Within the same species of warm-blooded animals, there is a tendency for darker, more heavily pigmented skin to occur in animals near the equator and lighter pigmented skin farther from it. This is because of the selective advantage the coloration provides. Heavy pigmentation protects from skin cancer caused by ultraviolet radiation from the sun. However, too much pigmentation can significantly reduce the skin's ability to produce vitamin D, which is necessary for the absorption of calcium and phosphorus from food. Open country close to the equator typically experiences high amounts of intense ultraviolet radiation while temperate and arctic regions have relatively little. Subsequently, heavy skin pigmentation is an advantage near the equator but a disadvantage farther away from it. Gloger's Rule was named after Wilhelm Gloger, a 19th century naturalist.