On the Ecological Significance of Bergmann's Rule

A positive correlation of weight with latitude in homoiotherms (Bergmann's rule) cannot normally depend upon the physics of heat exchange. Most latitudinally widespread mammals in North America do not follow this rule. Those that do are usually carnivores or granivores; a change in their body size reflects a change in the size of their prey. A latitudinal change in the size of available prey is due either to the distribution of the prey species or to the distribution of other predators utilizing the same prey species. Only the smallest species of a set of similar predators normally will conform to Bergmanns rule, and then only beyond the limits of distribution of the largest species. These changes in size seem to be another example of character displacement.