Bergmann's rule

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, 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.

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. 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.