Artificially raised roost temperatures lead to larger body sizes in wild bats.

Climate warming has major consequences for animal populations, as ambient temperature profoundly influences all organisms' physiology, behavior, or both. Body size in many organisms has been found to change with increased ambient temperatures due to influences on metabolism and/or access to resources. Changes in body size, in turn, can affect the dynamics and persistence of populations. Notably, in some species, body size has increased over the last decades in response to warmer temperatures. This has primarily been attributed to higher food availability, but might also result from metabolic savings in warmer environments. Bechstein's bats (Myotis bechsteinii) grow to larger body sizes in warmer summers, which affects their demography as larger females reproduce earlier at the expense of a shorter life expectancy. However, it remains unclear whether larger body sizes in warmer summers were due to thermoregulatory benefits or due to increased food availability. To disentangle these effects, we artificially heated communal day roosts of wild maternity colonies over four reproductive seasons. We used generalized mixed models to analyze these experimental results along with 25 years of long-term data comprising a total of 741 juveniles. We found that individuals raised in heated roosts grew significantly larger than those raised in unheated conditions. This suggests that metabolic savings in warmer conditions lead to increased body size, potentially resulting in the decoupling of body growth from prey availability. Our study highlights a direct mechanism by which climate change may alter fitness-relevant traits, with potentially dire consequences for population persistence.