Facial attractiveness among rhesus macaques (Macaca mulatta) : manipulating and measuring preferences for conspecifics' facial characteristics
The face holds a central role in both human and nonhuman primate social interactions, through the communication of feelings and intentions via facial expressions and by acting as a means of recognising individuals. Humans, however, also employ their faces in mate attraction and assessment, an area that has received little attention in nonhuman primates. Many researchers have proposed that human aesthetic judgments of facial attractiveness have a biological basis, and these preferences have evolved via sexual selection processes during human evolution. The use of the face in attractiveness assessments need not be limited to humans. Rather, there is good reason to suggest that this may also apply to other nonhuman primates, based on homologies in the way in which primates use their faces, and on evidence that the face is a site of sexual selection for many primate species. It was the aim of this thesis to explore whether facial traits may also play a role in judgements of attractiveness in a nonhuman primate, the rhesus macaque( Macaca mulatta), in an effort to understand whether humans are unique in utilising the face as a mechanism of mate assessment. Three factors that are reported to influence facial attractiveness in humans are facial symmetry, sexual dimorphism, and averageness. To assess whether they also play a role in nonhuman primates, a series of experiments were conducted where digital images of adult male and female rhesus macaque faces were altered for these features. Opposite-sexed images were then displayed to adult males and females in a captive setting. Eye gaze measures were utilised to assess visual preference for, and the relative importance of, these traits. These experiments yielded mixed results. Increasing facial symmetry of opposite-sexed conspecifics positively influenced the dependent gaze measures employed here. Manipulating degree of facial sexual dimorphism had little influence on the visual gaze of either sex. Facial averageness positively influenced visual preferences for opposite-sexed conspecifics among both sexes, although increasing degree of averageness did not. The last topic to be explored was facial colouration. Rhesus macaques like, various other species of anthropoid primates, possess facial displays of red secondary sexual colouration. As above, animals viewed digitally altered pale and red versions of opposite-sexed conspecifics. Although females displayed preferences for red male faces, males displayed no clear preferences based on female facial colour. This raises the possibility that male and female facial colour may serve different roles in intraspecific signaling. While it cannot be concluded that visual preferences are indeed indicative of real-life preferences, the results do indicate that animals are not indifferent to variations in conspecific facial features. The present findings have important implications regarding the evolution of facial attractiveness, as they provide the first experimental evidence suggesting that facial features may serve as a mechanism for mate selection across primate taxa and that both human and nonhuman primates may employ similar criteria to appraise facial attractiveness.