Daily torpor and energetics in a tropical mammal, the northern blossom-bat Macroglossus minimus (Megachiroptera)

W. Bartels, B. S. Law, F. Geiser

Little is known about torpor in the tropics or torpor in megachiropteran species. We investigated thermoregulation, energetics and patterns of torpor in the northern blossom-bat Macroglossus minimus (16 g) to test whether physiological variables may explain why its range is limited to tropical regions. Normothermic bats showed a large variation in body temperature (Tb) (33 to 37 degrees C) over a wide range of ambient temperatures (Tas) and a relatively low basal metabolic rate (1.29 ml O2 g-1 h-1). Bats entered torpor frequently in the laboratory at Tas between 14 and 25 degrees C. Entry into torpor always occurred when lights were switched on in the morning, independent of Ta. MRs during torpor were reduced to about 20-40% of normothermic bats and Tbs were regulated at a minimum of 23.1 +/- 1.4 degrees C. The duration of torpor bouts increased with decreasing Ta in non-thermoregulating bats, but generally terminated after 8 h in thermoregulating torpid bats. Both the mean minimum Tb and MR of torpid M. minimus were higher than that predicted for a 16-g daily heterotherm and the Tb was also about 5 degrees C higher than that of the common blossom-bat Syconycteris australis, which has a more subtropical distribution. These observations suggest that variables associated with torpor are affected by Ta and that the restriction to tropical areas in M. minimus to some extent may be due to their ability to enter only very shallow daily torpor.