Gloger’s rule: what it is and how it explains the coloring of animals
We will try to decipher the key to this theory as well as the biological explanations behind this phenomenon. Likewise, we will know more details about the trajectory of its author and other contributions of interest to his field of knowledge.
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 pigmented color, while those that live in dry environments will tend to have a paler-looking skin, coat 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 set of animals or at least to the majority. In this case, this group would be that of 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 affected 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, a zoologist born in the now defunct Kingdom of Prussia (now Germany) in 1803, He mentioned for the first time what is known as Gloger’s rule in his publication, “The modification of birds by the influence of the weather”, which was published in 1833. And is that most of Gloger’s research was based on the observation of different species of birds, as he specialized in ornithology.
CWL Gloger was a man with a passion for biology and zoology. In fact, another of his most outstanding works responds to the name of Non-profit Manual and auxiliary book of natural history, an example of his devotion to expanding the frontiers of science and making knowledge reach the whole world, without looking for a profit 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, a 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 frequent that we see specimens of species with paler, yellow tones, etc.
The next step would be to delve into the biological roots that lie behind Gloger’s rule to understand why it works. Although it is not a fully proven mechanism and therefore has a part of the researchers’ intuition, there is a consensus on the adaptive objective that this process would follow for animals.
According to studies by Constantin Golger, birds with darker plumage have a 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 in arid sectors will see their feathers dyed with lighter pigments, thanks to pheomelanins.
There is a second reason that can cause birds in dry habitats to have lighter feathers, sandy tones or pale red.. The second key by which Gloger’s rule could happen 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 predators and as possible prey.
This would explain the reason for these lighter coats and plumages in areas that are usually desert or arid, as 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 it will have less probability of be seen for its potential prey and in turn the prey will be less conspicuous, so that the predators will have more difficulty finding them.
Is it true in humans?
Although we have focused on bird species so far, 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 strike almost perpendicularly, have to be better protected against UV radiation. This protection is achieved thanks to darker shades in skin and fur. Likewise, the further we move from the equator and get closer to the poles, that pigmentation should descend more and more.
Not only does it decrease because that 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. In this way, adaptively the 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 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. On the contrary, the greater the distance from these surroundings, the paler the skin will be.
Obviously, in modern human society, where each individual has the ability to move freely around practically anywhere in the world, we will find people with skin of any hue regardless of the area in which we are. Gloger’s rule refers to a form of adaptation that has been in place 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 the color of the skin of individuals. For example, Tibetan people have a darker pigmentation than would, in principle, fit the area in which they live, 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 seen previously, having a 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 that of 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 living 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, so it would not have been necessary for them to adapt to acquire less pigmented skin and generate this element from exposure to the sun.