Nature isn’t inert or homogeneous; thousands of processes and phenomena take place in it every second, leading to significant environmental changes. Nature is a living medium, with different habitats in which countless physical, chemical and, of course, biological interactions happen on a daily basis, creating an environment full of nuances and shades. Thus, it’s extremely difficult to discover laws that determine the workings of nature (and its inhabitants).
It’s reasonable to think that the different populations of the millions of species (and subspecies) that inhabit the planet don’t all follow the same patterns and rules, but rather present significant variations depending on their environment. In other words, important differences can be found in the features of different populations of the same species. Such variations may refer both to animal behaviour and to physical features and alterations, visible through phenotypes: different colouration, patterning, calls, body weight, etc.
In this article, we’re going to focus on one of these distinguishing features: body size. We’ll explain the mechanisms that cause size variations in different populations and individuals of the same species, and we’ll clarify some considerations.
Bergmann’s rule and the geography of body size
Carl Bergmann (1814-1865) was a German naturalist and biologist who, in 1847, developed one of the most important biogeographical rules in the field of biology and ecology: a pattern known as “Bergmann’s rule“. Bergmann described a series of patterns related to the body mass of certain animals (he focussed on endothermic mammals and birds), and reached the conclusion that there’s a tendency for the average body size of a species population (or species within a single taxon) to increase towards higher latitudes and altitudes. That is to say, larger body sizes are observed in populations that live farther from the equator or in high-altitude regions.
This tendency was initially attributed to the fact that there is a decrease in the surface area to volume ratio (S/V) when species increase in size, allowing them to retain body heat more efficiently in cold climates. Thus, in terms of thermoregulation, a larger body size would be an advantage for species in regions with low temperatures. However, the principles of Bergmann’s rule have been criticised and questioned by many.
Criticism of Bergmann’s rule
Most critics of Bergmann’s rule believe that there is no correlation between temperature and body mass, and that many other factors can affect body size and cause variations between populations.
The first criticism is that an increase in volume doesn’t necessarily mean that temperature is regulated more efficiently, but will rather result in more body heat loss and a requirement for more energy (and, consequently, more food intake). Thus, body mass would decline in high latitudes and altitudes, where food becomes a limiting factor.
By comparison, there are several observations on hibernating mammals that contradict the theory that an increase in size reduces body heat loss. In the case of this group of animals, heat loss could be prevented through more effective methods that would require less energy; for example, by isolating and protecting themselves, may that be through hibernation or thanks to a thick fur coat.
A third criticism points out the fact that the same tendency is found in some ectothermic or cold-blooded animals (amphibians, reptiles, insects, fish, etc.), which in principle shouldn’t follow the rule, as they’re unable to generate internal heat by themselves and depend on external heat sources.
Are there alternative arguments that support Bergmann’s rule?
Despite the criticism, there are alternative explanations that justify an increase in body size within a species. One of them is that a larger body will generally be safer in cold climates, as the minimum temperature threshold (the temperature below which an endotherm must dedicate more metabolic energy to maintaining body heat) decreases as body mass increases. This means that, the larger the animal, the less energy it’ll need to control its temperature.
Another hypothesis argues that animals with higher body mass can store more reserve substances, allowing them to go without food for longer periods. Antlions, for example, benefit from this advantage.
There is also evidence of a latitudinal gradient of diversity: species richness decreases towards the poles of our planet. It’s logical to think, therefore, that if species abundance decreases towards high latitudes, so does the number of ecological interactions, including competition and predation risk. This can reduce the pressure on certain traits of species, allowing them to develop a larger body size. In this sense, the body size of certain carnivores seems to be related to the density of prey and the absence of larger competitors.
What animals follow Bergmann’s rule?
As stated at the beginning of this article, nature is full of variations and nuances. Therefore, it’s important to highlight that Bergmann’s rule is simply a rule, not a law… so not all species comply with it. This pattern doesn’t apply equally to all species (there is more evidence for it in endotherms than ectotherms), but it’s safe to say that Bergmann’s rule is an intraspecific and interspecific pattern in 70% of mammals and birds. Two examples of species that follow Bergmann’s rule are elks and muskrats.
In contrast, there are many species that work the other way around; the largest vascular plants, for example, as well as the largest individuals of many invertebrate species, are found in warm and tropical regions (close to the equator), where there are no limiting factors. Additionally, species with short life cycles (such as insects in temperate regions) have fewer generations per year, rather than a smaller body size.
Translated by Carlos Heras