Heat tolerance in desert rodents is correlated with microclimate at inter- and intraspecific levels

Physiological diversity in thermoregulatory traits has been extensively investigated in both endo- and ectothermic vertebrates, with many studies revealing that thermal physiology has evolved in response to selection arising from climate. The majority of studies have investigated how adaptative variation in thermal physiology is correlated with broad-scale climate, but the role of fine-scale microclimate remains less clear. We hypothesised that the heat tolerance limits and evaporative cooling capacity of desert rodents are correlated with microclimates within species-specific diurnal refugia. We tested predictions arising from this hypothesis by comparing thermoregulation in the heat among arboreal black-tailed tree rats (Thallomys nigricauda), Namaqua rock rats (Micaelamys namaquensis) and hairy-footed gerbils (Gerbillurus paeba). Species and populations that occupy hotter diurnal microsites tolerated air temperatures (T-a) similar to 2-4 degrees C higher compared to those species occupying cooler, more thermally buffered microsites. Inter- and intraspecific variation in heat tolerance was attributable to similar to 30% greater evaporative water loss and similar to 44 % lower resting metabolic rates at high T-a, respectively. Our results suggest that microclimates within rodent diurnal refugia are an important correlate of intra- and interspecific physiological variation and reiterate the need to incorporate fine-scale microclimatic conditions when investigating adaptative variation in thermal physiology.