The interplay of temperature and circadian periodicity in winter activity of non-cavernous hibernator, Nyctalus noctula

Winter activity of hibernating mammals is likely to be influenced by climate change. Our study focuses on Nyctalus noctula, a non-cavernous hibernator using artificial roosts in a recently colonized winter region. Using continuous acoustic monitoring and temperature measurements inside and outside the roosts, we found that bats exhibit a circadian cycle (active at night, resting during the day) even during hibernation season. Activity duration and intensity changed in response to ambient temperature, photoperiod, and hibernation progression. Warm ambient temperatures led to increased nighttime activity, extending the duration of the active phase. As photoperiod increased, the rest phase lengthened, while the overall magnitude of activity decreased from the beginning to the end of the hibernation period. Below 0 °C vocal activity was nearly zero indicating a minimal probability of bat activity during both day and night. The species recent success in extending its hibernation range northward may be attributed to its flexible adjustment to prevailing environmental conditions. Nevertheless, it remains uncertain whether engaging in daily activity at temperatures above 0 °C confers any advantages at northern latitudes to prevent premature energy depletion. The persistence of circadian activity during winter could be a relic behavior, adapted from historical patterns of wintering in insect-rich and warm southern latitudes.