Field metabolic rates and water uptake in the blossom-bat Syconycteris australis (Megachiroptera).

Blossom-bats, Syconycteris australis (18 g) are known to be highly active throughout the night. Since this species frequently enters torpor, we postulated that their use of heterothermy may be related to a high energy expenditure in the field. To test this hypothesis we measured field metabolic rates (FMR) of S. australis at a subtropical site using the doubly labelled water (DLW) method. We also measured DLW turnover in captive animals held at constant ambient temperature (Ta) with ad libitum food to estimate whether Ta and food availability affect energy expenditure under natural conditions. The FMR of S. australis was 8.55 ml CO2 g-1 h-1 or 76.87 kJ day-1 which is 7.04 times the basal metabolic rate (BMR) and one of the highest values reported for endotherms to date. Mass-specific energy expenditure by bats in the laboratory was about two-thirds of that of bats in the field, but some of this difference was explained by the greater body mass in captive bats. This suggests that foraging times in the field and laboratory were similar, and daily energy expenditure was not strongly affected by Ta or ad libitum food. Water uptake in the field was significantly higher than in the laboratory, most likely because nectar contained more water than the laboratory diet. Our study shows that S. australis has a FMR that is about double that predicted for its size although its BMR is lower than predicted. This supports the view that caution must be used in making assumptions from measurements of BMR in the laboratory about energy and other biological requirements in free-ranging animals.