Abstract:
Many small mammals bask in the sun during rewarming from heterothermy, but the implications
of this behaviour for their energy balance remain little understood. Specifically, it remains
unclear whether solar radiation supplements endogenous metabolic thermogenesis
(i.e., rewarming occurs through the additive effects of internally-produced and external
heat), or whether solar radiation reduces the energy required to rewarm by substituting (i.e,
replacing) metabolic heat production. To address this question, we examined patterns of
torpor and rewarming rates in eastern rock elephant shrews (Elephantulus myurus) housed
in outdoor cages with access to either natural levels of solar radiation or levels that were experimentally
reduced by means of shade cloth. We also tested whether acclimation to solar
radiation availability was manifested via phenotypic flexibility in basal metabolic rate (BMR),
non-shivering thermogenesis (NST) capacity and/or summit metabolism (Msum). Rewarming
rates varied significantly among treatments, with elephant shrews experiencing natural
solar radiation levels rewarming faster than conspecifics experiencing solar radiation levels
equivalent to approximately 20% or 40% of natural levels. BMR differed significantly between
individuals experiencing natural levels of solar radiation and conspecifics experiencing
approximately 20% of natural levels, but no between-treatment difference was evident
for NST capacity or Msum. The positive relationship between solar radiation availability and
rewarming rate, together with the absence of acclimation in maximum non-shivering and
total heat production capacities, suggests that under the conditions of this study solar radiation
supplemented rather than substituted metabolic thermogenesis as a source of heat during
rewarming from heterothermy.
