Waking to drink: rates of evaporative water loss determine arousal frequency in hibernating bats

Bats hibernate to cope with low ambient temperatures (T-a) and low food availability during winter. However, hibernation is frequently interrupted by arousals, when bats increase body temperature (T-b) and metabolic rate (MR) to normothermic levels. Arousals account for more than 85% of a bat's winter energy expenditure. This has been associated with variation in T-b, T-a or both, leading to a single testable prediction, i.e. that torpor bout length (TBL) is negatively correlated with T-a and T-b. T-a and T-b were both found to be correlated with TBL, but correlations alone cannot establish a causal link between arousal and T-b or T-a. Because hydration state has also been implicated in arousals from hibernation, we hypothesized that water loss during hibernation creates the need in bats to arouse to drink. We measured TBL of bats (Pipistrellus kuhlii) at the same T-a but under different conditions of humidity, and found an inverse relationship between TBL and total evaporative water loss, independent of metabolic rate, which directly supports the hypothesis that hydration state is a cue to arousal in bats.