Arousals are energetically costly, and amount to 80–90% of total energy expenditure during the winter (Thomas et al. Torpor is interrupted with euthermic bouts-arousals-which allow the bat to counter the ecological and physiological costs caused by torpor (Thomas et al. Hibernation consists of extended bouts of torpor, during which bats lower their body temperature close to ambient temperature of the hibernaculum and decrease their metabolic rate (Geiser 2004 Guppy and Withers 2007). The hibernation period of temperate bats can last more than 8 months (Norquay and Willis 2014) and is elapsed in underground hibernation sites, hibernacula. Bats can respond to this challenge either by migrating to warmer areas, where food is occasionally available throughout the winter and hibernation is less risky (Popa-Lisseanu and Voigt 2009), or they can hibernate in situ, by utilizing fat reserves accumulated before the winter (Geiser 2013). Insectivorous bats living at high latitudes face enormous fluctuations in the seasonal availability of food. Furthermore, our findings underline the suitability of using acoustic monitoring in homing on hibernation sites that are not otherwise accessible. Our results suggest that rock outcrops may be more important to bats than previously acknowledged, highlighting the need to take these sites in account in planning of conservation measures. auritus may be using low barometric pressure as a proxy for suitable foraging conditions during the winter. We also found that ambient temperature had a positive effect on bat activity and found evidence that P. We recorded significantly more activity near rock outcrops compared to other habitats, excluding root cellars. Our results revealed echolocation calls of Eptesicus nilssonii, Plecotus auritus and Myotis sp. These sites included four rock outcrops with crevices and cave features, three glacial erratics or boulder fields, three ancient shores, three root cellars and three control sites where we did not expect bats to be overwintering. To investigate hibernation site selection and winter activity of boreal bats, we recorded bat calls using passive acoustic monitoring at 16 sites in South-Western Finland. Natural hibernation sites used by bats in areas that lack cave features have long remained unresolved.
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