Physiological ecology of cheirogaleid primates: variation in hibernation and torpor

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. Show more

The Madagascan fat-tailed dwarf lemur, Cheirogaleus medius, hibernates in tree holes for seven months of the year, even though winter temperatures rise to over 30 degrees C. Here we show that this tropical primate relies on a flexible thermal response that depends on the properties of its tree hole: if the hole is poorly insulated, body temperature fluctuates widely, passively following the ambient temperature; if well insulated, body temperature stays fairly constant and the animal undergoes regular spells of arousal. Our findings indicate that arousals are determined by maximum body temperatures and that hypometabolism in hibernating animals is not necessarily coupled to a low body temperature. Show more

The Malagasy primate Cheirogaleus medius hibernates in tree holes for 7 months, although ambient temperatures during hibernation rise above 30 degrees C in their natural environment. In a field study we show that during hibernation the body temperature of most lemurs fluctuates between about 10 degrees C and 30 degrees C, closely tracking the diurnal fluctuations of ambient temperature passively. These lemurs do not interrupt hibernation by spontaneous arousals, previously thought to be obligatory for all mammalian hibernators. However, some lemurs hibernate in large trees, which provide better thermal insulation. Their body temperature fluctuates only little around 25 degrees C, but they show regular arousals, as known from temperate and arctic hibernators. The results from this study demonstrate that maximum body temperature is a key factor necessitating the occurrence of arousals. Furthermore, we show that hibernation is not necessarily coupled to low body temperature and, therefore, low body temperature should no longer be included in the definition of hibernation. Show more