Bergmann’s Rule: What It Is And How It Describes Animals
Throughout its history, human beings have already described 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 outlook 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, It 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 at many points. If you want to know what this interesting idea consists of and what its practical applications are, continue reading.
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 (able to maintain a metabolically favorable body temperature regardless of the environment) are larger in cold climates than in hot areas.
This rule has been attempted to be explained in several ways. We show them briefly below:
It is the last two points that draw our attention the most because, indeed, 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 (due to their greater energy reserves in more voluminous tissues), in addition to allowing them to preserve their body heat more effectively.
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 It has been demonstrated that a living being with a high body surface/volume ratio is “more” in contact with the environment. That is why humans have 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-to-volume ratio radiates less body heat per unit of mass, which is why it will remain warmer in cold environments. Hot 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.
Examples
It is surprising to learn that Bergmann’s rule is perfectly applicable to human beings under certain specific conditions. For example, It has been shown that the human populations that live at the poles are of a heavier constitution than those closer to the Equator in general a fact completely consistent with the postulation presented here.
On the other hand, a 2019 study reported on BBC News showed that a group of monitored birds reduced the length of certain body structures by up to 2.4% over generations (1978-2016), a completely different result. significant. This could be explained based on 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 “textbook” case of Bergmann’s rule. It has been observed that deer species from northern regions tend to be larger and more robust, while those that live in areas closer to the equator tend to be smaller and thinner. Again, the postulation is fulfilled.
Notably This rule is generally applicable to birds and mammals , although we must also take into account the intrinsic genetic properties of populations, natural selection pressures other than temperature and stochastic events such as genetic drift. In nature there are generalities, but of course these hypotheses cannot be applied in an immovable 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, since Allen’s rule also provides us with various concepts to take into account when it comes to this topic. This hypothesis postulates that, Even with the same body volume, homeothermic animals must show different surface areas that will help or impede 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 with a considerable amount of hair. On the other hand, a desert fox or fennec has ears of disproportionate size compared to the rest of its body. In multiple studies in laboratory environments it has been shown that Cartilage size can increase or decrease in species depending on the environmental conditions to which they are exposed throughout the 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 enormous, 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 meanings
As we have previously said, conditioning the size of 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? 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 rival? Nature is not based on black and white, but each factor represents another 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 met 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 depending on the latitude of the ecosystem in which they live. From all this terminological conglomeration, it is enough for us to make a single concept clear: smaller animals are theoretically more efficient when it comes to dissipating heat, while larger 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 a product of temperature, but also of humidity, relationships with other living beings, competition, food chains, sexual selection and many other parameters, both biotic and abiotic. .
