Bergmann’s Rule

Bergmann’s Rule by Kevin A. Gardner
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There exists in nature, a phenomenon regarding animal body size within the same species in two different parts of the globe. A whitetail deer in the southern part of the United States, for example will have a smaller body size, or less overall mass, than that of a whitetail deer in the far northern states. This difference or variance can be somewhere from marginal, to noticeable, to extreme depending on the species compared. Research has determined that it has a great deal to do with the total body mass of the animal for the purpose of cooling the body and is actually an evolution within the species based on geographical distribution. At times this difference, coupled with pigmentation variances and other forms of habitat adaptation have undoubtedly been the cause of a subspecies designation; well that’s what the research indicates.

Most Wildlife Biologists and members of the Zoological circle are familiar with something called Bergmann’s Rule. While controversial and having been formed in the more than recent past, the rule applied by Bergmann is simple to state, but not to understand.

Christian Bergmann (May 18, 1814– April 30, 1865) was a well respected biologist, physiologist and anatomist who through research established Bergmann’s Rule. The rule is applied to mammals and is in its simplest terms stating that the further from the equator a given species is distributed, the larger in body size will be found in the regions furthest north.

This rule has come under fire repeatedly and has been disproved in isolated cases. However in a broader scope the rule seems to hold up quite well. This can explain the southern deer (more proximate to the equator) versus the northern deer (more distant to the equator) and their body size differences. Bergmann’s rule, however, was not validated within a small segment of the globe, like the United States, for example as the proving ground, it was better proven crossing many country boundaries from mid planet to the poles using specimen of a particular phylum, versus specimen of an order or suborder. What that means is that animals of the same body “form”, much like that of people having a common body “form”, were the study subject as opposed to a specific species like the whitetail deer, where there would not have been a population to span the study distance. Where the broader scope of “deer” (meaning deer in general not one species) facilitated the research much better.

Bergmann noted that the closer to the equator, the smaller in overall body mass a member of the same specie would have. It is believed to be directly related to the body’s ability to cool itself as needed. Many people like to look at Bergmann’s Rule the opposite way, as that the farther north, the larger they get. You decide how you want to view the position, because a belief in Bergmann’s Rule can be a belief in how the animal kingdom populated the earth from the start, did they populate upward (away) or downward (toward) the equator, a place I do not really want to take this article.

If Bergmann’s Rule is solid, we should seek larger bodied animals in the northern regions. There is however something to be said for distribution, habitat (or lack thereof) and density, in the 21st century that were not present when Bergmann was busy formulating, that can cause effect. Let’s look at another factor that can affect body size and mass within a species, barring the geographical element.

A study:

Much of this change in body structure has to do with things like competition within a species, terrain and climate. Look at the bighorn sheep for example; in just the state of Colorado, there exist the Rocky Mountain Bighorn Sheep and the Desert Bighorn Sheep. They exist within the confine of a “region” of each other, meaning the “southwest region” of the state. Yet comparatively, the Rocky Mountain sheep has a much blockier, heavy body and a smaller horn in comparison. The Desert sheep has a leaner body mass and a larger horn. While there would exist a degree of optical illusion because of the body mass variance, the animals do indeed use their food energy resources differently based on extreme climatic and population differences within a small range.

There are much fewer Desert bighorn than there are Rocky Mountain bighorn, thus there is less competition to bulk up against for dominance relative to breeding rights. While we are only talking about a single state, Colorado has both mountains and arid high desert within its boundaries. Therefore two distinct habitats exist back to back for easy comparison. The Rocky Mountain sheep has steep snowy mountains to contend with and can transition up to twenty miles from its summer to winter range in places, interacting with many others of the species along the way to compete with. The Desert sheep has a much smaller geography, with more stable climatic conditions which facilitates home ranges with minimal migration need, and much less encounter with others to compete with. The desert sheep in this case, uses less of its food energy for endurance and bulk to keep warm and compete, and thus allowing the excess food energy to transfer into horn mass. Remember, only that food energy not needed for sustenance can be transferred into horn or antler mass energy. This model supports Bergmann’s Rule in a small way, making it easier for us to wrap our brain around the concept.

To take this further, what is also unique about the two bighorn species is that they are so closely related, that the actual desert adaptation seems to be what has separated them after years of evolving and genetic resilience against drift within the subspecies. This makes them an awesome study in food energy use compared to animals with a several state distribution.

This same theory when overlaid to a situation like that of the deer population in Pennsylvania, helps to explain the spotty success of the program that Dr. Gary Alt initiated, where less animals are available to harvest, but the state now has a greater population of larger antlered animals than in the past. Competition has diminished in many areas, leaving more available food source, less actual need for body mass (however the animals are getting larger overall)and excess nutrition to make antlers. This is also why Pennsylvania is at a pivotal point in their management process and must decide if they want to propagate bigger bucks or more bucks, as the immediate future management plans will dictate how the population will “rebound” and mix. This trophy quality management plan, thus far, has come at the cost of great discontent among the hunters who want higher harvest opportunities.

Knowing that density and available food sources are driving body size within a given geography, we now start to see a greater predator population with animals, in body size, far exceeding what traditionally would have been supported by the latitude line.

Why does all of this matter? What does Bergmann’s rule and body size have to do with my hunting opportunities? It matters because we are seeing changes in the distribution of many animals into isolated populations which is dictated by human made boundaries in the name of development. With Bergmann’s rule looming to put a cap on body size, it would stand to reason that habitat and food sourcing will become critical in maximizing potential body size in our herd. To the contrary of Bergmann, adequate food sourcing will allow body mass increase if abundant (especially in animals that lack horns or antlers like coyote, bobcat and lion), allowing animals like the predators to maximize in body size and counter management objectives. This countering becomes significant as concentrations of animals grow in ever decreasing habitat size. This is already starting to play out. We see large predators in areas that have been devoid of them for years, now being a thriving habitat.

A study: While conducting research recently into an area in south-central Pennsylvania that is believed by some to contain a remnant population of Mountain Lion, I came across a print in the snow that I found interesting enough to document. The print appears to be that of an large cat, and based on the actual size of the print, has spiraled more than a few recent conversations into the realm of Bergmann’s rule and the existence of Lions in Pennsylvania. In later measuring against the print, it was found to be approximately 9 cm in length which is at the far end of that of a Bobcat, which are known to exist here, but on the small end of that of a Lion which is only believed to exist here. I added a comparison print to the image for reference from a known print from the Southwestern part of the country because the track orientation seems so similar that it made a great comparison. Many similarities exist, in fact enough to consider further research.

The relevance of this situation is that if a Bobcat made the print, it would be consistent with what Bergmann’s Rule would dictate at much higher latitude’s than that of Pennsylvania (largest Bobcat I can find on record is in the state of Maine at 76 pounds, again, up north). If in fact a Mountain Lion track, it would have to be from a young lion, yet one that is out of place and inhabiting an area believed to be devoid of the species, still obviously thriving enough to have young.

Are our actions creating island populations that have the potential to show remarkable growth and that support predators that contradict Bergmann’s Rule, a study that was conducted when nature held the reins more so than today? Is Bergmann’s Rule a limiting factor against our management objectives, or support for it as we work our plans? What can we learn from the bighorn sheep example about body size versus horn size, versus population, versus competition…etc. to aid our management plans? If nothing else, Christian Bergmann built a great model for teaching wildlife management, but he would be scratching his head if he saw what we’ve been able to do to it.

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