What are Bergmann’s and Allen’s rules?

Bergmann's rule is one of the best-known generalizations in zoology.

It is generally defined as a within-species tendency in warm-blooded animals to have increasing body size with increasing latitude and decreasing ambient temperature.

Bergmann's rule states that among mammals and birds, individuals of a particular species in colder areas tend to have greater body mass than individuals in warmer areas.

This principle is named after a nineteenth-century German biologist, Karl Bergmann (1814-1865, ⬆️ on the left), who published observations along these lines in 1847.

A corollary of Bergmann's rule is Allen's rule, observed in 1877 by Joel Asaph Allen (1838-1921, ⬆️ on the right) which indicates a trend in terms of appendages of warm-blooded animals according to latitude. Allen's rule holds that individuals in populations of the same species have a tendency to have shorter limbs than the corresponding animals in warmer climates.

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