Macroevolution of body extremities reveals an integrated phenotypic response of coloration and morphology to temperature in a large clade of Neotropical passerines (Furnariida)

ABSTRACT Endotherms show smaller extremities (Allen’s rule) and larger body sizes (Bergmann’s rule) in colder climates, thereby reducing heat loss. Coloration can also be important in thermoregulation, as darker coloration absorbs more radiant energy than lighter coloration (Bogert’s rule or thermal melanism hypothesis). Extremities lacking or bearing thinner covers of feathers or fur—such as ears, legs and bills—can be important thermal windows to exchange heat. Yet, no study to date has simultaneously investigated the coloration and morphology of body extremities in relation to temperature. In 566 species of Furnariida (Neotropical ovenbirds and allies), we tested with phylogenetic comparative methods whether body extremity coloration, morphology and plumage coloration jointly respond to temperature. We predicted that, in colder climates, bills and tarsi would be darker and smaller as well as associated with darker plumage and larger body sizes. Consistent with this prediction, we found that bills are darker, smaller and are associated with darker plumage and larger body sizes in colder climates. Tarsi are darker and associated with darker plumage and larger body sizes in colder climates, but tarsus coloration is not associated with tarsus length, which, contrary to predictions of Allen’s rule, was longer in colder climates. Using phylogenetic path analyses, we found that climate affects body extremity coloration both directly and indirectly through its effects on habitat type, plumage coloration, and morphology. Our study suggests that temperature promotes an integrated phenotypic response of coloration and morphology across body extremities, plumage, and body size.