As the Northern Hemisphere continues to warm up, natural selection will reverse a long-term trend in warm-blooded animal evolution known as Allen’s Rule. Mammals that stay active in the winter tend to have thicker fur than those that hibernate, just as the plumage of seabirds is thicker than that of land birds in general.
Water is a much better conductor than air and the waterfowl that rest on the water or dive under it must be well insulated or else they would perish from hypothermia. They don’t make pillow and coat stuffing out of the feathers of land birds. They use eiderdown, which is a very good insulator.
The appendages of mammals radiate body heat into the atmosphere. The longer or larger they are, the better radiators they are. The size of the ear serves as a good example. With the exception of humans, perhaps, in northern latitudes mammalian ears tend to be much smaller than those in tropical and subtropical areas. Tails are also heat radiators, that’s why most mammals with tails curl up when sleeping. The closer the tail and the legs are to the body, the less the heat loss.
The furs of northern fur-bearing animals such as fishers, martins, lynxes, and minks were highly sought after by the French and English settlers in the 1600s, not just for local use, but for export to Europe. The fur trade was a very profitable industry for those seeking out livings in the New World.
Allen’s Rule states that for any group of mammals, say, bears, rabbits, or squirrels, the more boreal the mammal, the shorter its ears and the shorter its tail. Polar bears have smaller ears than southern black bears; snowshoe rabbits have shorter ears than jack rabbits. Jack rabbits released on the tundra for stocking purposes would not fare well at all.
Just like the primates we stem from, humans are tailless and have comparatively small ears. Nevertheless, these small human ears radiate a lot of heat. Touch your unprotected ears on a very cold night and they can feel ice cold. Many mammals with longish ears can fold them back against the body when the going gets cold. We wear hats with flaps covering the ears for the same reason.
Another rule that applies to mammals residing more northerly than their southern look-alikes is that their bodies are more massive. Think of the polar bear and the Alaskan brown bear: They are far more massive than the black bears that range in the Smoky Mountains or southern Rockies. It seems to be a paradox. On the one hand their ears and tails are smaller, but their bodies are larger.
The larger bodies have to do with the surface area to mass ratio. The surface, or integument, of the mammal’s body is responsible for most of the heat loss. That is why we insulate it on cold days with layers of fabric, sometimes in a sandwich filled with down. Rotund bodies have less surface area per unit of mass than slim ones. Take shrews for example. They’re smaller than mice and their surface area to body of mass ratio is among the highest in all mammals. They lose so much heat through their integument that they have to eat four or five times their weight every day in the winter in order to keep from losing out to hypothermia.
Yes, obesity is not popularly represented in the Styles section of The New York Times, but in older times it could mean the difference between survival and loss in cold climates. All mammals in colder climates store up body fat to get through the winter, why not humans? It’s a good insulator and energy source.
There is a downside to the larger bodies and smaller ears. Smaller ears don’t hear as well as larger ones, especially those that are both large and can be directed toward a sound source. As the climate heats up, heat loss will not be as important. Those individuals born with larger ears and developing smaller bodies will have an advantage over their littermates. In time evolution will move in the opposite direction.
In humans it will be different. We are on the verge of making the selection ourselves with the help of genetic counselors, based on sex, I.Q., size, and other attributes. Couples that want athletes will be able to select for them, those that want girls and not boys will have girls, and vice versa. You want your son or daughter to be eligible to join Mensa? That will be possible, too. In other words, we are coming very close to being able to produce superhuman progeny. It’s been done in livestock, dogs, cats, and other domestic mammals and birds for centuries.