The effects of solar radiation on heterothermy and metabolic thermogenesis capacity in the eastern rock elephant shrew Elephantulus myurus.

Abstract

Recent studies of heterothermy in free-ranging mammals have revealed that solar radiation is an important variable influencing torpor patterns. The interaction between solar radiation (SR) and arousal costs can be thought of as a continuum from passive increases in body temperature (Tb) (rewarming at a slower rate but energetically less costly), to supplementation of endogenous heat production (rewarming more rapidly but with costs similar to that in the absence of solar radiation). To better understand the importance of solar radiation I experimentally manipulated its availability for E. myurus rewarming from torpor under natural conditions of air temperature and photoperiod. Tb was recorded for E. myurus housed in deep shade (20 % SR), partial shade (40% SR) or full sun (100% SR), and torpor frequency, rewarming rates, minimum body temperature, torpor bout duration and heterothermy index compared among treatments. Animals in unshaded cages rewarmed more rapidly than individuals in partially shaded cages. Torpor bouts were less frequent, but overall levels of heterothermy were higher in E. myurus receiving natural solar radiation compared to those in partially shaded treatments. This study, as far as I am aware, is the first demonstrating that solar heat gain, separated from the effects of an increase in ambient temperature (Ta), plays an important role in torpor arousal. Taken with the direct evidence for elephant shrews basking while rewarming, this demonstrates that animals in the unshaded treatment used solar radiation to supplement, rather than replace, endogenous metabolic thermogenesis. It is clear that E. myurus does not necessarily depend on solar radiation for rewarming, but will take advantage of this resource if available, primarily by reducing time taken to rewarm.