Capuchin monkeys do not differentiate between images of different facial width

Previous research has shown links between the facial Width-to-Height Ratio (fWHR) and Assertiveness in brown capuchins, Sapajus apella. Similar to research in humans that suggests the male face may be a cue to dominance-related traits such as aggression, in capuchins, wider faces are associated with higher Assertiveness. In the current study we assessed whether capuchins differentiate between wide versus narrow faces. Using facial photographs manipulated for higher or lower fWHR, we examined capuchins' latency to approach a wide or narrow face, and compared this with their latency to approach an alpha versus non-alpha group member. We predicted that if capuchins perceive wider faces of unfamiliar individuals as more assertive than those with narrower faces, then they would exhibit a longer latency to approach the wide over the narrow face condition. Whilst capuchins did exhibit a significantly longer latency to approach alpha versus non-alpha group members, there was no significant difference in their latency to approach either manipulated facial condition. These results suggest that capuchins do not respond to static images in the same way as they respond to real conspecifics, or that they do not use facial width as a cue to Assertiveness in their approach behaviour. We propose alternative methodology to explore whether face width is a cue to social status or assertive traits in nonhuman primates.