In zoology, Bergmann's Rule is a principle that correlates environmental temperature with body mass in warm-blooded animals. Among mammals and birds, individuals of a particular species in colder areas tend to have greater body mass than individuals in warmer areas. For instance, white-tailed deer are larger in Canada than in the Florida Keys. The rule is named after a nineteenth-century German biologist, Carl Bergmann .
This rule operates as larger animals have a lower surface area to volume ratio than smaller animals, so they radiate less body heat, and stay warmer in cold climates. Conversely, smaller animals in warmer climates can avoid overheating by radiating heat.