Energy metabolism and evaporative water loss in the European free-tailed bat and Hemprich's long-eared bat (Microchiroptera): species sympatric in the Negev Desert.

We compared the thermoregulatory abilities of two insectivorous bat species, Tadarida teniotis (mean body mass 32 g) and Otonycteris hemprichii (mean body mass 25 g), that are of different phylogenetic origins and zoogeographic distributions but are sympatric in the Negev Desert. At night, both were normothermic. By day, both were torpid when exposed to ambient temperatures (T(a)) below 25 degrees Celsius, with concomitant adjustments in metabolic rate (MR). Otonycteris hemprichii entered torpor at higher T(a) than T. teniotis, and, when torpid, their body temperatures (T(b)) were 1 degrees -2 degrees Celsius and 5 degrees -8 degrees Celsius above T(a), respectively; MR was correspondingly reduced. At night, the lower critical temperature of T. teniotis was 31.5 degrees Celsius, and that of O. hemprichii was 33 degrees Celsius. Mean nocturnal thermoneutral MR of T. teniotis was 37% greater than that of O. hemprichii. At high T(a), evaporative water loss (EWL) increased markedly in both species, but it was significantly higher in T. teniotis above 38 degrees Celsius. In both species, the dry heat transfer coefficient (thermal conductance) followed the expected pattern for small mammals, by day and by night. Total EWL was notably low in normothermic and torpid animals of both species, much lower than values reported for other bats, indicating efficient water conservation mechanisms in the study species. Comparing thermoregulatory abilities suggests that O. hemprichii is better adapted to hot, arid environments than T. teniotis, which may explain its wider desert distribution. By both standard and phylogenetically informed ANCOVA, we found no differences in basal metabolic rate (BMR) between desert and nondesert species of insectivorous bats, substantiating previous studies suggesting that low BMR is a characteristic common to insectivorous bats in general.