Vol. 43, No. 1
David M. Larson
Red-Bellied Woodpecker (Photograph by Peter Oehlkers)
Bergmann’s Rule indicates that, for birds and mammals, individuals in colder climes will be larger in body size and mass than those in warmer climes (Bergmann 1847). Although Bergmann described the relationship of species within a genus, this rule has been shown to be mostly valid for populations within species as well. Hence, resident Bald Eagles and white-tailed deer in Florida are considerably smaller than those in Massachusetts, and similar size gradients have been demonstrated in other widespread species, including Song Sparrows, Downy and Hairy woodpeckers, and many others. The classical explanation for Bergmann’s Rule is based on surface-to- volume ratio. For a given body shape, as linear dimensions increase, volume increases faster than surface area. Larger individuals, with relatively more volume to create heat, should have a selective advantage in colder climates because heat generation is a function of volume, whereas heat loss is a function of surface area.