Gloger's Rule in long-tailed weasels (Mustela frenata)

    The coloration of the face, chest, and nape vary among ten subspecies of long-tailed weasel, Mustela frenata, proceeding south (1) to central Mexico (5) [left column] and thence from northern Mexico (6) to Oregon (10) [right column]. In tropical regions of heavy rainfall [1 & 2] the pelage is darker and there is a decrease in extent of the white facial markings. In the more arid regions of central Mexico and the Sonoran Desert [3 - 8], the pelage is lighter and the white facial markings steadily more distinct. In the rain forests of northern coastal California and Oregon, pelage again becomes darker and facial markings less distinct.

    The suggested adaptive value of Gloger's Rule is that the lighter colors reflect sunlight and allow better thermoregulation in arid areas.