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.