The pattern of metabolic rate follows the pattern of body temperature. Therefore, as is the case for body temperature, the pattern of metabolic rate and the extent of its daily fluctuations are exogenously determined by the pattern of hibernacu-lum temperature and thus the properties of the tree hole used as hibernaculum. However, in lemurs that hibernate in well-insulated tree holes, the high costs of increased metabolic rate during the regular arousals seem to be balanced-out by an otherwise steady, relatively low metabolic rate. Surprisingly, therefore, the choice of hibernaculum seems to be of relatively little energetic relevance, despite the great implications with respect to the pattern of thermoregulation for the lemurs.
Due to the overall higher levels of body temperature in the tropical hibernator C. medius, mass specific metabolic rate is about tenfold higher than that of temperate hibernators, which hibernate at body temperature close to the freezing point. This corresponds to the finding that overall energetic savings of tropical hibernation do not reach the high levels of temperate hibernation (over 90%), but rather lie within the range of temperate daily torpor. Nevertheless, overall energetic savings of tropical hibernation in C. medius amounts to about 70% (Dausmann et al., submitted).
Temperate hibernators increase their body weight by about 40-50% before hibernation (Humphries et al., 2003), compared to about 90% in C. medius (Fietz and Ganzhorn, 1999). Based on the measurement of metabolic rates, 0.77 g body lipids are consumed by C. medius on average per day during the hibernation period. Given 5 months of deep hibernation (a 30 days), this means that they should have at least 115.5 g lipid at their disposal during hibernation. This corresponds to the observed increase in body mass before hibernation of about 120 g. However, it also shows that the energy supply of C. medius is fairly limited, clearly restricting the extension of the hibernation phase (Dausmann et al., submitted).
Besides the amount of fat accumulated before hibernation, the composition of the fatty acids seems to be equally important to the hibernation ability and quality in temperate hibernators (Frank, 1991; Geiser, 1993). During the prehiber-nation fattening period, temperate hibernators selectively store polyunsaturated, essential fatty acids in their white adipose tissue (Armitage, 1979; Geiser and Kenagy, 1987). This is thought to represent an adaptation to low body temperature during hibernation (Frank, 1991; Geiser, 1993). Considering the relatively high and diurnally fluctuating body temperature of C. medius during tropical hibernation, polyunsaturated fatty acids would not be expected to play a key role under these conditions. Indeed, C. medius have an unusually low content (<3%) of essential fatty acids in their white adipose tissue compared to 35-45% found in temperate hibernators (Geiser and Kenagy, 1987) before the onset of hibernation (Fietz et al., 2003).
The changes in thermoregulation and energy expenditure compared to the activity period show that tropical hibernation in C. medius is an important, wellregulated adaptive response to survive the unfavorable dry season.
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