Ecogeographical variation in black bear ( Ursus americanus ) colourmorphs suggests sex-selective support for Gloger’s rule

Mammalian coat colour is adaptive, conferring benefits that enhance fitness. While most species have converged to an optimal coat colour it remains variable in others. Gloger’s rule predicts darker coats in humid than in arid areas; yet this rule requires further testing. Using coat colour data from 2279 black bears ( Ursus americanus Pallas, 1780) from the Yukon, I examined the spatial prevalence of black and non-black colourmorphs. An information-theoretic approach was used to rank candidate models including age, sex, and ecoregion, as predictors of colourmorph prevalence. Ecoregions differed with respect to humidity, habitat openness, and grizzly bear ( Ursus arctos Linnaeus, 1758) densities, and were used as proxies to test ecogeographical hypotheses. Most black bears were black (64%). Sex and ecoregion were the highest ranked predictors of colourmorph prevalence. Colourmorphs of male black bears—but not females—differed significantly by ecoregion. Males were more apt to be black in humid ecoregions. These results provide partial support for Gloger’s rule operating at ecoregional scales depending on sex. From an evolutionary perspective, the sex-biased conformity to Gloger’s rule suggests that selection for coat colour may operate differently on males and females, providing an intriguing avenue for further research with polychromatic mammals.