Bergmann's rule is an ecogeographic principle that states that within a broadly distributed genus, species of larger size are found in colder environments, and species of smaller size are found in warmer regions. Although originally formulated in terms of species within a genus, it has often been recast in terms of populations within a species. It is also often cast in terms of latitude. The rule is named after a nineteenth-century German biologist, Christian Bergmann, who was among the first to describe the pattern in 1847 (but see [1]). Bergmann's rule is most often applied to mammals and birds which are endotherms, but some researchers have also found evidence for the rule in studies of ectothermic species.[2] While Bergmann's rule appears to hold true for most mammals and birds, there are exceptions.[3][4][5]
There seems to be a tendency for larger-bodied animals to conform more closely than smaller-bodied animals, at least up to certain latitudes, perhaps reflecting a reduced ability to avoid stressful environments by burrowing or other means.[6] In addition to being a general pattern across space, Bergmann’s rule has been reported in populations over historical and evolutionary time when exposed to varying thermal regimes.[7][8][9] Note that Bergmann's rule describes a tendency of body mass variation within groups; it does not suggest that large-bodied animals do not occur in warm climates.
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Human populations who live near the poles, including the Inuit, Aleut, and Sami people, are on average heavier than populations from mid-latitudes, consistent with Bergmann's rule.[10] They also tend to have shorter limbs and broader trunks, consistent with Allen's rule.[10] According to Marshall T. Newman in a 1953 article for the Journal of the American Anthropologist, Native American populations are generally consistent with Bergmann's rule although the cold climate and small body size combination of the Eastern Eskimo, Canoe Indians, Yuki, Andes natives and Harrison Lake Lillouet runs contrary to the expectations of Bergmann's rule.[11] Newmann contends that Bergmann's rule holds for the populations of Eurasia, but it does not hold for those of sub-Saharan Africa.[11]
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 therefore stay warmer in cold climates. 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-volume ratio of smaller animals in hot and dry climates facilitates heat loss through the skin and helps cool the body.
In 1937 German zoologist and ecologist Richard Hesse proposed an extension of Bergmann's rule. Hesse's rule, also known as the heart–weight rule, states that species inhabiting colder climates have a larger heart in relation to body weight than closely related species inhabiting warmer climates.[12]
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