One such rule holds that subspecies living in colder climates attain a greater body size than subspecies of the same species living in warmer climates. The reason would be that the volume of an object increases in proportion to the cube of its radius, whereas the surface area varies with the square of the radius. Because most heat loss takes place through exposed surface, a larger body would lose less heat. Another empirical rule proposes that extremities of animals are relatively short in the cooler regions of the range of a species, because considerable heat loss takes place through these more poorly insulated parts of the body.
It has also been proposed that, among human beings, dark skin is a necessary adaptation in tropical regions in order to protect against excessive vitamin D production. This notion has not received full scientific support. It is hard to establish whether climate alone, or climate in combination with other factors (such as food), is responsible for skin differences. Furthermore, the ways in which humans have learned how to deal with widely varying climatic conditions have proved so efficient that adaptation through marked physical changes was perhaps never necessary.