The 85th Annual Meeting of the American Association of Physical Anthropologists (2016)

Evidence that male face shape in rhesus macaques (Macaca mulatta) is subject to inter-sexual selection


1Department of Life Sciences, University of Roehampton, 2Department of Zoology, University of Cambridge, 3Department of Anthropology, Northwestern University, 4Institute for Mind and Biology, University of Chicago

April 16, 2016 4:30, A 703/704 Add to calendar

Sexual selection acts on traits underlying within-sex variation in mating success. Under inter-sexual selection, traits that are attractive to females can grant a mating advantage to males. Morphological traits may be attractive because they are associated with physiological, morphological, or behavioral characteristics indicating male quality. In this study, we tested the hypothesis that the evolution of sexually dimorphic male face shape in rhesus macaques is subject to inter-sexual selection. We analyzed facial photographs of rhesus macaques (N=71 male, 28 female) and found significant sex differences in 6 of 8 facial measures. We used these to calculate an index of facial masculinity (FM) and selected 20 male photos–ten each with low and high FM scores. Paired low-high FM images were shown to free-ranging female rhesus macaques during the mating season in a differential looking-time experiment.

Females looked significantly longer at high FM stimuli when the difference in masculinity scores within the stimulus pair was high, and this attentional bias toward high FM stimuli correlated positively with the absolute difference in FM. Furthermore, contrary to what one would expect if the attentional bias were due to threat, we found no difference in latency to disengage depending on FM. This supports the view that females’ attentional bias was explained by attraction rather than fear. Further research should elucidate the information content of high FM, in regards to its function as a potential indicator of genetic and/or phenotypic quality, as well as its possible relation with male mating and reproductive success.

Funding: Partially supported by grants from the American Society of Primatologists, The International Primatology Society and The Leakey Foundation (to AVG).