Ectothermic Pattern Of Body Temperature Regulation During Hibernation

Besides reproduction another important energy expenditure in endotherms is the maintenance of their relatively high body temperature, especially when ambient temperature is low. Hibernation is the most powerful means for endotherms to reduce this cost (Heldmaier et al., 2004) and is therefore seen as an important adaptation to survive predictably unfavorable periods. It is usually defined by a controlled reduction of metabolic rate down to a fraction of the euthermic level, and a substantial decrease of body temperature down to the level of ambient temperature (Lyman et al., 1982; Heldmaier, 1989; Geiser and Ruf, 1995). Temperate animals, however, cannot enjoy several months of continuous hibernation, but must awaken regularly to experience phases of euthermic body temperature for one to several days. The function of these so-called arousals remains an unsolved mystery, but they are presumably necessary for the maintenance of vital body functions during hibernation with otherwise permanently low body temperature.

The tropical C. medius show a fascinatingly different thermal behavior during hibernation compared to that of their temperate counterparts. The hibernation phase of C. medius can be divided into 5 months of deep hibernation (May -September) and 2 months of transition (April and October; Figure 2). Before the entrance into deep hibernation, lemurs leave their tree holes occasionally, and therefore do not yet have to rely exclusively on their endogenous fat reserves during this time. During the coldest hours of the night, they employ short bouts of torpor, which means that their body temperature drops to almost ambient values for some hours during the early morning, but reaches euthermic levels again the next night (Dausmann et al., 2005). Combined with their decreased locomotor activity, this allows them additional fat storage during the last weeks before hibernation. From May onward, the adult animals retreat into tree holes and engage in continuous hibernation. Juveniles remain active for a few more weeks, but also suspend their activities by the end of May (Figure 2).

The pattern of body temperature and metabolic rate during hibernation in C. medius is astonishingly flexible for a mammal, and depends on the insulation capacities of the tree hole used during hibernation (hibernaculum). The lemurs adjust their body temperature to the prevailing ambient temperature in the tree holes, and their thermal behavior resembles an ectothermic pattern, as observed in reptiles (Figure 4; Dausmann et al. 2004, 2005).

Whenever a tree hole has relatively thin walls or the sleeping chamber is very close to the entrance, the sleeping chamber is then poorly insulated against the strong daily fluctuations of ambient temperature. C. medius using these kinds of tree holes passively heat and cool during hibernation, closely tracking the daily fluctuations of ambient temperature with its diurnal increase and nocturnal fall. This results in strong daily fluctuations of body temperature of up to 20°C

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