Rock solid: winter ecology of boreal bats at natural hibernation sites

Overwintering bats at northerly latitudes spend up to eight months a year in a state of dormancy: a period characterised by extended use of torpor, but not an absolute lack of activity. Although this period constitutes a majority of their yearly cycle, little is known about their ecology during this time, because of the cryptic behaviour of bats. Knowledge gaps are particularly emphasised in karst‐free areas, such as in Fennoscandia, where the lack of mines and caves does not facilitate the monitoring of hibernating bats. Previous research in these areas shows bats use a variety of rock habitats to overwinter but to better provide information for conservation planning in these environments under great anthropogenic pressure, details are needed on species‐specific differences in overwintering behaviour. Here, we monitored the activity of overwintering bat taxa (genus Myotis , Eptesicus nilssonii and Pipistrellus nathusii ) at nine boulder fields and eleven rock outcrops from September to May in southwestern Finland, in northern Europe. Our results show species‐specific patterns in using rock habitats to overwinter. Myotis ‐bats begin overwintering early compared to other taxa and display low activity over the winter. Both E. nilssonii and P. nathusii initiate their hibernation period later and show more activity throughout the winter. Both boulder fields and rock outcrops are used by the studied taxa, but there are species‐specific temporal differences in activity in these habitats. The taxa varied greatly in activity in response to ambient temperature during the winter, with both E. nilssonii and Myotis‐ species expressing strong positive responses to increasing temperature. Our study provides novel information on natural hibernation sites in the boreal zone and emphasises the need for further research investigating the microclimatic conditions favoured by bats, in particular as climate change will have an impact on the suitability of these sites in the future.