Red Fox Skull Size Variability Along the Ural’s Meridian: Bergmann’s Rule or the Optimum Rule?

Abstract

Changes in the skull size of the red fox (Vulpes vulpes L.) along the eastern macroslope of the Ural Range from the forest-tundra to the steppe are analyzed. The sizes varied from minimum in the north to maximum in pre-forest-steppe pine-birch forests. Further south, the size of the skull decreased again. The average fox population density changed similarly. Based on these data, it was concluded that the sizes change in accordance with the optimum rule, and not with Bergman’s rule. Probably, microevolutionary adaptations to living conditions are influenced by general factors that influence both body size and the population density of the species.