Winter activity of pipistrelle bats
The fieldwork for this study of pipistrelle bats Pipistrellus pipistrellus was carried out in the winters of 1980/81, 1981/82 and 1982/83 in East Anglia. Pipistrelle bats were observed feeding in all winter months, showing that hibernating bats can become fully active at any time during the winter. Feeding rates were measured using a bat-detector and were shown to increase with ambient temperature. A model which assumed that pipistrelles should only be active on nights when their expected feeding success is sufficiently high to exceed the extra energetic expenditure entailed by active feeding was successful in predicting the nights on which activity occurred. The frequency with which winter activity occurs suggests that pipistrelles retain considerable physiological flexibility during hibernation and that continuous torpor is not a pre-requisite of successful sperm storage. Observations were made of occasional winter activity by noctules Nyctalus noctula, indicating that winter activity may be widespread amongst hibernating bats. Weight loss between 1 October and 30 April amounted to about 25% of initial winter weight (1 October) for males and females. Bats who were lighter than average for the time of year were not less likely than average to be recaptured in subsequent years. This suggests that death from starvation may not be a major source of mortality for this species. Activity outside the roost began earlier on warm nights than on cold ones. Once the effect of temperature on the timing of emergence was discounted I found no effect of cloud cover on the timing of arrival, of bats at a feeding site. My analysis of data published by Venables (1943) confirmed the effect of temperature on emergence times but also indicated that emergence occurred earlier on cloudy nights. Captures of emerging bats indicated that males emerged earlier than females. The number of emerging bats was not significantly correlated with the time at which emergence began. Within nights there was no consistent effect of body weight on emergence order. However, across nights, emergence of light bats was earlier than that of heavy ones. Functional models of periodic arousal from hibernation were developed which investigate the theoretical ecological consequences of arousal frequency.