LAUSR

Simple Summary Every summer, in Italy and other temperate regions, many bats fall from their shelters (roosts), and, especially in urban areas, they are rescued by the public and admitted to wildlife rehabilitation centres. Why this happens is unclear, but bats might be victims of heatwaves due to climate change causing hyperthermia and dehydration. We used 5842 records provided by four Italian rehabilitation centres covering >20 years and found that for both the total sample and most species, the number of admitted bats strongly increased in the summer weeks in which temperatures rose above 30 °C. The species involved were those living in urban areas, and the effect was, in most cases, evident for both juveniles and adults. Although our study relies on correlative evidence, heat stress is the best explanation for peaks of bats falling from roosts. Bats admitted to rehabilitation centres are likely a small fraction of all those affected, so heatwaves might have important negative effects on bat populations in cities, where these mammals eat many harmful insects, providing great benefits to people. We urge that large-scale monitoring of bats in urban areas is carried out to unveil the causal mechanisms and dynamics behind this phenomenon and inform bat conservation. Abstract Climate change is exerting a broad range of (mostly adverse) effects on biodiversity, and more are expected under future scenarios. Impacts on species that deliver key ecosystem services, such as bats, are especially concerning, so their better understanding is key to preventing or mitigating them. Due to their physiological requirements, bats are especially sensitive to environmental temperatures and water availability, and heatwave-related mortality has been reported for flying foxes and, more anecdotally, other bat species. For temperate regions, to date, no study has highlighted an association between temperature extremes and bat mortality, mostly due to the difficulty of relying on data series covering long timespans. Heatwaves may affect bats, causing thermal shock and acute dehydration so bats can fall from the roost and, in some cases, are rescued by the public and brought to wildlife rehabilitation centres (WRCs). In our work, we considered a dataset spanning over 20 years of bat admittance to Italian WRCs, covering 5842 bats, and hypothesised that in summer, the number of admitted bats will increase in hotter weeks and young bats will be more exposed to heat stress than adults. We confirmed our first hypothesis for both the overall sample and three out of five synurbic species for which data were available, whereas hot weeks affected both young and adults, pointing to an especially concerning effect on bat survival and reproduction. Although our study is correlative, the existence of a causative relationship between high temperatures and grounded bats is still the best explanation for the recorded patterns. We urge such a relationship to be explored via extensive monitoring of urban bat roosts to inform appropriate management of bat communities in such environments and preserve the precious ecosystem services such mammals provide, especially insectivory services.