Sexual dimorphism in primates
It is taken for granted in human societies that men are, on average, slightly taller, larger and heavier than women, despite a considerable overlap in their respective weight distributions. However, among mammals in general, intersexual size differences within a single species do not always favour males and range from leopard seals, with the female 20% longer and correspondingly heavier than the conspecific male, to gorillas with the male almost twice the size of his mate. There is ample evidence that body size plays a fundamental role in relation to an animal's survival. Consequently, when males and females of the same species attain different adult body weights, these should be seen in the overall context of divergent life history strategies, as emphasised by the typically later achievement of sexual maturity in the larger bodied sex. Most explanations of sexual size dimorphism in primates tend to be male-centred. They typically emphasize competition between males for females and protection of the social group by larger bodied males. However, such accounts are commonly marred by circular arguments and post hoc rationalisations. They are also self-defeating in their neglect of the possible effects of natural selection acting on females. The present research examines the ontogeny of sexual size dimorphism in terms of the divergent energetic needs of males and females. An allometric approach has been adopted, and the frustrations of circularity overcome by exploiting the special relation which exists between brain size and body weight. The results indicate that, at least for simian primates, body size reduction in females has played a major role in the evolution of sexual size dimorphism. For several species of larger bodied primates this difference has apparently been enhanced by body size increase in males. The scaling of molar tooth area with body weight corroborates these findings.