Anthropogenic land use changes, such as deforestation and commercial forestry, have substantially reduced natural roost sites for European bats. A common conservation solution is to provide artificial roosts (i.e., bat boxes), but there are concerns that these can become hotter than natural roosts in summer and could be death traps during heat waves. Nevertheless, females of several bat species form maternity colonies in these boxes, thus occupying hotter and more humid microclimates than solitarily roosting males. We tested if cooling efficiency and heat tolerance differ between sexes in European bats, and estimated the evaporative water requirements of living in bat boxes during hot summer days.

We used indirect calorimetry and thermometry to quantify thermoregulation at high air temperatures (T_a) in four species of verspitilionid bats that regularly occupy artificial roosts. We measured resting metabolic heat production, evaporative water loss rates (EWL), and body temperature (T_b) at T_a between 28 °C and 48 °C during summer. We predicted that females have higher evaporative cooling capacity (evaporative heat loss/metabolic heat production) than males, allowing them to reach their heat tolerance limit at higher T_a.

We found no sex differences in maximum evaporative cooling efficiency, maximum T_b, and maximum T_a tolerated. However, the patterns of increasing EWL with T_a differed between sexes. Females tolerated higher T_a before increasing EWL than males and then rapidly increased EWL to higher values than males at the maximum T_a tolerated. These sex differences in heat‑dissipation strategies may reflect varying ecological and physiological constraints associated with different summer roosting habits.

Our study revealed that small European bat species are already at risk of succumbing to lethal dehydration during present-day heat waves. For conservation managers working with common European bat species, particularly those in monoculture forests with woodcrete bat-boxes, our physiologically-informed recommendations include positioning boxes in diverse locations varying in aspect and sun exposure. This will ensure thermal heterogeneity of roost sites and provide a wide gradient of microclimate conditions, allowing for roost switching when necessary.