Gloger's rule
Gloger's Rule is a zoological 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 who found out that birds in more humid habitat tended to be darker than their relatives from regions with higher aridity. Over 90% of the species researched conform to this rule.[citation needed]
The most likely explanation why Gloger's rule applies in such cases is that it increases the resilience against feather- or hair-degrading bacteria (such as Bacillus licheniformis). Humid environments are more suitable to microbes, but dark feathers or hair are more difficult to break down (Burtt & Ichida, 2004).
In addition, there is a marked tendency for mammals 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 utilize the sun's UV radiation with increasing latitude. Absorption of a certain amount of UV radiation is necessary for the production of certain vitamins, notably Vitamin D (see also Osteomalacia).
This principle is also vividly demonstrated amongst human populations (Ember et al., 2002). 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, which have darker skin than might be expected from the latitude they evolved at. In the first case, this is apparently an adaptation to the extremely high UV irradiation on the Tibetan Plateau, whereas in the second case, the necessity to absorb UV radiation is alleviated by a diet that is naturally rich in Vitamin D.
References
- Burtt, Edward H. Jr. & Ichida, Jann M. (2004): Gloger's Rule, feather-degrading bacteria, and color variation among Song Sparrows. Condor 106(3): 681-686. doi:10.1650/7383 (HTML abstract)
- Ember, Carol R.; Ember, Melvin & Peregrine, Peter N. (2002): Anthropology (10th ed.) Prentice Hall.
- Gloger, Constantin Lambert (1833): Das Abändern der Vögel durch Einfluss des Klimas. August Schulz, Breslau.