This means that if you live in a cold climate you will find a certain advantage in being somewhat spherical -- being short and thick -- which is essentially the case for, say, walruses, Snowshoe Hares, and Eskimos. The Black-tailed Jackrabbit at the upper left and the Arctic Hare at the upper right clearly take advantage of this phenomenon. The Black-tailed Jackrabbit, a resident of hot, arid areas, has long ears, long legs and a longish head. The Arctic Hare's ears are much shorter relative to the head size, plus the head itself is more spherical, and the legs are relatively shorter than the Black-tailed Jackrabbit's.
In fact, in ecology there's a "rule" recognizing this principle at work among geographical races of single species. Called Allen's Rule, it states that certain extremities of animals are relatively shorter in the cooler parts of a species' range than in the warmer parts. By "extremities" is mainly meant arms, legs, ears, and snout or nose.
It's also a matter of basic physics that the larger a sphere, the less is its surface area relative to its total volume. Therefore, large balls lose heat more slowly, relative to their size, than small ones. You might guess, then, that animals tend to be larger in cold areas than in tropical ones. In fact, Bergmann's Rule asserts that geographic races of a species possessing smaller body size are found in the warmer parts of the range, and races of larger body size in cooler parts.
Turning from spheres, Gloger's Rule states that dark pigments increase in races of animals living in warm and humid habitats. Of course, among the animal species supporting this "rule" is humankind.
With special reference to birds in the Northern Hemisphere, the Egg Rule states that the average number of eggs in a set, or clutch, laid by songbirds and several other kinds of birds increases as one moves north in latitude. This state of affairs probably evolved to balance the fact that in the north songbirds may be able to raise only one brood per season, while farther south, two, three, or more broods may be possible.
Sometimes, when we are alone in the forest or at the beach, or even during a quiet moment in the backyard, thinking on these "rules" as wild plants and animals live their lives around us, it's almost as if we can hear the old philosophers' "music of the spheres."