Bergmann’s rule: what it is and how it describes animals

Bergmann's rule is a famous biogeographical rule that associates temperature and body mass.

In the course of its history, man has already described a total of 1,326,337 animal species. This value fluctuates continuously because, in addition to the newly discovered living beings, the experts of the United Nations (UN) point out that about 150 species become extinct every 24 hours. Of course, as far as biodiversity is concerned, the current outlook is not encouraging.

Zoology is a branch of biology that is in charge of imposing a little order in all this vital conglomerate, since it studies, mainly, the physiology, morphology, behavior, distribution and ecology of each of the species that inhabit our planet.

One of the oldest biological rules of zoological and ecological nature, coined in 1847, is known as Bergmann's rule. This postulation is linked to the distribution and morphology of species according to environmental temperature, two clearly different concepts but interconnected in many points. If you want to know what this interesting idea consists of and what its practical applications are, read on.

What is Bergmann's rule?

Bergmann's rule is defined simply: the tendency for a positive association between the body mass of species in a monophyletic higher taxon and the latitude inhabited by those species.. Put a little more kindly, endothermic animals (capable of maintaining a metabolically favorable body temperature independent of the environment) are larger in cold climates than in warm areas.

Several attempts have been made to explain this rule. We show them briefly below:

• It has been tried to demonstrate as an artifact of phylogenetic relationships between species, i.e., different species are distributed in different latitudes.
• It has been tried to explain it as a consequence of an ability to migrate (larger animals will do it more effectively).
• Its application could be based on a resistance to starvation, i.e., larger homeotherms will go longer without food.
• The ability of species of different sizes to conserve or dissipate heat.

It is the last two points that attract our attention the most, because Bergmann's rule could explain an extreme adaptation to an inclement climate.. At least on paper, larger species would have a greater capacity to survive periods of resource scarcity (because of their greater energy reserves in more voluminous tissues), as well as allowing them to preserve their body heat more efficiently.

The physics of postulation

It's time to get a little technical, but don't worry: you'll understand the following lines perfectly. According to Bergmann, large animals have a smaller surface/volume ratio.. Demonstrably, a living being with a high body surface/volume ratio is "more" in contact with the environment. This is why humans have multi-chambered lungs, as it is an effective way to increase the tissue surface area in contact with the air, which allows us to capture oxygen more efficiently.

Thus, an animal with a low surface/volume ratio radiates less body heat per unit mass, which is why it will remain warmer in cold environments. Warm environments pose just the opposite problem, since the heat produced by metabolism must be dissipated quickly to avoid overheating of the living being. For this reason, animals are "interested" in being smaller the closer they are to the equator: more heat is lost through the skin and the body stays cooler.

Examples

It is surprising to learn that Bergmann's rule is perfectly applicable to humans under certain specific conditions. For example, it has been shown that human populations living at the poles are generally heavier in constitution than those closer to the equator.This fact is completely consistent with the postulation presented here.

On the other hand, a study in 2019 reported in BBC News showed that a group of monitored birds reduced over the generations (1978-2016) the length of certain body structures by up to 2.4%, a completely significant result. This could be explained on the basis of climate change: the warmer it gets on Earth, the more size reduction species experience.

As far as mammals are concerned and beyond humans, deer are a textbook case of Bergmann's rule. It has been observed that deer species in northern regions tend to be larger and more robust, while those inhabiting areas closer to the equator tend to be smaller and leaner. Again, the postulation holds true.

It should be noted that this rule is generally applicable to birds and mammals.However, intrinsic genetic properties of populations, natural selection pressures other than temperature, and stochastic events such as genetic drift must also be taken into account. There are generalities in nature, but of course these hypotheses cannot be applied unmovably to all living things.

Allen's rule

We do not want to stay on the surface and we delve a little deeper into the world of thermoregulation, since Allen's rule also provides us with several concepts to take into account in this subject. This hypothesis postulates that, even with the same body volume, homeothermic animals should show different surface areas that will help or hinder their heat dissipation.. Let's take a simple example.

If we look at an arctic fox, we can observe that it has flat, small ears and a considerable amount of fur. On the other hand, a desert fox or fennec has ears that are disproportionately large compared to the rest of its body. Multiple studies in laboratory environments have shown that cartilage size can increase or decrease in cartilage size can increase or decrease in species depending on the environmental conditions to which they are exposed over generations..

This makes perfect sense: at the same amount of volume from a theoretical standpoint, a fennec has much more body surface area due to its huge, flattened ears. This allows it to dissipate heat efficiently, as these structures are also often heavily irrigated by Blood vessels. On the other hand, the arctic fox is interested in accumulating its metabolic temperature, which is why the less it leaves exposed to the environment, the better.

Skepticism and meanings

As we have said previously, conditioning the size of animals exclusively on the latitude of the environment can be misleading. We can theorize that, perhaps, a larger animal would have a clear evolutionary advantage over a predator in a hot environment.

What happens in that case: is it more worthwhile for it to have to seek ancillary methods to dissipate its body temperature (behavioral changes, for example) and still be able to cope with its rival? Nature is not based on black and white, but each factor represents another point on a scale of grays that models what we know as natural selection..

On the other hand, it is also necessary to note that this rule does not hold true in many cases of ectothermic animals, such as turtles, snakes, amphibians, macroalgae and crustaceans. The non-applicability of this postulation in various cases has caused multiple professionals and thinkers to subject it to scrutiny throughout history.

Summary

As we have seen in these lines, Bergmann's rule can explain, to a certain extent, the reason for the variability in size between species according to the latitude of the ecosystem in which they live. From all this terminological conglomerate, it is enough to make one concept clear: smaller animals are theoretically more efficient at dissipating heat, while larger animals excel in their ability to store it.

Again, it is essential to emphasize that there is no universal rule or postulation (beyond natural selection and genetic drift) that fully explains the morphological characteristics of a species. Yes, animals and their characters are a product of temperature, but also of humidity, relationships with other living beings, competition, food chains, sexual selection and many, many other biotic and abiotic parameters.

Bibliographical references:

• Adams, D. C., & Church, J. O. (2008). Amphibians do not follow Bergmann's rule. Evolution: International Journal of Organic Evolution, 62(2), 413 - 420.
• Bergmanns rule, britannica.com.
• Birds Shrinking as the climate warms, BBC news.
• Figueroa-de León, A., & Chediack, S. E. (2018). Patterns of richness and latitudinal distribution of caviomorph rodents. Mexican journal of biodiversity, 89(1), 173 - 182.
• L'heureux, G. L., & Cornaglia Fernández, J. (2016). Ecomorphological variations in guanaco populations of Patagonia (Argentina).
• Mousseau, T. A. (1997). Ectotherms follow the converse to Bergmann's rule. Evolution, 51(2), 630 - 632.
• Bergmann's rule-introduction for educators, fieldmuseum.org.