Nightly clustering in communal roosts and the regular presence of adult females at night provide thermal benefits for juvenile Bechstein's bats

Thermal benefits from clustering during the reproductive period are assumed to explain the almost ubiquitous group living behaviour in female temperate zone bats. While there is evidence for such benefits in adult female bats in the field, to our knowledge no studies have quantified the thermal benefits of clustering in juvenile, non-volant bats. Using thermography, we assessed in the field (1) whether juvenile Bechstein's bats thermally benefitted from clustering during the absence of adult females at night; and (2) whether the regular nightly visits of adult females led to an increase in juveniles’ skin temperatures. We show that the juveniles did indeed thermally benefit from clustering when their mothers were absent. With increasing age, clustering juveniles experienced lower heat loss during the absence of adult females. Also the influence of roost temperatures on the juveniles’ skin temperature decreased when they became older, indicating improved thermoregulation abilities. We also observed a small, although significant increase in the juveniles’ surface skin temperature after visits by adult females. In conclusion, our study provides evidence that clustering at night is advantageous for non-volant juvenile bats, because it improves their thermoregulation at times when mothers are not present in the communal roost.