One Species Hibernates Shorter, the Other Longer: Rapid but Opposing Responses to Warming Climate in Two Sympatric Bat Species.

The timing of hibernation represents a key seasonal transition in the annual cycle of hibernators, directly impacting their survival and reproductive success. Ongoing climate change affects many of the factors that influence hibernation phenology, such as weather patterns, food availability and the timing of mating and reproduction. The extent to which individuals should adjust their hibernation phenology in response is likely to vary between species, sexes and age classes. Consequently, long-term, individualised datasets are essential for capturing individual responses to climate change and understanding the underlying mechanisms. However, such datasets remain exceedingly rare for wild hibernators. Using 13 years of individual-level RFID-logging data from over 4000 marked bats, we discovered strikingly different shifts in hibernation phenology in two sympatric species. While Myotis nattereri shortened its hibernation period with warming winters, Myotis daubentonii surprisingly prolonged its hibernation duration. These opposing trends were primarily driven by shifts in hibernation entry in autumn. Within both species, phenological shifts varied by sex and age, with adult males exhibiting the most pronounced changes. In M. daubentonii, adult males extended the duration of hibernation by 1.8 days/year, equating to almost a full month over the study period. In M. nattereri, adult males reduced the duration of hibernation by 2.3 days/year, resulting in a one-third decrease in total hibernation duration. These remarkable shifts were strongly correlated with autumn temperatures: for each 1°C rise, hibernation entry in M. daubentonii advanced by 9.3 days, whereas in M. nattereri it was delayed by 6.2 days. Our results highlight that hibernation phenology can shift rapidly and in unforeseen, species-specific ways in response to climate change. As mismatches between hibernation timing and prey availability can impact survival and reproduction, these phenological shifts could significantly affect individual fitness and population viability.