Gloger's rule: what it is and how it explains the coloration of animals

Gloger's rule tries to explain the curious color distribution of animals according to the area in which they live. Therefore, it has been studied from Biology and from disciplines associated with Anthropology.

We will try to decipher the key to this theory as well as the biological explanations behind this phenomenon. Likewise, we will learn more details about the trajectory of its author and other contributions of interest to his field of knowledge.

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What is Gloger's rule?

Gloger's rule, sometimes written as Golger's rule, is a law described by author Constantin Wilhelm Lambert Gloger, with which tries to explain why animals that live in more humid climates tend to have a darker or more pigmented color, while those living in dry environments will tend to have paler-looking skin, fur, or plumage due to less pigmentation.

Gloger's rule would therefore be a biological rule, that is, a general principle that applies to all members of a group of animals or at least to the majority. In this case, that set would be that of homeothermic or warm-blooded animals, that is, those that maintain a stable body temperature and generally above the surrounding temperature, thanks to a series of processes metabolic.

Homeothermic animal species are all those that are classified within birds and mammals. It is, therefore, these types of vertebrates that would be affected by Gloger's rule and in which it should be fulfilled. the maximum of greater pigmentation the more humid the natural habitat of the animal species in question that we are studying.

Gloger, a zoologist born in the now-defunct Kingdom of Prussia (now Germany) in 1803, He first mentioned what is known as Gloger's rule in his publication, "The Modification of Birds by the Influence of Climate", which was published in 1833.. And it is that most of Gloger's investigations were based on the observation of different species of birds, since he specialized in ornithology.

C.W.L. Gloger was a man with a passion for biology and zoology. In fact, another of his most outstanding works is called the Nonprofit Manual and Auxiliary Book of Natural History, an example of his devotion to expanding the frontiers of science and bringing knowledge to the whole world, without seeking a benefit while traveling said path.

It is important to mention that, although this author was the first to formulate Gloger's rule and its implications, the relationship between the level of pigmentation of the body and the degree of humidity in the area where the animal lives, had already been mentioned in some way by Peter Simon Pallas, precisely another Prussian zoologist. The author who noticed this first mention was Erwin Friedrich Theodor Stresemann, a German naturalist.

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Biological foundations of Gloger's rule

We already know how Gloger's rule works for practical purposes and why, thanks to it, it is normal that in humid environments we find more animal species with shades of feathers or black, dark brown or other similar shades, while in dry areas it will be more frequent to see specimens of species with paler, yellow, etc

The next step would be to delve into the biological roots behind Gloger's rule to understand why it works. Although it is not a completely proven mechanism and therefore has a part of researchers' intuition, there is a consensus on the adaptive objective that this process would follow for animals.

According to Constantin Golger's studies, birds with darker plumage present a greater natural resistance to the action of a series of bacteria that spoil the feathers or the hair. An example of this organism is bacillus licheniformis. The point is that these types of bacteria are much more common in humid areas, forming many more colonies in the plumage and fur of animals than in dry environments.

Following this reasoning, birds that live in humid areas will probably have pigmented plumage with eumelanins, which provides dark tones and at the same time makes them more resistant to attack by bacteria, as we have already seen. In contrast, birds from arid sectors will see their feathers dyed with lighter pigments, thanks to pheomelanins.

There is a second reason why birds in dry habitats can have lighter, sandy-toned or pale red feathers.. The second key by which Gloger's rule could occur would be crypsis, another adaptive mechanism that provides greater chances of surviving those animals that blend in with their surroundings so as not to be seen, both as predator and possible prey.

This would explain the reason for these lighter coats and plumage in areas that are usually desert or arid, since it makes it easier for the animal has colors similar to those of the environment through which it moves, so that in the case of the hunter it will have less chance of be seen by its potential prey and in turn the prey will be less conspicuous, so predators will have a harder time spotting it. find them.

Is it true in humans?

Although up to now we have focused on bird species, the truth is that Gloger's rule also applies to mammals. In fact, for them, we would find another powerful explanation for this mechanism, which is none other than protection against potentially harmful ultraviolet radiation from the sun.

According to this principle, mammals that live in equatorial regions, where the sun's rays hit almost perpendicularly, have to be more protected against UV radiation. This protection is achieved thanks to darker shades of skin and fur. Likewise, the more we move away from the equator and we get closer to the poles, that pigmentation should decrease more and more.

It not only decreases because this protection against ultraviolet radiation is no longer needed, but also to be able to acquire the valuable vitamin D that organisms need and that is produced after a metabolic process that this same radiation triggers. This way, adaptively, species need a balance between protection against excessively intense radiation but at the same time requiring certain doses to acquire vitamin D.

Within mammals, human beings are no exception, so Gloger's rule would apply equally to our species. Following the same reasoning, human populations that have developed in areas closer to the equator show a tendency to acquire a more pigmented skin tone. Conversely, the greater the distance from these environments, the paler the skin will be.

Obviously, in modern human society, where each individual has the ability to move freely through virtually anywhere in the world, we will find people with skin of any color regardless of the area in which we let's find. Gloger's rule refers to a form of adaptation that has been in effect for thousands of years and hundreds and hundreds of generations, before we had today's mobility.

Even so, there are some exceptions to the generality of Gloger's rule regarding the distribution of the human population on our planet and the color of the skin of individuals. For example, Tibetan people have darker pigmentation than would initially fit their area, the Tibetan Plateau. But there is a very plausible explanation, and that is that it is an area with a high incidence of ultraviolet radiation.

Therefore, as we have previously seen, having a darker skin tone serves as a natural protection and therefore an adaptive advantage to counteract the effects of UV radiation excessive. The other exception would be the Inuit people, inhabitants of Greenland and the northernmost areas of Alaska (United States) and Canada.

Inuit individuals also have a more pigmented skin tone than would be expected from people who live far from the equator.. Likewise, there is an explanation for this deviation from Gloger's rule, and that is that the Inuit diet is already very rich in vitamin D, Therefore, it would not have been necessary for them to adapt to acquire less pigmented skin and generate this element as a result of exposure to sun.

Bibliographic references:

• Burtt Jr, E.H., Ichida, J.M. (2004). Gloger's rule, feather-degrading bacteria, and color variation among song sparrows. The Condor.
• Delhey, K. (2017). Gloger's rule. Current Biology
• Delhey, K. (2019). A review of Gloger's rule, an ecogeographical rule of colour: Definitions, interpretations and evidence. Biological Reviews. Wiley Online Library.
• Gloger, C. W. L (1833). Abänderungsweise der einzelnen, einer Veränderung durch das Klima unterworfenen Farben. Das Abändern der Vögel durch Einfluss des Klimas. Breslau: August Schulz.