Effects of Sex and Age on Heterothermy in Goodman’s Mouse Lemur (Microcebus lehilahytsara)

Daily torpor and hibernation are the most powerful measures of endotherms to reduce their energy expenditure. During entrance into these torpid states metabolic rate is suppressed to a fraction of euthermic metabolism, paralleled by reductions in ventilation and heart rate. Body temperature gradually decreases towards the level of ambient temperature. In deep torpor body temperature as well as metabolic rate are controlled at a hypothermic and hypometabolic level. Torpid states are terminated by an arousal where metabolic rate spontaneously returns to normal levels again and euthermic body temperature is established by a burst of heat production. In recent years some of the cellular mechanisms which contribute to hypometabolism have been disclosed. Transcription, translation, as well as protein synthesis are largely suppressed. Cell proliferation in highly proliferating epithelia like the intestine is suspended. ATP production from glucose is reduced and lipids serve as the major substrate for remaining energy requirements. All these changes are rapidly reverted to normometabolism during arousal. Hibernation and daily torpor are found in small mammals inhabiting temperate as well as tropical climates. It indicates that this behaviour is not primarily aimed for cold defense, instead points to a general role of hypometabolism, as a measure to cope with a timely limited or seasonal bottleneck of energy supply.

In many long-lived vertebrates (including humans), adult males have shorter lifespans than adult females, partly as a result of higher annual rates of mortality in males and partly owing to sex differences in the rate of ageing. A probable explanation of the evolution of sex differences in ageing is that, in polygynous species, intense intrasexual competition between males restricts the number of seasons for which individual males are able to breed successfully, weakening selection pressures favouring adult longevity in males relative to females. If this is the case, sex differences in adult longevity and in the onset and rate of senescence should be greater in polygynous species than in monogamous ones and their magnitude should be related to the duration of effective breeding males compared with females. Here, we use data from longitudinal studies of vertebrates to show that reduced longevity in adult males (relative to females) is commonly associated with a more rapid decline in male than female survival with increasing age and is largely confined to polygynous species. The magnitude of sex differences in adult longevity in different species is consistently related to the magnitude of sex differences in the duration of effective breeding, calculated across surviving adults. Our results are consistent with the suggestion that sex differences in senescence in polygynous species are a consequence of weaker selection for longevity in males than females.