Gloger's rule

One explanation of Gloger's rule in the case of birds appears to be the increased resistance of dark feathers to feather- or hair-degrading bacteria such as Bacillus licheniformis. Feathers in humid environments have a greater bacterial load, and humid environments are more suitable for microbial growth; dark feathers or hair are more difficult to break down. More resilient eumelanins (dark brown to black) are deposited in hot and humid regions, whereas in arid regions, pheomelanins (reddish to sandy color) predominate due to the benefit of crypsis.

Gloger's rule concerns coloration visible to humans. However, about 55% of solar energy is in the near-infrared. A study found that northern bobwhite populations whose back plumage was equally dark in visible light absorbed different shares of sunlight across the ultraviolet-to-near-infrared range, with birds from Iowa, the coolest of the three areas sampled, absorbing the most.

Among mammals, there is a marked tendency in equatorial and tropical regions to have a darker skin color than poleward relatives. In this case, the underlying cause is probably the need to better protect against the more intense solar UV radiation at lower latitudes. However, absorption of a certain amount of UV radiation is necessary for the production of certain vitamins, notably vitamin D (see also osteomalacia).

Gloger's rule is also found among human populations. Populations that evolved in sunnier environments closer to the equator tend to be darker-pigmented than populations originating farther from the equator. There are exceptions, however; among the most well known are the Tibetans and Inuit, who have darker skin than might be expected from their native latitudes. In the first case, this is apparently an adaptation to the extremely high UV radiation on the Tibetan Plateau, whereas in the second case, the necessity to absorb UV radiation is alleviated by the Inuit's diet, which is naturally rich in vitamin D, and the year-round snow and ice, which effectively reflects UV into the environment.