Evolutionary History and Climatic Correlates of Hypermelanism in Viperidae

Body colorations have been investigated intensely concerning their adaptive significance from the ecological and evolutionary perspectives. Studies on melanism have caught growing interest thanks to its marked variability across space, time and taxon and, in ectotherms, it has been hypothesised to be driven by thermal advantages. Among reptiles, vipers show conspicuous inter- and intraspecific patterns of variation, making them excellent models to address evolutionary and adaptive patterns. We investigated the thermal melanism hypothesis across Viperidae by performing a phylogenetic comparative approach to assess whether its occurrence is phylogenetically driven or, alternatively, whether it is influenced by climate. Phylogenetic signal was detected and reconstructed the ‘non-melanistic’ form as the ancestral state at the root of their phylogeny, whereas a climatic effect was found so that melanism is more frequent in colder environments. With this work, we provide on a large geographical scale strong support for the putative advantages provided by melanism in colder climates; moreover, melanism appears to have evolved in multiple events throughout the diversification of vipers, but it has been rarely maintained over time. We hypothesise that it is maintained only when environmental conditions, such as low thermal regimes, render it favourable; nevertheless, experimental evidence is necessary to further support this hypothesis.