Differences in the physiological responses to temperature among stonechats from three populations reared in a common environment

The physiological response to variation in air temperature (Ta) can provide insights into how animals are adapted to different environments. I measured metabolic rate, total evaporative water loss (TEWL) and body temperature (Tb) as a function of Ta in stonechats from equatorial Kenya, temperate central Europe and continental Kazakhstan, environments where stonechats have evolved different life histories. All birds were raised and kept under identical captive conditions to highlight genetically based differences and to exclude phenotypic plasticity as explanatory factor. The slope relating metabolic rate to Ta was steepest in Kazakh stonechats and lowest for birds from Kenya, indicating that, counterintuitively, the tropical stonechats were best insulated. Taking into account variation in Tb in response to Ta, the lower critical temperature for the three populations fell between 32.0 and 34.9 °C, values higher than previously assumed. Whole organism BMR did not differ among populations, but because body mass was significantly higher in the Kenyan stonechats, their mass-specific BMR was lower compared with conspecifics from higher latitude. Whole organism or mass-specific TEWL did not differ among populations. Possibly, Kenyan birds are better insulated to compensate for their limited capacity to elevate metabolic rate.