Bergmann's rule is an ecogeographic principle 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. The rule is named after nineteenth-century German biologist Carl Bergmann, who described the pattern in 1847, though 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.
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. 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.
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. They also tend to have shorter limbs and broader trunks, consistent with Allen's rule. 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. Newman contends that Bergmann's rule holds for the populations of Eurasia, but it does not hold for those of sub-Saharan Africa.
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 marine crustaceans, it has been proposed that an increase in size with latitude is observed because decreasing temperature results in increased cell size and increased life span, both of which lead to an increase in maximum body size (continued growth throughout life is characteristic of crustaceans). The size trend has been observed in hyperiid and gammarid amphipods, copepods, stomatopods, mysids, and planktonic euphausiids, both in comparisons of related species as well as within widely distributed species. Deep-sea gigantism is observed in some of the same groups, probably for the same reasons.
Hesse's rule 
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.
According to study by Valerius Geist in 1986, Bergmann's rule is false, the correlation with temperature is spurious, instead he found body size is proportional to the duration of the annual productivity pulse, or food availability-per-animal during the growing season.
See also 
- Gigantothermy – large ectothermic animals more easily maintain constant body temperature
- Insular dwarfism – small populations promote reduction in size of individuals
- Allen's rule – populations have shorter appendages in colder climates
- Animal migration
- Gene flow
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