correlations alone cannot establish a causal link between arousal and Tb or Ta. Since hydration state too has been implicated in arousal from hibernation, we hypothesized that water loss while hibernating creates the need to restore body water and electrolyte balance, and bats arouse to drink. We measured the TBL of bats (Pipistrellus kuhlii) kept in different conditions of humidity, but at the same Ta. We found an inverse relationship between TBL and total evaporative water loss that is independent of metabolic rate, thus supporting the hypothesis that hydration state is a cue to arousal in bats.