Do predators influence the behaviour of temperate-zone bats? An analysis of competing models of roost emergence times

Temperate-zone bats likely feed in an environment with few dangerous nocturnal predators, suggesting a life largely free of the antipredator trade-offs faced by most other animals. Such bats may, however, encounter dangerous diurnal raptors with an early start to their nightly feeding, and thus may face predation-related trade-offs in deciding when to emerge from a daytime roost. Non-predator-related factors could also come into play in this nightly decision. We used an information-theoretic approach to examine which combination of a wide range of variables exerts the greatest influence over emergence times from maternity roosts in a population of Indiana bats, Myotis sodalis, covering a 12-year period. Our competing statistical models were based on two broad environmental influences on emergence times: predators and physical (meteorological and astronomical) variables. The best-performing model included most of the variables associated with antipredator trade-offs, assuming that bats emerging before or near sunset experience a foraging benefit and a substantial risk of predation by diurnal raptors. Accordingly, state-dependent behaviour was apparent, with emergence beginning earlier during the more energetically demanding portion of the reproductive cycle. During lactation (the most demanding period), about 20% of emergence events began before sunset, compared to only 5% during colony formation. Earlier emergence was also associated with high cloud cover (darker conditions), larger roost populations and roosts located deep into the forest. Moonlight at sunset, however, was absent from the best-supported model and had no apparent influence on emergence times. Overall, the emergence behaviour of our species is consistent with a trade-off between energetic needs and predation risk, most likely due to the temporal overlap of early emergences with the activity period of diurnal raptors. This behaviour may be more common than expected in temperate-zone bats.