Torpor and thermal energetics in a tiny Australian vespertilionid, the little forest bat (Vespadelus vulturnus)

Craig K. R. Willis, Christopher Turbill, Fritz Geiser

Data on thermal energetics for vespertilionid bats are under-represented in the literature relative to their abundance, as are data for bats of very small body mass. Therefore, we studied torpor use and thermal energetics in one of the smallest (4 g) Australian vespertilionids, Vespadelus vulturnus. We used open-flow respirometry to quantify temporal patterns of torpor use, upper and lower critical temperatures (T (uc) and T (lc)) of the thermoneutral zone (TNZ), basal metabolic rate (BMR), resting metabolic rate (RMR), torpid metabolic rate (TMR), and wet thermal conductance (C (wet)) over a range of ambient temperatures (T (a)). We also measured body temperature (T (b)) during torpor and normothermia. Bats showed a high proclivity for torpor and typically aroused only for brief periods. The TNZ ranged from 27.6 degrees C to 33.3 degrees C. Within the TNZ T (b) was 33.3+/-0.4 degrees C and BMR was 1.02+/-0.29 ml O(2) g(-1) h(-1) (5.60+/-1.65 mW g(-1)) at a mean body mass of 4.0+/-0.69 g, which is 55 % of that predicted for a 4 g bat. Minimum TMR of torpid bats was 0.014+/-0.006 ml O(2) g(-1) h(-1) (0.079+/-0.032 mW g(-1)) at T (a)=4.6+/-0.4 degrees C and T (b)=7.5+/-1.9. T (lc) and C (wet) of normothermic bats were both lower than that predicted for a 4 g bat, which indicates that V. vulturnus is adapted to minimising heat loss at low T (a). Our findings support the hypothesis that vespertilionid bats have evolved energy-conserving physiological traits, such as low BMR and proclivity for torpor.