Winter Activity of the Western Barbastelle (Barbastella barbastellus, Chiroptera) at a Hibernation Site in the Swiss Jura

In bats of the temperate zone, survival throughout hibernation strongly depends on fine tuning the balance between the amount of fat reserves built in late summer or fall and the energy expenses. The latter are affected to a large extent by occasional arousals and active episodes. Reports of the activity of bats during the hibernation period and of its relationships with environmental conditions are therefore of utmost importance. The winter activity of the threatened western barbastelle (Barbastella barbastellus) is poorly described, but indirect observations (body mass loss, intra-cave displacements in winter) suggest that it might be frequent. We describe the phenology of acoustic activity (number of passes per night) displayed by the western barbastelle during hibernation (October–March) at Grotte aux Fées, a natural limestone cave of the Swiss Jura. Calls were recorded automatically at the cave's entrance and within the cave's main chamber with the aim to test whether the phenology of acoustic activity throughout the hibernation period would be similar at the two sites. We also test for the effect of weather variables (ambient temperature and precipitation) on acoustic activity in mid-hibernation. Acoustic activity followed a bimodal pattern at the entrance porch, with peaks corresponding to arrival at- and departure from the cave. Minimum acoustic activity was observed in January (median 2–3 passes/night), but even then, passes were recorded during most nights. Within the cave's main chamber, the phenology of acoustic activity differed from that at the entrance. Activity remained consistently high throughout the winter (median 27 passes/night in January). In January, acoustic activity was significantly and positively affected by ambient temperature, but not by precipitation. Even intra-cave activity was positively affected by external ambient temperature. At Grotte aux Fées, barbastelles deposited feces in winter and it is known that a rise of ambient temperature stimulates the activity of moths, the principal food resource of the western barbastelle. Our results suggest that B. barbastellus often has high energy expenses during hibernation and the extent to which these expenses are partly balanced by opportunistic feeding in winter awaits further study.