The island of Spitzbergen, where I find myself at present, is a mere 700 miles from the North Pole. Karl Bergen's pub, the one and only hostelry within the "settlement", displays a notice in Norwegian on the door which reads: "The risk from polar bears in this establishment is rather small. We therefore ask our guests to deposit their weapons in the weapons cabinet. The key may be requested from the bar." Reminiscent of the old wild and woolly west, it sets the scene for life some distance north of the Arctic Circle.
Most of the polar bears, it seems, prefer to live some distance to the east of where I am at present. But they do appear occasionally, and you are warned that you may encounter one at any time on the island, and that they suddenly attack. You are advised, therefore, not to venture outside the settlements without a gun, or if this is impracticable - as indeed it is for most of us greenhorns from the south - then without an armed guard you must not wander farther than Karl Bergen's pub.
A full-grown polar bear is about eight feet tall when rearing up. A near namesake of the owner of the pub, a 19th century German biologist called Carl Bergmann, had an interesting theory on why they are so big. He noticed a curious pattern, related to temperature, to the average size attained by warm-blooded animals of the same species.
His theory, known as Bergmann's Rule, states that the smaller subspecies, a particular species, are to be found in the warmer parts of the ecological range, and that the larger examples inhabit the cooler districts. In the case of the cat family, for example, the largest is the Siberian tiger; in the case of bears, the polar bear, found only inside the Arctic Circle, is bigger than any others of its kind.
Bergmann's Rule is assumed to be an evolutionary consequence of the mathematical fact that as a three-dimensional object increases in size, the ratio between its surface area and its volume diminishes. If, for example, you double the length, width and height of an object, its volume increases by a factor of eight while the surface area grows only by a factor of four.
It follows that the bigger an animal happens to be, the smaller the area of skin it has in relation to its bulk. Since body heat is lost mainly through the skin, a large body therefore becomes an advantage in a cold environment where heat conservation is important. This, it seems, is why the local polar bears are so very, very big.