Testing the thermal melanism hypothesis: a macrophysiological approach

Abstract

Summary 1. The thermal melanism hypothesis (TMH) predicts that dark (low skin reflectance) individuals are at an advantage in cool climates as they heat faster and reach higher equilibrium temperatures than lighter (higher reflectance) individuals. However, tests of the TMH have yielded mixed support, especially in ectothermic vertebrates. 2. Most studies to date have been undertaken at small spatial scales or using a few, closely related populations or species. Here, we therefore examine the TMH at large scales in heliothermic lizard species, testing two of its major predictions and a corollary thereof, using standard and phylogenetically corrected analyses. 3. First, we test the prediction that skin reflectance and climate variables such as mean annual temperature (MAT) and global solar radiation are positively related across species. Second, we determine whether a positive relationship exists between skin reflectance and body mass. Third, since physiology, behaviour and morphology should be co-adapted, we test the prediction that skin reflectance and traits of thermal biology are positively related. 4. We find strong support for a positive relationship between skin reflectance and mean annual radiation even after adjusting for phylogeny. Moreover, radiation was a better predictor of skin reflectance than MAT. We also find support for a positive relationship of skin reflectance with body size, although this was non-significant after accounting for phylogeny. 5. Skin reflectance was not related to measures of thermal biology, although confounding effects such as methodological differences could not be ruled out. 6. In summary, this study provides novel support for the TMH operating interspecifically at large geographic scales, suggesting adaptive variation of skin reflectance among lizards.