Sexual dimorphism in body size: Are larger species more dimorphic?

Abstract

It is frequently stated that larger species show greater sexual dimorphism in size. Here the evidence for this statement is reviewed and the theories which predict an association between size and dimorphism considered. In several taxa larger species are more dimorphic, though there are exceptions. Indeed in the Mustelidae (weasels, otters, etc.) smaller species are more dimorphic. The main reason why an association between size and sexual dimorphism sometimes exists is probably because, on an evolutionary timescale, ecological factors such as food distribution affect both size and the opportunity for polygyny—polygynous species tend to be dimorphic—rather than because of a direct causal link between size and dimorphism. When the effects of polygyny on sexual dimorphism are removed, in only primates and small mammals is there still evidence of a link between size and dimorphism. The theories that predict a causal link between size and dimorphism are generally unconvincing.