Within the same species of warm-blooded animals, individuals from populations living in colder environments usually have shorter appendages than do those from populations in warmer areas. This is because of the selective advantage it provides. Short arms, legs, and other appendages have relatively less skin surface area that can radiate heat into the surrounding environment. Subsequently, the body retains more of it. Allen's Rule is a corollary of Bergmann's Rule. Allen's Rule was named after Joel Allen, a 19th century naturalist.