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 For this reason, 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 author’s career and other contributions of interest to his field of knowledge.
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 wetter climates tend to have a darker or more pigmented color while those that live 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 group would be homeothermic or warm-blooded animals, that is, those that maintain a stable body temperature and generally above the temperature of the environment, thanks to a series of metabolic processes.
Homeothermic animal species are all those that are classified within birds and mammals. It is, therefore, these types of vertebrates that would be covered by Gloger’s rule and in which the maxim of greater pigmentation should be fulfilled the more humid the natural habitat of the animal species in question that we are studying.
Gloger, zoologist born in the now defunct Kingdom of Prussia (today, 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 most of Gloger’s research was based on the observation of different species of birds, since he specialized in ornithology.
CWL Gloger was a man passionate about biology and zoology. In fact, another of his most notable works is called the Non-Profit Manual and Auxiliary Book of Natural History, an example of his devotion to expanding the frontiers of science and bringing knowledge to the entire world, without seeking a benefit while traveling that 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 of 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, German naturalist.
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 black, dark brown or other similar shades of feathers or hair, while in dry areas It will be more common for us to see specimens of species with paler, yellow tones, 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 demonstrated 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 have greater natural resistance to the action of a series of bacteria that damage feathers or 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 on the plumage and fur of animals than in dry environments.
Following this reasoning, birds that live in humid areas will probably have plumage pigmented 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 areas will see their feathers dyed with lighter pigments, thanks to pheomelanins.
There is a second reason that can cause birds from dry habitats to have lighter, sandy toned or pale red feathers The second key to why Gloger’s rule could occur would be crypsis, another adaptive mechanism that provides greater chances of survival to those animals that camouflage themselves with their environment so as not to be seen, both as a predator and as a possible prey.
This would explain the reason for these lighter coats and plumages in areas that are usually desert or arid, since it makes it easier for the animal to have colors similar to those of the environment through which it moves, so that in the case of the hunter he will be less likely to be seen by its potential prey and in turn the prey will be less conspicuous, so predators will have more difficulty finding them.
Is it true in human beings?
Although until 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 tones in the skin and coat. Likewise, the further we get from the equator and closer to the poles, this pigmentation should decrease more and more.
It not only decreases because 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 too intense radiation but at the same time require certain doses to acquire vitamin D
Within mammals, humans are no exception, so Gloger’s rule would equally apply 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. On the contrary, 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 anywhere in the world, we will find people with skin of any shade regardless of the area in which we are. Gloger’s rule refers to a form of adaptation that has existed for thousands of years and hundreds and hundreds of generations, before we had the mobility of today.
Even so, There are some exceptions to the generality of Gloger’s rule regarding the distribution of the human population on our planet and skin color of individuals. For example, Tibetan people have darker pigmentation than would, in principle, fit with the area in which they live, the Tibetan plateau. But there is a very plausible explanation, and it is that it is an area with a high incidence of ultraviolet radiation.
Therefore, as we had previously seen, having darker skin tone serves as a natural protection and therefore an adaptive advantage to counteract the effects of excessive UV radiation. 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 it is that the Inuit diet is already very rich in vitamin D, so the adaptation of acquiring less pigmented skin would not have been necessary for them. and generate this element as a result of exposure to the sun.
