Bergmann's Rule

In zoology, Bergmann's rule is an ecogeographic rule that correlates latitude with body mass in animals. Broadly it asserts that within a species the body mass increases with latitude and colder climate, or that within closely related species that differ only in relation to size that one would expect the larger species to be found at the higher latitude. The rule is named after a nineteenth-century German biologist, Christian Bergmann, who first formulated the rule in 1847. The rule is often applied only to mammals and birds (endotherms), but some researchers have also found evidence for the rule in studies of ectothermic species. Bergmann's rule is controversial amongst researchers and its validity has often been called into question, and there is division amongst scientists about whether the rule should be interpreted to within species variation or among species variation. Although several mechanisms have been suggested to explain the rule there is no clear explanation for why the pattern exists.

Explanations for Bergmann's Rule
The earliest explanation, given by Bergmann when originally formulating the rule, is that larger animals have a lower surface area to volume ratio than smaller animals, so they radiate less body heat per unit of mass, and stay warmer in cold climates. On the other hand, warmer climates impose the opposite problem: body heat generated by metabolism needs to be dissipated quickly rather than stored within. Thus, the higher surface area-to-weight ratio of smaller animals in hot and dry climates facilitates heat loss through the skin and helps cooling of the body.

However, some notable exceptions of species with large mass and small surface-to-volume ratios that reside in warm climates exist, such as the African elephant. In this case, similar thermoregulatory optimizations may be operating, such as mass homeothermy to resist a significant rise in core body temperature in warm climates. Anecdotally, elephants are more frequently found in the shelter of shade when they are accompanied by calves, which have a significantly higher surface-to-volume ratio, and are much more prone to changes in temperature from radiant sources in the environment. (For similar arguments with references, see ).

For humans, the rule is true to a certain extent, but differing cultural practices including local diet, migration and gene flow between populations must obviously account for much of this. For example, northern Asians are on average larger than their Southeast Asian counterparts. The Inuit of Alaska and northern Canada are known for their compact bodies (relatively short arms and legs compared to height of the torso) as adaptation to severe cold (which is Allen's Rule). Moreover, pygmies are found only in tropical rain forests. There are, however, counterexamples. Furthermore, the ability of humans to cope in colder climates can be mostly attributed to appropriate clothing and dwellings, whereas animals cope mainly with genetic adaptations.