The dark side of coloration: Ecogeographical evidence supports Gloger's rule in American marsupials

Geographical distribution of color phenotypes and associations with ecological predictors remains poorly understood. An important geographic pattern concerning this topic is Gloger's rule, which predicts the increase of pigmentation in endothermic animals from cold and dry to warm and wet environments. Didelphid marsupials exhibit a variety of color patterns, ranging from light and dark uniform to more complex colorations. However, surprisingly little is known about the adaptive significance of dark coloration in this singular group of mammals. Using a phylogenetic comparative approach, we investigated whether coloration in different body regions of didelphids (i.e., dorsum and face) is associated with variables representing heat and humidity of the environment, as predicted by Gloger's rule. We demonstrated that Gloger's rule explains the interspecific color variation in American marsupials, especially when considering the facial region. Thus, dark coloration was more frequent among didelphid species occupying warm and wet environments than cold and dry environments. We also discuss the selective forces that can potentially explain coat color variation in didelphid marsupials, including camouflage, pathogen resistance, and pleiotropy hypotheses.