Modified wintering strategy in Daubenton’s bat ( <i>Myotis daubentonii</i> , Kuhl 1817) in response to climate change, a case study in “Nietoperek” bat reserve (W. Poland)

Climate change is impacting on wildlife. Animals that hibernate, such as bats (Chiroptera), are particularly susceptible to the adverse effects of shortened winters, reduced snow cover, weather anomalies and increased underground temperatures. To measure the influence of climate change on hibernation strategies of bats we studied body condition change in Daubenton’s bats Myotis daubentonii, a temperate-zone species with a declining population, in one of Europe’s largest hibernation sites (Natura 2000 site PLH080003 “Nietoperek” – Lubuskie Province, W. Poland). Body mass (M) and forearm length (FL) were measured monthly during the 2021/2022 and 2022/2023 winter seasons. The body condition index (BCI = M/FL) was calculated for 1048 bats. We compared BCI with microclimatic conditions in the areas where the bats were captured. Sub-adult bats (age: 0–1) were observed more frequently at lower temperatures (5°C), while adults (age 1+) preferred higher temperatures (8°C). We found a significant positive trend for temperature preferences for hibernating male bats, but not for females. Fluctuating BCI values did not follow an expected consistent decreasing trend, but reflected increases in external temperatures suggesting the widespread use of winter-feeding strategies during favourable weather conditions. The expected preference of individuals with lower BCI to hibernate at lower temperatures did not occur. This may be because the cool part of the corridor close to the entrance is selected not for thermal conditions but for the ability of bats to respond more quickly to changes in weather.