Wildlife needs to adapt to its place on the globe to survive, not to mention thrive.
In this installment of Wildlife Matters, the Masked Biologist takes a look at another generally accepted principle of adaptation to life in the north: Allen’s Rule.
Earlier this winter, actually last December, I wrote about Bergmann’s rule, a generally accepted principle that relates to wildlife adaptation and survival in northern climates in winter as compared to southern climates in summer. In it’s simplest form, Bergmann’s rule states that body size is larger at northern latitudes than in the same species or closely related species at southern latitudes. The principle relates specifically to body heat regulation; larger animals have more mass, and are able to keep body heat in. The tradeoff then is that in the summer, these animals have a harder time shedding heat. My example was the wolf; the Mexican gray wolf is considered the smallest of the five subspecies of gray wolf in North America, and the Canadian timber wolf is considered the largest. This makes sense, not only because they need to thermoregulate (manage body temperature) but because of the relative size of their prey. If you are going to take down large moose, caribou or elk you are going to have to be well-equipped for it.
Allen’s rule is very similar. In fact, it is based on the identical thermoregulation principle: the smaller your ratio of surface area to volume, the less heat you will lose, and the more slowly you will lose it. In its simplest form, Allen’s rule states that body form or shape is linear in warm climates and more rounded and compact in cold climates. So, while Bergmann’s rule is focused on “larger” Allen’s rule refines it a step, if you will, to “more compact.” This point occurred to me one day while I was selecting something to post on Facebook, and I came across a great photo of an arctic fox. Arctic foxes are not big animals. Do they violate Bergmann’s rule? Well, if you recall from my previous article (it’s ok if you don’t), if you violate this rule, you need an adaptation, like small animals up north hibernating or large animals (like elephants) having built in radiators to stay cool.
The arctic fox is a small fox, but not the smallest on the continent. The kit fox of the desert southwest is far smaller. So Bergmann’s rule applies. Taking it a step further, Allen’s rule also applies. If you compare the two side by side, the arctic fox has much smaller ears, a shorter snout, and shorter stouter legs. Having smaller body parts and reduced extremities helps reduce how much surface area allows the wind and snow to strip precious heat from the arctic fox.
To apply Allan’s rule strictly across the board would be inappropriate, I think, especially without using Bergmann’s rule in concert. Using the example of wolves, the largest wolves on the continent are in the arctic, and the smallest are significantly closer to the equator. Does this mean that the legs of Canada timber wolves are shorter than the legs of Mexican gray wolves? Remember, this is a rule of ratios; you would need to measure the length of the largest bone of the leg, the femur, and compare it to another measurement of size (overall body length, for example). It would be interesting to drill into this further, but perhaps that will have to wait for a future article.
When we see a clear violation of Allen’s rule, we expect a special adaptation to be involved. The best example I can think of regionally is the moose. We have moose in the northern U.S., with resident populations in Minnesota and Michigan as well as annual observations here in Wisconsin. Moose are large…very large. They have heavy massive bodies, and they can only survive in areas where it gets very cold in winter. In fact, they have a lethal upper limit of winter temperatures, which means if it doesn’t regularly get below zero in the winter their chances of surviving is reduced. Yet they have very long thin legs. Well, their adaptation is fascinating. They have cross-current blood flow in their legs. This means they allow their blood to cool dramatically as it flows down to their feet, then comes back up and is warmed by outgoing blood so that as it continues its trip back into the chest cavity the blood returns to internal body temperature and the animal loses relatively little body heat in the process. However, it doesn’t do a great job of shedding heat in the summer, which is why this amazing animal needs to stay far north or in higher altitudes in order to thrive.
Striving to make new things familiar and familiar things new, this is the Masked Biologist coming to you from the heart of Wisconsin’s great Northwoods.