Most heat is lost near a body's surface.
Therefore, the less surface area a body has, relative to its total size, the less heat it will lose from its surface. It happens that of all the geometrical forms in nature, the one with the least surface area relative to its total size, or unit volume, is the sphere . The Black-tailed Jackrabbit at the upper left, and the Arctic Hare at the upper right, clearly take advantage of this phenomenon. The jackrabbit, a resident of hot, arid areas, has long ears, long legs and a longish head. The 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 jackrabbit's.