Deep Torpor Patterns and Body Mass Loss of Above-Ground Hibernating Bats (Nyctalus noctula) in Captivity ― Effects of Sex, Age, and Grouping

(Uploaded by Plazi for the Bat Literature Project) Hibernation patterns of European bat species, especially those that hibernate above ground, remain under-explored. Our study focused on Nyctalus noctula, a common species among above-ground hibernating bats. We conducted the study on 22 individuals, manipulating group size and sex-age ratio in a captive setting, and recorded skin temperature data using temperature loggers. Bats hibernating in groups awoke less frequently and had more prolonged torpor bouts than solitary hibernating bats. The latter cooled faster during periods of arousal, but their time in the warming and euthermic stages was similar to that of individuals hibernating in groups. A general linear model (GLM) analysis revealed that all the factors we examined, including ambient temperature, sex, age, group size, and body mass, significantly affected the likelihood of a bat being in a state of hibernation or arousal. Young females with high body mass and hibernating in a group had a notably higher probability of arousal. Bats in groups maintained skin temperature (Ts) above 0ºC, while it dropped around 0ºC in some solitary hibernating bats. However, median Ts was similar in all bat groups. Body mass was higher in females compared to males and in adult bats compared to sub-adults. We hypothesized that adult males have greater fat reserves and can afford a longer total arousal time to effectively seek a mate and mating compared to sub-adult males with low body mass. Heavier females used more fat for more frequent arousals to reduce the negative metabolic consequences of hibernation for fast conception and successful pregnancy in spring. At the same time, sub-adult males entered the wintering period with lower body mass and balanced the use of energy resources for survival and mating. It seems that N. noctula, as an above-grounded hibernator, benefits more from group hibernation than solitary hibernation.