Gloger's rule is an ecogeographical rule which states that within a species of

endotherms An endotherm (from Ancient Greek, Greek ἔνδον ''endon'' "within" and θέρμη ''thermē'' "heat") is an organism that maintains its body at a metabolically favorable temperature, largely by the use of heat released by its internal bodily ...

, more heavily pigmented forms tend to be found in more

humid Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity depe ...

environments, e.g. near the

equator The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...

. It was named after the zoologist Constantin Wilhelm Lambert Gloger, who first remarked upon this phenomenon in 1833 in a review of

covariation In probability theory and statistics, covariance is a measure of the joint variability of two random variables. The sign of the covariance, therefore, shows the tendency in the linear relationship between the variables. If greater values of one ...

of climate and avian

plumage Plumage () is a layer of feathers that covers a bird and the pattern, colour, and arrangement of those feathers. The pattern and colours of plumage differ between species and subspecies and may vary with age classes. Within species, there can b ...

color.

Erwin Stresemann Erwin Friedrich Theodor Stresemann (22 November 1889, in Dresden – 20 November 1972, in East Berlin) was a German naturalist and ornithologist. Stresemann was an ornithologist of extensive breadth who compiled one of the first and most comprehe ...

later noted that the idea had been expressed even earlier by

Peter Simon Pallas Peter Simon Pallas Fellow of the Royal Society, FRS FRSE (22 September 1741 – 8 September 1811) was a Prussia, Prussian zoologist, botanist, Ethnography, ethnographer, Exploration, explorer, Geography, geographer, Geology, geologist, Natura ...

in ''Zoographia Rosso-Asiatica'' (1811). Gloger found that birds in more humid habitats tended to be darker than their relatives from regions with higher

arid Aridity is the condition of geographical regions which make up approximately 43% of total global available land area, characterized by low annual precipitation, increased temperatures, and limited water availability.Perez-Aguilar, L. Y., Plata ...

ity. 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 ''Bacillus licheniformis'' is a bacterium commonly found in the soil. It is found on bird feathers, especially chest and back plumage, and most often in ground-dwelling birds (like sparrows) and aquatic species (like ducks). It is a gram-positi ...

''. 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

eumelanin Melanin (; ) is a family of biomolecules organized as oligomers or polymers, which among other functions provide the pigments of many organisms. Melanin pigments are produced in a specialized group of cells known as melanocytes. There are ...

s (dark brown to black) are deposited in hot and humid regions, whereas in arid regions,

pheomelanin Melanin (; ) is a family of biomolecules organized as oligomers or polymers, which among other functions provide the Biological pigment, pigments of many organisms. Melanin pigments are produced in a specialized group of cells known as melan ...

s (reddish to sandy color) predominate due to the benefit of

crypsis In ecology, crypsis is the ability of an animal or a plant to avoid observation or detection by other animals. It may be part of a predation strategy or an antipredator adaptation. Methods include camouflage, nocturnality, subterranean life ...

. Among

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

s, there is a marked tendency in equatorial and

tropical The tropics are the regions of Earth surrounding the equator, where the sun may shine directly overhead. This contrasts with the temperate or polar regions of Earth, where the Sun can never be directly overhead. This is because of Earth's ax ...

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 Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of t ...

at lower latitudes. However, absorption of a certain amount of UV radiation is necessary for the production of certain

vitamin Vitamins are Organic compound, organic molecules (or a set of closely related molecules called vitamer, vitamers) that are essential to an organism in small quantities for proper metabolism, metabolic function. Nutrient#Essential nutrients, ...

s, notably

vitamin D Vitamin D is a group of structurally related, fat-soluble compounds responsible for increasing intestinal absorption of calcium, magnesium, and phosphate, along with numerous other biological functions. In humans, the most important compo ...

(see also

osteomalacia Osteomalacia is a disease characterized by the softening of the bones caused by impaired bone metabolism primarily due to inadequate levels of available phosphate, calcium, and vitamin D, or because of resorption of calcium. The impairment of b ...

). 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 Inuit (singular: Inuk) are a group of culturally and historically similar Indigenous peoples traditionally inhabiting the Arctic and Subarctic regions of North America and Russia, including Greenland, Labrador, Quebec, Nunavut, the Northwe ...

, 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 The Tibetan Plateau, also known as the Qinghai–Tibet Plateau or Qingzang Plateau, is a vast elevated plateau located at the intersection of Central Asia, Central, South Asia, South, and East Asia. Geographically, it is located to the north of H ...

, whereas in the second case, the necessity to absorb UV radiation is alleviated by the Inuit's diet, which is naturally rich in

, and the year-round snow and ice, which effectively reflects UV into the environment.

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See also

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Further reading