It is safe to say that we are experiencing a genuine Wisconsin winter right now—the snow, the cold temperatures, short days and seemingly endless nights.
In this week's episode of Wildlife Matters, the Masked Biologist examines Bergmann’s rule, which helps explain how our wildlife can handle Northwoods winters.
In my travels around the country, when people learn where I live, they frequently ask how I can stand our winters here. Well, I grew up here. These are the winters I need to feel like we have moved through our seasons, through our year. When I lived in other states that were west, southwest, or south of here, I was genuinely dissatisfied with the winters. Some were bitter cold, but had no snow. Some (like Kansas) had more freezing rain than snow. I like to have a nice blanket of snow, and freezing temperatures. Besides, living somewhere with cold snowy winters helps assure me that our summers won’t be terribly hot or dry. I may be able to handle cold temperatures, but I cannot handle heat. You might say I am adapted to our climate here. When I lived in Kansas for a few years, I may have begun to adapt; I was too warm in the winter, rarely wearing more than an insulated flannel. In the summers, though, with temperatures over 100 degrees and tropical humidity levels, I had no mechanism to help me stay comfortable other than air conditioning at home. At work I would sweat so severely that I would get rashes, and had to bring a second change of dry clothes to put on at lunchtime.
We had white-tailed deer in Kansas, the same exact deer that we have here. Well, maybe not exactly the same. Genetically there are some differences, adaptations for climate. If you took the deer from Vilas County and moved some to Kansas, they would likely die from heat stress in summer. Likewise, if you brought deer from Kansas up here, they would need some special help to get through our extremely cold and snowy winters. This is an example of the application of one of my favorite ecological principles, Bergmann’s Rule.
Bergmann’s Rule put simply would state that the further north in latitude an animal lives, the greater body mass it needs to survive or thrive. It has to be bigger. 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. So a large animal in warm climates would need special additional adaptations to stay cool. Elephants are a great example; they have huge ears, way bigger than they would need to find their food (plants) or detect predators. Their ears have a large complex network of blood vessels that act like car radiators. When the elephants flap their ears, it cools the blood before returning to the body. Without such adaptations, they would probably need to spend a lot more time in the water (like hippos) or they would not be able to survive. The smallest elephants would be most likely to survive because they would stay cooler. Over time, these smaller elephants would breed and the result would be an adapted species that is a smaller size due to its environment.
So if you look at species that persist here in the Northwoods and other parts of the country, they would in theory be bigger. Deer and wolves are two obvious examples. In fact, 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 moose, caribou or elk you are going to have to be well-equipped for it.
Not all animals get larger up north, though. Rodents are an example; northern chipmunks and flying squirrels are notably smaller than their southern relatives. Bergmann’s rule seems to have a notable exception in rodents, or more specifically in creatures that hibernate, because they have a mechanism to offset the effects of winter cold and consequently have less concerns about conserving body heat.
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.