Functional significance of emergence timing in bats

Abstract

We investigated intraspecific differences in evening emergence time of northern bats Eptesicus nilssonii, greater horseshoe bats Rhinolophus ferrumequinum and lesser horseshoe bats R. hipposideros. Significant differences in emergence time were associated with presumed variation in predation risk, related to light intensity, and energetic benefits of early emergence, caused by differences in age, reproductive state (energetic demands), and body condition. Females of both species emerged progressively later as pregnancy advanced, perhaps because of decreased flight performance, and earlier as lactation proceeded, probably because of increased energy demands and low reserves. Bats under energetic stress, due to persistent low ambient temperatures during pregnancy, or when body reserves were low, emerged relatively early, and hence appeared to take higher risks, than other bats. Young bats emerged much later than the adults at first, but progressively earlier as their flight skills improved. Lesser horseshoe bats emerged later at exposed roost exits than in more protected situations. The results largely corroborate the hypothesis that emergence time, and therefore feeding performance, of insectivorous bats is constrained at bright light conditions, possibly by predation risk (from birds), and modified by energetic considerations.