How weather triggers the emergence of bats from their subterranean hibernacula

Abstract

Hibernation is one of the most important behaviours of bats of the temperate zone. During winter, when little food or liquid water is available, hibernation in torpor lowers metabolic costs. However, the timing of emergence from hibernation is crucial for the resumption of the reproductive process in spring. Here, we investigate the spring emergence of six bat species or pairs of bat species of the genera Myotis and Plecotus at five hibernation sites in Central Europe over 5 years. Using generalized additive Poisson models (GAPMs), we analyze the influence of weather conditions (air and soil temperature, atmospheric pressure, atmospheric pressure trend, rain, wind, and cloud cover) as predictors of bat activity and separate these extrinsic triggers from residual motivation to emerge from hibernation (extrinsic factors not studied; intrinsic motivation). Although bats in a subterranean hibernaculum are more or less cut off from the outside world, all species showed weather dependence, albeit to varying degrees, with air temperature outside the hibernaculum having a significant positive effect in all species. The residual, potentially intrinsic motivation of species to emerge from their hibernacula corresponds to their general ecological adaptation, such as trophic specialization and roosting preferences. It allows the definition of three functional groups (high, medium and low residual activity groups) according to the degree of weather dependence of spring activity. A better knowledge of the interplay of extrinsic triggers and residual motivation (e.g., internal zeitgebers) for spring emergence will help to understand the flexibility of a species to adapt to a changing world.

Bat Hibernation: In Groups or Individually?

“…Both "warm" and "cold" underground systems can be found in each of the mentioned climate zones. Weather also influences the behavior of bats hibernating in underground systems [61]. The authors show that air and soil temperature, atmospheric pressure, the atmospheric pressure trend, precipitation rate, wind speed, and cloud coverage affect the spring activity of bats in wintering sites.…”

“…Bats may form clusters or leave the hibernaculum in conditions causing excessive energy loss [6,58]. The climatic conditions of the underground are influenced by factors such as the height of the opening above sea level, the exposure of the opening, the existence of vegetation in front of the opening, the morphology of the cave (length and shape of the passages and halls), the existence of heat-transporting water streams, and heat energy from within the earth [24,[59][60][61]. Both "warm" and "cold" underground systems can be found in each of the mentioned climate zones.…”

“…They used temperature and air humidity measurements to determine the conditions of the roost site climate. Koch et al [61] claim that social thermoregulation can compensate for the unfavorable microclimatic conditions prevailing in roosts in late spring. According to our observations, bats, especially Barbastella barbastellus, hibernate both in clusters and individually throughout the hibernation period.…”