The influence of gender on thermoregulation in pouched mice, Saccostomus campestris.

Abstract

Saccostomus campestris display sexual disparity in the use of summer daily torpor in response to energy stress. The hypothesis that males may compensate for a limited heterothermic capacity with lower normothermic body temperatures by maintaining lower resting metabolic rates relative to females was tested. Furthermore, the influence of testosterone on torpor incidence in males was investigated. Body temperature (T[b]) and oxygen consumption (VO₂) were measured at various ambient temperatures (Tₐ) and were compared between the sexes under food ad libitum and food restriction treatments. There were no significant differences in T[b] and VO₂ between sexes under food ad libitum treatment. Under food restriction there were pronounced sex differences in the employment of heterothermy. Females defended a lower setpoint T[b] for torpor (ca. 25°C), than males (ca. 29°C), and also employed torpor more frequently than males. Non-torpid males did, however show slight reductions in VO₂ under food restriction. The effect of testosterone on daily torpor was investigated by comparing minimum T[b]and torpor frequency of castrated mice implanted with testosterone-filled (experimental) and saline-filled (control) silastic capsules in response to food ad libitum and food restriction treatments. Testosterone inhibited torpor in males. The majority of control animals employed torpor under both food ad libitum and food restriction diets. It was concluded that although the animals were capable of shallow, summer torpor, it was confined to moderate ambient temperatures and was not used at low Tₐ's where several animals became pathologically hypothermic. Females derive energetic benefits from the use of torpor whereas males may partially compensate for their limited heterothennic capacity by a reduction in resting metabolic rates, accompanied by moderate reductions in body temperature during energetically stressful periods. The difference in the capacity for daily heterothenny between sexes was attributed to differences in their reproductive physiology.