Gloger's rule
Encyclopedia
Gloger's Rule is a zoological
Zoology
Zoology |zoölogy]]), is the branch of biology that relates to the animal kingdom, including the structure, embryology, evolution, classification, habits, and distribution of all animals, both living and extinct...

 rule which states that within a species of endotherms
Warm-blooded
The term warm-blooded is a colloquial term to describe animal species which have a relatively higher blood temperature, and maintain thermal homeostasis primarily through internal metabolic processes...

, more heavily pigmented
Biological pigment
Biological pigments, also known simply as pigments or biochromes are substances produced by living organisms that have a color resulting from selective color absorption. Biological pigments include plant pigments and flower pigments...

 forms tend to be found in more humid environments, e.g. near the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....

. It was named after the zoologist Constantin Wilhelm Lambert Gloger
Constantin Wilhelm Lambert Gloger
Constantin Wilhelm Lambert Gloger was a German zoologist and ornithologist.Gloger was the first person to recognise the structural differences between swallows and swifts, and also the first to put up artificial bat boxes.He was the originator of Gloger's rule, which states that dark pigments...

, who first remarked upon this phenomenon in 1833 in a review of covariation of climate and avian plumage
Plumage
Plumage refers both to the layer of feathers that cover a bird and the pattern, colour, and arrangement of those feathers. The pattern and colours of plumage vary between species and subspecies and can also vary between different age classes, sexes, and season. Within species there can also be a...

 color. (Erwin Stresemann
Erwin Stresemann
Erwin Stresemann was a German naturalist and ornithologist.Stresemann was one of the outstanding ornithologists of the 20th century...

 notes that the idea was already expressed by Pallas
Peter Simon Pallas
Peter Simon Pallas was a German zoologist and botanist who worked in Russia.- Life and work :Pallas was born in Berlin, the son of Professor of Surgery Simon Pallas. He studied with private tutors and took an interest in natural history, later attending the University of Halle and the University...

 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
Arid
A region is said to be arid when it is characterized by a severe lack of available water, to the extent of hindering or even preventing the growth and development of plant and animal life...

ity. Over 90% of the 52 North American bird species researched 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
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 and aquatic species ....

. 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
Crypsis
In ecology, crypsis is the ability of an organism to avoid observation or detection by other organisms. It may be either a predation strategy or an antipredator adaptation, and methods include camouflage, nocturnality, subterranean lifestyle, transparency, and mimicry...

.

Among mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s, 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 use the sun's UV radiation with increasing latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

. Absorption of a certain amount of UV radiation is necessary for the production of certain vitamin
Vitamin
A vitamin is an organic compound required as a nutrient in tiny amounts by an organism. In other words, an organic chemical compound is called a vitamin when it cannot be synthesized in sufficient quantities by an organism, and must be obtained from the diet. Thus, the term is conditional both on...

s, notably vitamin D
Vitamin D
Vitamin D is a group of fat-soluble secosteroids. In humans, vitamin D is unique both because it functions as a prohormone and because the body can synthesize it when sun exposure is adequate ....

 (see also Osteomalacia
Osteomalacia
Osteomalacia is the softening of the bones caused by defective bone mineralization secondary to inadequate amounts of available phosphorus and calcium, or because of overactive resorption of calcium from the bone as a result of hyperparathyroidism...

).

This principle is also vividly demonstrated 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 Tibetan
Tibetan people
The Tibetan people are an ethnic group that is native to Tibet, which is mostly in the People's Republic of China. They number 5.4 million and are the 10th largest ethnic group in the country. Significant Tibetan minorities also live in India, Nepal, and Bhutan...

s and Inuit
Inuit
The Inuit are a group of culturally similar indigenous peoples inhabiting the Arctic regions of Canada , Denmark , Russia and the United States . Inuit means “the people” in the Inuktitut language...

, 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 irradiation on the Tibetan Plateau
Tibetan Plateau
The Tibetan Plateau , also known as the Qinghai–Tibetan Plateau is a vast, elevated plateau in Central Asia covering most of the Tibet Autonomous Region and Qinghai, in addition to smaller portions of western Sichuan, southwestern Gansu, and northern Yunnan in Western China and Ladakh in...

, whereas in the second case, the necessity to absorb UV radiation is alleviated by a diet naturally rich in Vitamin D.

See also

  • Allen's rule
    Allen's rule
    Allen's rule is a biological rule posited by Joel Asaph Allen in 1877. It states that endotherms from colder climates usually have shorter limbs than the equivalent animals from warmer climates.- Theory :...

  • Bergmann's Rule
    Bergmann's Rule
    Bergmann's rule is an ecogeographic principle that states that within a broadly distributed genus, species of larger size are found in colder environments, and species of smaller size are found in warmer regions. Although originally formulated in terms of species within a genus, it has often been...

    – that correlates latitude with body mass in animals.
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