Janis Wolf

• Throughout evolutionary history, species have always been faced with changing environments, but the unprecedented rate of change induced by current human-induced climate change poses significant challenges to many recent organisms. Besides altering large-scale weather patterns and increasing the frequency and intensity of extreme weather events, climate change is characterised by a significant rise in global temperatures. Projections indicate an increase of 1.5°C to 5°C by the end of the century, largely depending on the extent of greenhouse gas emissions. As temperature is intimately linked to nearly all biological processes, global warming may impact many aspects of animal physiology, including thermoregulation, reproduction, and morphology. In this doctoral thesis, I investigate how global warming affects the physiology of Bechstein’s bats (Myotis bechsteinii), a typical European forest-dwelling species of high conservation concern. I focus particularly on the mechanisms underlying a previously observed increase in body size among individuals born during warmer summers. Using individualised long-term data on morphological and reproductive traits from four wild bat colonies, combined with two field experiments, together with my co-authors, I examined the effects of temperature on body size and metabolism across multiple reproductive seasons. In a first experiment, we artificially raised and maintained roost temperatures at temperatures within the thermoneutral zone of the bats, simulating optimal thermal conditions for growth. Directly heating the bat boxes that were used as day roosts during the maternity season, allowed us to isolate the effects of temperature on body size from potential confounding factors such as insect availability. Our findings reveal that body size in Bechstein’s bats is directly and positively related to warmer roost temperature, likely due to reduced thermoregulatory costs, and thus, increased energy availability for growth. In other words, bats in heated roosts should have exhibited greater thermoregulatory efficiency, requiring less energy to maintain normothermia, which allowed more energy to be allocated toward juvenile growth. In a second experiment, we aimed to unveil the underlying mechanisms that drive the established direct effect of temperature on body size in M. bechsteinii. In order to do so, we used field respirometry to assess the metabolic response of communally roosting Bechstein’s bats to roost temperatures during two lactation periods. Notably, average daily metabolic rates were not different on heated than on unheated days, suggesting that the saved energy from thermoregulating more efficiently is directly reinvested in other aspects, such as growth and/or maternal care. Furthermore, the results highlight the capacity of Bechstein’s bats to cope with cooler temperatures through metabolic and behavioural mechanisms such as torpor, digestion-induced thermogenesis and social thermoregulation. However, as temperature exceeds the thermoneutral zone, the bats’ metabolic rate increased sharply, suggesting that individuals may struggle to cope with extended periods of extreme heat. Prolonged exposure to high temperatures could significantly elevate energy expenditure and lead to dehydration, jeopardising bat populations in an ever warming climate. Thermal imaging data also indicated minimal torpor use during lactation in heated colonies, with only two instances of torpor observed during the whole lactation period in one colony. A reduction in torpor use may alleviate the negative effects of torpor on milk production and growth processes and thus additionally promote offspring development. In a third analysis, we leveraged 14 years of individualised data to test whether heating had an effect on body condition in reproductive females, additionally comparing them to non-reproductive females. Interestingly, despite the thermoregulatory benefits, mothers from heated colonies did not show improved body condition compared to those from unheated colonies, suggesting that the energy saved in warmer conditions is instead allocated to maternal care, further enhancing juvenile growth. We additionally show that body condition in spring is higher in those females that had offspring the subsequent summer than non-reproductive females, suggesting that resources left after hibernation may act as capital for the decision whether or not to reproduce in a given year. In conclusion, the studies I present in this thesis provide key insights into the mechanisms behind the temperature-dependent increase in body size in Bechstein’s bats and, more broadly, how temperature influences their physiology, growth, and reproduction. These findings highlight the complex trade-offs between the apparent immediate benefits of warmer conditions for growth and the potential associated long-term negative effects posed by global warming. Given the ongoing climate crisis, understanding these physiological responses is crucial for the conservation of temperate-zone bat species such as Bechstein’s bats.