Summit metabolism and metabolic expansibility in Wahlberg's epauletted fruit bats (Epomophorus wahlbergi): seasonal acclimatisation and effects of captivity

Summit metabolism (M-sum), the maximum rate of resting metabolic thermogenesis, has been found to be broadly correlated with climatic variables and the use of heterothermy in some endotherms. Far less is known about Msum and metabolic expansibility [ME, the ratio of Msum to basal metabolic rate (BMR)] in bats compared with many other endotherm taxa. We measured BMR and Msum during winter and summer in captive and wild populations of a pteropodid from the southern subtropics, Wahlberg's epauletted fruit bat (Epomophorus wahlbergi) in Pretoria, South Africa. The Msum of fruit bats ranged from 5.178 +/- 0.611 W (captive, summer) to 6.006 +/- 0.890 W (captive, winter), and did not vary significantly between seasons. In contrast, BMR decreased by 17-25% in winter. The combination of seasonally stable Msum but flexible BMR resulted in ME being significantly higher in winter than in summer, ranging from 7.24 +/- 1.49 (wild, summer) to 13.11 +/- 2.14 (captive, winter). The latter value is well above the typical mammalian range. Moreover, both Msum and ME were significantly higher in captive bats than in wild individuals; we speculate this represents a phenotypic response to a reduction in exercise-associated heat production while in captivity. Our data for E. wahlbergi, combined with those currently available for other chiropterans, reveal that Msum in bats is highly variable compared with allometrically expected values for other mammals.