Gloger’s rule: what is it and how does it explain the coloration of animals?

Let's see how this rule proposed by Constantin Wilhelm Lambert Gloger is studied in biology.

Gloger's rule tries to give an explanation for the curious distribution of the color of animals according to the area in which they inhabit. 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. Also, 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 the author Constantin Wilhelm Lambert Gloger, with which. tries to explain why animals living in more humid climates tend to have a darker or more pigmented color, while those living in dry environments will tend to have a paler looking skin, coat or plumage due to less pigmentation.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 set of animals, or at least to most of them. In this case that set would be homeothermic or warm-blooded animals, i.e., those that maintain a stable body temperature generally above the temperature of the environment, thanks to a series of metabolic processes.

The homeothermic animal species are all those that are classified within birds and mammals. It is, therefore, these types of vertebrates that would be reached 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 (today Germany) in 1803, first mentioned Gloger's rule in his publication, "The Modification of Birds by the Influence of Climate", which was published in 1833.. Most of Gloger's research was based on the observation of different species of birds, as 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 responds to the name of Nonprofit Manual and Auxiliary Book of Natural History, an example of his devotion to expand the frontiers of science and make knowledge available to everyone, without seeking a profit while traveling that path.

It is important to mention that, although it was this author who 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 inhabits, had already been mentioned in some way by Peter Simon Pallasanother Prussian zoologist. The author who noticed this first mention was Erwin Friedrich Theodor Stresemann, a German naturalist.

Biological foundations of Gloger's rule.

We now know how Gloger's rule works in practical terms and why 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 we will more often see specimens of species with paler shades of yellow, etc.

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

According to Constantin Golger's studies, birds with darker plumage have a greater natural resistance to the action of a series of bacteria that spoil feathers or hair. An example of this organism is bacillus licheniformis. The point is that this type of bacteria is 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 living in humid areas will probably have a plumage pigmented with eumelanins, which provides dark tones and at the same time makes them more resistant to bacterial attack, as we have already seen. In contrast, birds from arid sectors will have their feathers dyed with lighter pigments, thanks to pheomelanins.

There is a second reason that may cause birds from dry habitats to have lighter, sandy or pale red feathers.. The second key to Gloger's rule could be crypsis, another adaptive mechanism that provides greater chances of survival to those animals that camouflage with their environment to avoid being seen, both as a predator and as possible prey.

This would explain the reason for these lighter coats and plumage 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 in which it moves, so that in the case of the hunter it will have less chance of being seen by its potential prey and in turn the prey will be less conspicuous, so that predators will have more difficulty finding them.

Does this hold true for humans?

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

According to this principle, mammals living in equatorial regions, where the sun's rays strike almost perpendicularly, should be more protected against UV radiation. This protection is achieved by darker shades of skin and fur. Likewise, the further we move away from the equator and towards the poles, the darker the pigmentation should become.

This is not only 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 triggered by this same radiation. In 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 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 in virtually any part of the world, we will find people with skin of any shade regardless of the area in which we find ourselves. Gloger's rule refers to a form of adaptation that has ruled 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, there are some exceptions to the generality of Gloger's rule in terms of the distribution of the human population across our planet and the skin color of individuals. of individuals. For example, Tibetan people have darker pigmentation than would, in principle, fit the area they inhabit, the Tibetan plateau. But there is a very plausible explanation, and that is that this 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 the Inuit people, inhabitants of Greenland and the northernmost parts of Alaska (United States) and Canada.

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

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). Modification of the individual colors subject to change by the climate. The Changing of Birds by Influence of Climate. Wroclaw: August Schulz.