The principle holding that in a warm-blooded animal species having distinct geographic populations, the body size of animals living in cold climates tends to be larger than in animals of the same species living in warm climates. Animals with larger bodies are generally more massive and thus produce more body heat. They also have smaller surface areas relative to their mass, resulting in a relatively lower rate of heat radiation. The Bergmann rule is named for the German biologist Karl Bergmann (1814-65). Compare Allen's rule.