Bergmann’s rule: what it is and how it describes animals
Bergmann’s rule: what it is and how it describes animals : The human being has already described, throughout its history, a total of 1,326,337 animal species. This value fluctuates continuously because, in addition to the new living beings discovered, experts from the United Nations (UN) point out that around 150 species become extinct every 24 hours. Of course, when it comes to biodiversity, the current picture is not encouraging.
Zoology is a branch of biology that is responsible for imposing a bit of order in this entire vital conglomerate, since it mainly studies 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 1847, is known as Bergmann’s rule. This postulation is linked to the distribution and morphology of the species according to the 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, keep reading.
What is Bergmann’s rule?
Bergmann’s rule is defined simply: the tendency for a positive association between the body mass of the species in a higher monophyletic taxon and the latitude inhabited by those species. In a slightly kinder way, endothermic animals (capable of maintaining a metabolically favorable body temperature regardless of the environment) are larger in cold climates than in hot areas.
This rule has been tried to explain in various ways. We show them briefly below:
- It has been tried to demonstrate as an artifact of the phylogenetic relationships between species, that is, different species are distributed in different latitudes.
- An attempt has been made to explain 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 endure longer without eating.
- Because of the ability of species of different sizes to conserve or dissipate heat.
It is the last two points that most attract our attention since, effectively, Bergmann’s rule could explain extreme adaptation to inclement weather. At least on paper, the larger species would have a greater capacity to survive periods of scarcity of resources (due to their greater energy reserves in more voluminous tissues), in addition to allowing them to preserve their body heat in a more efficient way.
The physics of the application
It’s time to get a little technical, but don’t worry: you will understand the following lines perfectly. According to Bergmann, large animals have a lower surface / volume ratio. In a demonstrated way, a living being with a high body surface / volume ratio is “more” in contact with the environment. That is why humans present lungs with multiple chambers, as it is an effective way to increase the tissue surface in contact with air, which allows us to capture oxygen more efficiently.
Thus, an animal with a low surface area / 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 colder.
It is surprising to know that Bergmann’s rule is perfectly applicable to human beings under certain specific conditions. For example, It has been shown that human populations that inhabit the poles are of heavier constitution than those closest to the Equator in general, made completely consistent with the postulation presented here.
On the other hand, a study in 2019 collected in the 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 result completely significant. This could be explained based on climate change: the hotter it is on Earth, the more size reduction the 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 the species of deer in the northern regions tend to be larger and more robust, while those that inhabit areas closer to the equator tend to be smaller and thinner. Again, the application is fulfilled.
Notably this rule is generally applicable to birds and mammalsHowever, the intrinsic genetic properties of populations, natural selection pressures other than temperature, and stochastic events such as genetic drift must also be taken into account. In nature there are generalities, but of course these hypotheses cannot be applied immovably to all living beings.
We do not want to stay on the surface and delve a little deeper into the world of thermoregulation, since Allen’s rule also provides us with various concepts to take into account when it comes to this issue. This hypothesis postulates that, even with the same body volume, homeothermic animals must show different surface areas that will help or prevent their heat dissipation. Let’s take a simple example.
If we look at an arctic fox, we can see that it has flat ears, small and with a considerable amount of hair. On the other hand, a desert or fennec fox has oversized ears 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 the environmental conditions to which they are exposed throughout generations.
This makes all the sense in the world: at the same amount of volume from a theoretical point of view, 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 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 meanings
As we have said previously, conditioning the size of the animals exclusively to the latitude of the environment can lead to error. 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? Does it pay you more to have to find accessory methods to dissipate your body temperature (behavioral changes, for example) and still be able to face your rival? 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 made multiple professionals and thinkers subject it to scrutiny throughout history.
Bergmann’s rule : Summary
As we have seen in these lines, Bergmann’s rule can explain, to some extent, the reason for the variability of size between species according to the latitude of the ecosystem in which they inhabit. From all this conglomeration of terminology, it is enough to make a 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 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 living beings, of competition, of food chains, of sexual selection and of many other parameters, both biotic and abiotic. .
Bergmann’s rule : Bibliographic references:
- Adams, DC, & Church, JO (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, SE (2018). Wealth patterns and latitudinal distribution of caviomorphic rodents. Mexican Journal of Biodiversity, 89 (1), 173 – 182.
- L’heureux, GL, & Cornaglia Fernández, J. (2016). Ecomorphological variations of the guanaco populations of Patagonia (Argentina).
- Mousseau, TA (1997). Ectotherms follow the converse to Bergmann’s rule. Evolution, 51 (2), 630-632.
- Bergmann’s Rule-Introduction for Educators, fieldmuseum.org.