Bergmann’s rule

Bergmann's rule is an ecogeographical rule that states that within a broadly distributed taxonomic clade, populations and 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. It is possible that the rule applies to some plants, such as Rapicactus.

The rule is named after nineteenth century German biologist Carl Bergmann, who described the pattern in 1847, although he was not the first to notice it. 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. such as the ant Leptothorax acervorum. While Bergmann's rule appears to hold true for many mammals and birds, there are exceptions.

Larger-bodied animals tend to conform more closely to Bergmann's rule than smaller-bodied animals, at least up to certain latitudes. This perhaps reflects a reduced ability to avoid stressful environments, such as by burrowing. 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. In particular, reversible dwarfing of mammals has been noted during two relatively brief upward excursions in temperature during the Paleogene: the Paleocene-Eocene thermal maximum and the Eocene Thermal Maximum 2.

Human populations near the poles, including the Inuit, Aleut, and Sami people, are on average heavier than populations from mid-latitudes, consistent with Bergmann's rule. They also tend to have shorter limbs and broader trunks, consistent with Allen's rule. According to Marshall T. Newman in 1953, 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 people, Andes natives and Harrison Lake Lillouet runs contrary to the expectations of Bergmann's rule. Newman contends that Bergmann's rule holds for the populations of Eurasia, but it does not hold for those of sub-Saharan Africa.

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