Abstract
Daily heterothermia is used by small mammals for energy and water savings, and seems to be preferentially exhibited during winter rather than during summer. This feature induces a trade-off between the energy saved during daily heterothermia and the energy cost of arousal, which can impact energy balance and survival under harsh environmental conditions. Especially, aging may significantly affect such trade off during cold-induced energy stress, but direct evidences are still lacking. We hypothesized that aging could alter the energetics of daily heterothermia, and that the effects could differ according to season. In the gray mouse lemur (Microcebus murinus), a non-human primate species which exhibits daily heterothermia, we investigated the effects of exposures to 25 and 12°C on body composition, energy balance, patterns of heterothermia and water turnover in adult (N = 8) and aged animals (N = 7) acclimated to winter-like or summer-like photoperiods.Acclimation to summer prevented animals from deep heterothermia, even during aging. During winter, adult animals at 12°C and aged animals at 25°C exhibited low levels of energy expenditure with minor modulations of heterothermia. The major effects of cold were observed during winter, and were particularly pronounced in aged mouse lemurs which exhibited deep heterothermia phases. Body composition was not significantly affected by age and could not explain the age-related differences in heterothermia patterns. However, aging was associated with increased levels of energy expenditure during cold exposure, in concomitance with impaired energy balance. Interestingly, increased energy expenditure and depth of heterothermia phases were strongly correlated.In conclusion, it appeared that the exhibition of shallow heterothermia allowed energy savings during winter in adult animals only. Aged animals exhibited deep heterothermia and increased levels of energy expenditure, impairing energy balance. Thus, an impaired control of the heterothermic process induced high energy costs in the aging mouse lemur exposed to cold.
Highlights
The high volume/surface ratios and low capacities for energy storage [1] observed in small mammals have been associated with the expression of daily heterothermia as this strategy efficiently reduces the costs of normothermia [for review, see 2,3]
Adult animals acclimated to winter exhibited higher Body Mass (BM) and fat mass (FM) than those acclimated to summer
Seasonal variations in BM and FM were slight in aged mouse lemurs, and cold exposure did not induce major change in body composition
Summary
The high volume/surface ratios and low capacities for energy storage [1] observed in small mammals have been associated with the expression of daily heterothermia as this strategy efficiently reduces the costs of normothermia [for review, see 2,3]. Heterothermia expression, which mainly consists in a reduction in metabolic rate [4], is useful when environmental conditions become harsh, e.g. when food is scarce or ambient temperatures (Tas) are low This process can be seen as an evolved strategy that allows animals to increase their fitness in changing environments with limited resources. It is likely that the trade-off between the energy saved during daily heterothermia and the energetic cost of arousal impacts energy balance In this manner, we recently observed from core temperature (Tc) recording and body mass changes indirect evidences that aging may significantly affect such trade-off during cold-induced energy stress in the gray mouse lemur [7].
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