Bergmann’s Rule (the third ecographic rule) was formulated in 1847 by Karl (also seen as Carl) Bergmann (1814 – 1865), a German biologist. It states that the same or closely related species tend to be larger in colder climates. It was originally formulated for species within a genus, but now it also applies to populations within a species.
As noted with Allen’s Rule (see “Allen’s Rule” on 02-26-19), all mammals give off approximately the same amount of heat per unit surface area. The surface of an object increases as the square of the linear dimension and the volume increases as the cube. Thus the larger the surface area in relation to the volume, the less heat is lost. The lower the surface area, the better the adaptation to colder climates and higher latitudes. Larger animals conserve heat better than their smaller relatives.
Some zoologists argue that the relatively smaller surface of a larger animal would not give enough of a reduction in heat loss to be significant. However, although there are exceptions to Bergmann’s Rule, evolution seems to have favored larger animals with relatively short ears, limbs and tails in colder regions. Moose (Alces alces) in Sweden demonstrate Bergmann’s Rule as do white-tailed deer (Odocoileus virginianus). White-tailed deer in Michigan are significantly larger than those in Nicaragua.
There is evidence in the literature that humans, some ectothermic species (for example ants) and some plants also follow Bergmann’s Rule. Meiri and Dayan in the “Journal of Biogeography” (2003) reviewed the literature for various species and listed many that followed Bergmann’s Rule, including American kestrels (Falco sparverius) and bobcats (Lynx rufus).
The bobcat was photographed at sunrise on our ranch near Lookout CA (Modoc County) while the kestrel was on a fence in our barnyard.