Short-term versus seasonal temperature variation in bat hibernacula

Microsite selection in relation to thermal variability has been addressed in several studies. However, the distinction between long-term variability and short-term variability in temperature has rarely been made. Most studies assume that a higher seasonal variability, giving rise to lower mid-winter temperatures, implies a higher short-term variability.
We studied thermal variability in hibernacula used by a population of Barbastella barbastellus in Western Poland. Seasonal variation is higher in bunkers and thus temperatures get colder in winter than in the underground system. On the other hand, short-term variability in the bunkers was lower than in the underground system. This makes bunkers a more stable environment to hibernate for cold dwelling bats in warm winters, when temperatures in the bunkers do not get below freezing. During the last decade, a continuous series of warm winters occurred and the population of barbastelle bats partly moved from the underground system to the bunkers. The present temperature increases broadened the range of potential hibernation sites for barbastelles.
We further investigated the relationship between seasonal and short-term variation for different hibernation systems based on physical models, and showed that the relationship between seasonal and long term variability depends on the interplay between conductive heat transport through ground and walls, and heat transfer by air flow.
We combine theoretical insights gained by physical modeling with the results of the Barbastelle study to discuss the importance of short-term and seasonal dynamics in studying microsite selection of hibernating bats, with reference to the effects of climate change.