A competitive drive? Same-sex attentional preferences in capuchins

In primates, faces provide information about several characteristics of social significance, including age, physical health, and biological sex. However, despite a growing literature on face processing and visual attention in a number of primate species, preferences for same- or opposite-sex faces have not yet been examined. In the current study, we explore the role of conspecific sex on visual attention in two groups of capuchin monkeys. Subjects were shown a series of image pairs on a Tobii Pro TX300 eye tracker, each depicting an unfamiliar male and an unfamiliar female face. Given the behavioral evidence of mate choice in both sexes, we hypothesized that capuchins would preferentially attend to images of unfamiliar conspecifics of the opposite sex. Our alternative hypothesis was that capuchins would preferentially attend to same-sex individuals to assess potential competitors. Our results provide support for our alternative hypothesis. When comparing attention to each stimuli type across sexes, females spent significantly larger percentages of time than males looking at female photos, whereas males spent significantly larger percentages of time than females looking at male photos. Within each sex, females looked for significantly larger percentages of time to female versus male images. Males also looked for larger percentages of time to same-sex images, though not significantly. To our knowledge, these data are the first to demonstrate significant sex-biased attentional preferences in adult primates of any species, and suggest that, for capuchins, potential competitors garner more attention than potential mates. In addition, our findings have implications for studies of visual attention and face processing across the primate order, and suggest that researchers need to control for these demographic factors in their experimental designs.