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

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 (T-b) (33 to 37 degrees C) over a wide range of ambient temperatures (T(a)s) and a relatively low basal metabolic rate (1.29 mi O-2 g(-1) h(-1)). Bats entered torpor frequently in the laboratory at T(a)s between 14 and 25 degrees C. Entry into torpor always occurred when lights were switched on in the morning, independent of T-a. MRs during torpor were reduced to about 20-40% of normothermic bats and T(b)s were regulated at a minimum of 23.1 +/- 1.4 degrees C. The duration of torpor bouts increased with decreasing T-a in non-thermoregulating bats, but generally terminated after Sh in thermoregulating torpid bats. Both the mean minimum T-b and MR of torpid M. minimus were higher than that predicted for a 16-g daily heterotherm and the T-b 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 T-a 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.