The effect of torpor bout duration and ambient temperature on arousal from hibernation in wild common noctules (Nyctalus noctula)

As winters become increasingly milder, we expect rising winter temperatures to affect hibernating animals. However, it is hard to predict the effects of temperature increases on the activity of certain hibernators. In most species, hibernation is periodically interrupted by short instances of rewarming and euthermia known as “arousals”, which are energetically costly but mandatory. These arousals are influenced by ambient factors such as temperature, but the mechanisms that trigger them are poorly understood. That is especially true for the common noctule — a bat that hibernates in colonies in tree cavities, rock fissures and man-made structures. Due to the noctule’s complex hibernation ecology, its regulation of torpor and arousal is still poorly understood. In the first chapter of my PhD, we investigated the effect of temperature on the likelihood of arousing from torpor in wild common noctules. To do this, we captured 73 noctules from roosts in Switzerland and equipped them with bio-loggers, recording hourly body acceleration and body tempearature. We hypothesized that noctules are increasingly more likely to arouse the more they spend time in torpor, and that they will reduce the energetic cost of arousals by arousing with high ambient temperatures. Hence, we expected high temperatures to increase arousal probability — but not enough to force arousals when torpor bout duration is low. This hypothesis seems supported by the analyses done so far. Our results may allow us to predict the winter activity of noctules according to environmental conditions: this is important to predict how current and future changes in climate are affecting noctule energy expenditure during hibernation.