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Sexual dimorphism in apes and humans |
 One thing that humans clearly have in common with other apes is a difference in size between males and females, though this difference varies among different species. Male and female humans, for example, differ in body mass by around 13%.The degree to which male and female faces differ also varies among primate species. But can males’ larger head size explain the difference in form between male and female faces, or is the relationship more complicated? My research (pdf) on male and female human faces stemmed from an earlier study done on apes. The degree of difference between body size of male and female gorillas tends to be greater than that of male and female chimpanzees, yet male chimpanzees have broader faces for their size relative to females, whereas gorillas do not. This suggests that there is a selective advantage to male chimpanzees having relatively broader faces. Through examining different species of primate, I detected a link between canine tooth height and face breadth but the trend I observed was the opposite to what I had been expecting. Male primates with wide faces had relatively short canine teeth, whereas male primates without broad faces had relatively tall canine teeth. Large canines in primates are indicative of competitive and aggressive behaviour, and in some primate social systems this behaviour in males has a high reproductive payoff. However, my findings suggest that there may also be a reproductive payoff to having a wide face. Females may favour males with broad faces over males with large canines that signal aggression. Could a broad, shortened face increase male attractiveness? Might human faces tell the same story? Certainly human male canine teeth have become smaller as we have evolved – a fact which has never been fully explained.  In humans, the same type of sexual facial dimorphism as that found in the chimpanzee can be detected, but is less pronounced. Compared to women, men have shorter upper faces (taken to be the distance between the lip and the eyebrow) relative to their breadth. So in theory, an adult man with a lower ratio of upper facial height to breadth should appear more masculine.Psychologists have demonstrated that increased masculinity in male faces does not necessarily correspond to attractiveness. However, there maybe an optimal ratio of height to breadth of the male or female face that does correspond to attractiveness – one that is supported both by the human evolutionary record, and also by evidence from contemporary primates. Could these facial distinctions in men and woman just be part of our evolutionary legacy that although important to our ancestors has no real advantage today? Conversely, do the faces of our fossil ancestors and closest living relatives the chimpanzee hold the secret to human facial attractiveness? |