Gloger's Rule

Gloger's rule is an ecogeographical rule which states that within a species of endotherms, more heavily pigmented forms tend to be found in more humid environments, e.g. near the equator. It was named after the zoologist Constantin Wilhelm Lambert Gloger, who first remarked upon this phenomenon in 1833 in a review of covariation of climate and avian plumage color. (Erwin Stresemann notes that the idea was already expressed by Pallas in Zoographia Rosso-Asiatica (1811)) Gloger found that birds in more humid habitats tended to be darker than their relatives from regions with higher aridity. Over 90% of 52 North American bird species studies conform to this 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.

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 excessive 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).

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