The dark side of colouration: ecogeographical evidence supports Gloger's rule in American marsupials.

: Geographical distribution of colour 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 colour patterns, ranging from light and dark uniform to more complex colourations. However, surprisingly little is known about the adaptive significance of dark colouration in this singular group of mammals. Using a phylogenetic comparative approach, we investigated whether colouration 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 colour variation in American marsupials, especially when considering the facial region. Thus, dark colouration 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 colour variation in didelphid marsupials, including camouflage, pathogen resistance, and pleiotropy hypotheses. This article is protected by copyright. All rights reserved.