Hibernal Ecology of the Pipistrelle Bat: Energy Expenditure, Water Requirements and Mass Loss, Implications for Survival and the Function of Winter Emergence Flights

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Abstract
Body mass changes, energy expenditure and water requirements of captive colonies of twelve hibernating female pipistrelle of the mass at the start of each period. Mass losses greater than 1 g in bats with an initial mass less than 5.5 g were fatal. A critical mass of 4.0 g was identified at, or below, which female bats had to increase mass to survive. Food intake was inversely related to mean mass change. By extrapolation, the daily energy requirement when relying exclusively on body reserves, amounted to 0.93 kJ day-1. When all the energy was obtained from food, the requirement was 5.5 kJ day-1. Direct estimates of the energy demand of bats in torpor at 4.degree. C amounted to 0.45 KJ day-1. The difference in these estimates reflects differing times spent in torpor. By inference, bats relying exclusively on their own reserves spent only 5-20 min of each day out of topor compared with over 2 h for bats relying on ingested food. Free-water requirements averaged 0.19 ml day-1. Intake of free water was independent of food intake even though the food consisted of 59% water. Combining data on sustainable mass losses, mass change when relying on body reserves alone, water requirements and the body water pool size, survival times in the absence of food and water were calculated. Survival would be longer at greater body mass. Female bats weighing more than 4.2 g would die of dehydration sooner than of starvation whilst the converse is true at lower masses. For the whole of hibernation, in the wild, female pipistrelle bats exceed this mass. We suggest, therefore, that the primary bats were measured over 8-day periods between December and March in two winters. Mass change was independent function of winter emergence in this species is to drink. This hypothesis conflicts with previous studies which suggest hibernating bats emerge to feed. A review of the available evidence supports our interpretation better than an ''energy maximization'' hypothesis.