Male animals of many species use conspicuous coloration to attract mates. Among mammals, primates possess the most brilliant secondary sexual coloration. However, whether colour plays a part in primate female mate choice remains unknown. Adult male rhesus macaques undergo a hormonally regulated increased reddening of facial and anogenital skin during their mating season. We experimentally investigated whether red male facial coloration is preferred by simultaneously presenting female rhesus macaques (n = 6) with computer-manipulated pale and red versions of 24 different male faces. The duration and direction of gaze were measured to discern visual preferences. Females exhibited preferences for the red versions of male faces. It is proposed that male coloration might provide a cue to male quality.

BackgroundMany animals both display and assess multiple signals. Two prominently studied traits are symmetry and sexual dimorphism, which, for many animals, are proposed cues to heritable fitness benefits. These traits are associated with other potential benefits, such as fertility. In humans, the face has been extensively studied in terms of attractiveness. Faces have the potential to be advertisements of mate quality and both symmetry and sexual dimorphism have been linked to the attractiveness of human face shape.Methodology/Principal FindingsHere we show that measurements of symmetry and sexual dimorphism from faces are related in humans, both in Europeans and African hunter-gatherers, and in a non-human primate. Using human judges, symmetry measurements were also related to perceived sexual dimorphism. In all samples, symmetric males had more masculine facial proportions and symmetric females had more feminine facial proportions.Conclusions/SignificanceOur findings support the claim that sexual dimorphism and symmetry in faces are signals advertising quality by providing evidence that there must be a biological mechanism linking the two traits during development. Such data also suggests that the signalling properties of faces are universal across human populations and are potentially phylogenetically old in primates.

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Pink-to-red anogenital and facial sexual skin occurs in females of many primate species. Since female sexual skin color varies with reproductive state, it has long been assumed that color acts to stimulate male sexual interest. Although there is supportive evidence for this as regards anogenital skin, it is unclear whether this is also the case for facial sexual skin. In this study we experimentally manipulated digital facial and hindquarter images of female rhesus macaques (Macaca mulatta) for color within the natural range of variation. The images were presented to adult male conspecifics to assess whether the males exhibited visual preferences for red vs. non-red female coloration, and whether preferences varied with anatomical region. The males displayed significantly longer gaze durations in response to reddened versions of female hindquarters, but not to reddened versions of faces. This suggests that female facial coloration may serve an alternative purpose to that of attracting males, and that the signal function of sexual skin and the intended recipients may vary across anatomical regions.

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In human males and females, bilateral symmetry of facial shape influences assessments of attractiveness. It is possible, however, that other primate species also possess preferences for conspecific facial symmetry. To assess this experimentally, we presented 13 adult rhesus macaques (8 females, 5 males) with computer-manipulated images of symmetrical and asymmetrical versions of opposite-sexed conspecific faces. We utilized looking behavior to assess visual preferences for these factors. We found significant preferences for symmetry, raising the possibility that human preferences for facial symmetry are more deeply rooted in our evolutionary history than previously realized. Our results also have implications for the use of facial shape as a mechanism for attractiveness appraisals across the Primates.

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