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First published online August 25, 2003

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The Journal of Experimental Biology 206, 3381-3390 (2003)
doi: 10.1242/jeb.00574

Sex differences in the thermoregulation and evaporative water loss of a heterothermic bat, Lasiurus cinereus, during its spring migration

Paul M. Cryan^* and Blair O. Wolf

UNM Department of Biology, MCS03 2020, 1 University of New Mexico, Albuquerque, New Mexico 87131-0001 USA

^* Author for correspondence (e-mail: paulcryan{at}lycos.com)

Accepted 27 June 2003

This study quantifies sex differences in thermoregulation and water loss of a small (20-35 g) insectivorous heterothermic mammal, the hoary bat Lasiurus cinereus, during its spring migration. We measured body temperature, metabolic rate and evaporative water loss, and calculated wet thermal conductance, for bats exposed to air temperatures ranging from 0 to 40°C for periods of 2-5 h. Pregnant females maintained normothermic body temperatures (35.7±0.7°C; mean ± S.E.M.) independent of air temperature. In contrast, males became torpid during the majority (68%) of exposures to air temperatures <25°C. The thermal neutral zone (TNZ) ranged between approximately 30°C and 34°C in both sexes and, within the TNZ, females had lower mass-specific metabolic rates (6.1±0.2 mW g^-1) than males (9.0±0.9 mW g^-1). Wet thermal conductance values in torpid bats (0.7±0.5 mW g^-1 deg.^-1) were lower than those of normothermic individuals (1.1±0.3 mW g^-1 deg.^-1). Mass-specific rates of evaporative water loss in males were consistently higher than in females at most air temperatures and rates of water loss in torpid bats were 63±6% of normothermic values. These results suggest that male and pregnant female L. cinereus employ different thermoregulatory strategies during their spring migration. Females defend normothermic body temperatures, presumably to expedite embryonic growth, while males use torpor, presumably to minimize energy and water deficits. These variable thermoregulatory strategies may reflect continental differences in the summer distribution of the sexes.

Key words: Chiroptera, thermoregulation, energetics, body temperature, metabolic rate, conductance, evaporative water loss, torpor, sex difference, hoary bat, Lasiurus cinereus

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