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Figure 4. Patterns of body temperature during hibernation in Cheirogaleus medius (A) in a poorly insulated tree hole. Body temperature passively follows tree hole temperature in amplitude and slope and the lemur does not show arousals. (B) In a moderately insulated tree hole. Body temperature fluctuates with tree hole temperature, but is sometimes actively increased to euthermic levels. (C) In a well-insulated tree hole. Body temperature fluctuates little and the lemur shows regular arousals with euthermic body temperature. Inserted numbers give the daily maximum of body temperature (A, B) and the maximum body temperature during arousals (C). Vertical lines indicate midnight. Black horizontal bars show the dark phase. Black line: body temperature; gray line: tree hole temperature; dotted line: ambient temperature. Modified from Dausmann et al. (2004, 2005).

between about 10°C and 30°C (Figure 4A). Lemurs using these kinds of tree holes as hibernaculum do not interrupt hibernation by spontaneous arousals, as temperate hibernators have to, but show this pattern of passively fluctuating body temperature over many weeks or even months. However, the observations of predator attacks and other disturbances have proven that, contrary to reptiles, the ability for thermoregulation persists during hibernation. The lemurs are therefore at all times able to actuate the endogenous regulation of body temperature whenever they need to.

Some lemurs hibernate in large trees with thick walls, which provide better thermal insulation. Their body temperature fluctuates only by a few degrees (around 25°C), but contrary to the body temperature profile described above they exhibit regular arousals, similar to temperate hibernators (Figure 4C).

All transitions between completely passively fluctuating body temperature with high daily amplitude, and barely fluctuating body temperature with regular wake-up phases can occur during hibernation of C. medius (Figure 4B; Dausmann et al., 2004, 2005). Considering how meticulously body temperature is usually adjusted within a narrow range in mammals and birds, it is truly astounding how C. medius tolerate high daily fluctuations of body temperature and various patterns of thermoregulation, that can be changed from one day to the next.

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