Abstract
Aim Anurans' sensitivity to hydric stress has driven adaptive changes in multiple traits, potentially displaying geographical clines, as is the case with body size. A longstanding body of research suggests that skin resistance to water loss may also vary geographically in response to water loss. We build upon biophysical first principles to test for the existence of this yet elusive global‐level cline in skin resistance to dehydration that alternates with body size across the aridity gradient. Location Worldwide. Time Period Current. Major Taxa Studied Amphibian anurans. Methods We combined mechanistic biophysical modelling with a phylogenetic comparative approach within a model‐selection framework. We estimated species' sensitivity to variations in body mass and resistance to dehydration using biophysical equations for species worldwide. We then integrated these sensitivities into a weighted phylogenetic comparative approach with model selection to test the effect of the aridity index on trait variation. Results We provide the first evidence of a global‐level geographic gradient in skin resistance. Skin resistance varies with aridity gradients and alternates with body size along this gradient, as predicted by theory. These clines interact with microhabitat use, with clines in skin resistance predominating in ground‐dwelling species with low skin resistance. In contrast, body size variation is more pronounced in arboreal, small‐bodied species with higher skin resistance. Main Conclusions Our findings highlight how multiple functional traits can follow distinct adaptive routes under shared environmental pressures, thereby accounting for the lack of universality of ecogeographical rules. This study underscores the value of mechanistic models in generating and testing hypotheses about trait evolution and ecogeographical rules.