Within the same species of warm-blooded animals, individuals from populations living in colder environments usually have greater body mass than do those from populations in warmer areas. This is because of the selective advantage it provides. A massive body produces more internal heat and radiates relatively less of it into the surrounding environment because the skin surface area is relatively smaller. Subsequently, a massive body produces and retains more heat. Bergmann's Rule was named after Carl Bergmann, a19th century naturalist. See Allen's Rule.