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

Human beings have already described, throughout their history, a total of 1,326,337 animal species. This value fluctuates continuously because, in addition to the new living beings discovered, the experts of the The United Nations (UN) indicates that around 150 species become extinct every 24 hours. Of course, in terms of biodiversity, the current outlook is not encouraging.

Zoology is a branch of biology that is in charge of imposing a bit of order in all this vital conglomerate, because 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 a zoological and ecological nature, coined in the year 1847, This is known as Bergmann's rule.. This postulation is linked to the distribution and morphology of the species according to environmental temperature, two clearly different concepts but interconnected at many points. If you want to know what this interesting idea consists of and what its practical applications are, continue reading.

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What is Bergmann's rule?

Bergmann's rule is simply defined: the tendency for a positive association between the body mass of species in a monophyletic higher taxon and the latitude inhabited by those species. In a slightly kinder way, endothermic animals (capable of maintaining a body temperature metabolically favorable regardless of environment) are larger in cold climates than in areas hot.

Attempts have been made to explain this rule in various ways. We show them briefly below:

• It has tried to demonstrate as an artifact of the phylogenetic relationships between species, that is, different species are distributed in different latitudes.
• Attempts have been made to explain it as a consequence of an ability to migrate (larger animals will do so more effectively).
• Its application could be based on resistance to starvation, that is, larger homeothermic living beings will last longer without eating.
• By the ability of species of different sizes to conserve or dissipate heat.

The last two points are the ones that call our attention the most because, indeed, Bergmann's rule could explain an extreme adaptation to inclement weather. At least on paper, larger species would have a greater ability to survive periods of resource scarcity (for their greater energy reserves in more voluminous tissues), in addition to allowing them to preserve their body heat in a more effective.

The physics of the application

It's time to get a little technical, but don't worry: you'll understand perfectly the following lines. According to Bergmann, large animals have a lower surface area/volume ratio. In a demonstrated way, a living being with a high body surface/volume ratio is “more” in contact with the environment. For this reason, humans present lungs with multiple chambers, as it is an effective way of increase the tissue surface in contact with the air, which allows us to capture oxygen more effective.

Thus, an animal with a low surface area/volume ratio radiates less body heat per unit mass, which is why it will stay warmer in cold environments. Hot environments pose just the opposite problem, since the heat produced by metabolism must be quickly dissipated 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 colder.

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examples

It is surprising to learn that Bergmann's rule is perfectly applicable to human beings under certain specific conditions. For example, human populations inhabiting the poles have been shown to be heavier in build than those closer to the equator in general, a fact completely consistent with the postulation presented here.

On the other hand, a study in 2019 collected on BBC News showed that a group of monitored birds reduced over the course of 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 hotter it is on Earth, the more size reduction species experience.

As far as mammals are concerned and beyond humans, deer are a "book" case of Bergmann's rule. It has been observed that deer species from northern regions tend to be larger and robust, while those that inhabit areas closer to the equator tend to be smaller and thin. Again, the postulation is fulfilled.

Notably this rule is generally applicable to birds and mammals, although the intrinsic genetic properties of the populations must also be taken into account, pressures of natural selection other than temperature and stochastic events such as drift genetics. In nature there are generalities, but of course these hypotheses cannot be applied in an unchangeable way to all living beings.

Allen's rule

We do not want to stay on the surface and delve a little deeper into the world of thermoregulation, because Allen's rule also provides us with various concepts to take into account when it comes to this topic. refers. This hypothesis postulates that, even with the same body volume, homeothermic animals must 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 see that it has flat, small ears and a considerable amount of hair. On the other hand, a desert fox or fennec has ears that are disproportionate in size compared to the rest of its body. Multiple studies in laboratory settings have shown that cartilage size can increase or decrease in species depending on environmental conditions to which they are exposed over generations.

This makes all the sense in the world: at the same amount of bulk from a theoretical standpoint, a fennec has much more body surface area due to its huge, flattened ears. This allows it to dissipate heat effectively, since these structures are also usually highly 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 acceptances

As we have previously said, conditioning the size of the animals exclusively to 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 worth it to have to look for accessory methods to dissipate your body temperature (behavioral changes, for example) and still be able to face your opponent? Nature is not based on black and white, but each factor represents one more point on a gray scale that models what we know as natural selection..

On the other hand, it is also necessary to note that this rule is not fulfilled 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.

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Summary

As we have been able to see 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 inhabit. Of all this terminological conglomerate, it is worthwhile for us to make one single concept clear: the smallest animals are theoretically more efficient when it comes to dissipating heat, while the largest ones excel in their ability to store it.

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

Bibliographic references:

• Adams, D. C., & Church, J. EITHER. (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 Leon, A., & Chediack, S. AND. (2018). Patterns of richness and latitudinal distribution of caviomorph rodents. Mexican Biodiversity Magazine, 89(1), 173 - 182.
• L'heureux, G. L., & Cornaglia Fernández, J. (2016). Ecomorphological variations of guanaco populations in Patagonia (Argentina).
• Mousseau, T. TO. (1997). Ectotherms follow the converse to Bergmann's rule. Evolution, 51(2), 630-632.
• Bergmann's Rule-Introduction for Educators, fieldmuseum.org.