Comparative studies indicate that several mammalian clades obey Gloger's rule in that they exhibit darker coloration in humid warm climates, although the mechanisms responsible for this association still are poorly understood. We surveyed external appearances of a single species, the feral pig (Sus scrofa), shot at 48 hunting lodges across North America and matched these to potential abiotic drivers, namely: relative humidity, temperature, precipitation, and ultraviolet (UV) radiation, and to biotic factors of habitat shade and predation pressure. We found that darker animals occupy locations of greater precipitation and warmer temperatures, as expected from Gloger's rule. The recent range expansion of S. scrofa implies selection for pelage coloration has occurred very rapidly. Separating pelage coloration into eumelanin- and phaeomelanin-based pigmentation, we found more pronounced eumelanin-based pelage in areas of higher rainfall and temperatures and UV radiation, whereas pelage phaeomelanin is related to cool dry climates with lower UV radiation. This implies that humidity or UV protection but not crypsis are the mechanisms underlying Gloger's rule in this species and the factors driving eumelanin and phaeomelanin expression in mammalian pelage are different, reinforcing new interpretations of this venerable rule.