A comparative phylogenetic approach for the evolution of melanism in elapid snakes supports the Bogert's rule

Melanism is a common trait in vertebrates. While in endotherms, melanism has been primarily associated with the protection against oxidative stress caused by incoming UV radiation, in ectotherms, it is far less understood. Bogert's rule, the most widely supported biogeographical framework explaining the occurrence of melanism in reptiles, states that melanistic individuals should be favoured thanks to faster heating rates. It has led to the formulation of the Thermal Melanism Hypothesis to explain the evolution and maintenance of melanism in ectotherms. Although some support for this hypothesis exists, it is not broadly accepted, and the importance of the thermal advantage as an evolutionary driver of melanism in ectotherms needs to be specifically addressed. Melanism is common in squamate reptiles, where it occurs at various extents depending on the considered taxonomic level. Such variability opens the use of a phylogenetic comparative approach to test whether climatic conditions, by pivoting thermal advantage, could drive the evolution of melanism in reptiles. By focusing on the snake family Elapidae, we firstly reconstructed the evolutionary history of melanism and, subsequently, tested whether climate affects the frequency of melanism. Ancestral state reconstruction estimated that the ability to express melanism is the ancestral condition of elapid snakes. Furthermore, we found statistically significant effects of both average diurnal temperature and precipitation on the probability of a species to be melanistic, so that melanism is more frequent in colder and wetter climates. Melanism in Elapidae seems to be highly correlated with colder temperatures, so that it is most likely favoured compared with normal colourations to boost thermoregulation under less favourable conditions. Although broad-scale conclusions still must be drawn for squamates, with this research, we provide additional support for the TMH in ectotherms as a key adaptive interpretation of this phenotypic trait in wild populations.