Indirect rival assessment in a social primate, Theropithecus gelada

Identifying the cognitive challenges of sociality can be difficult because similar social interactions may be based on very different cognitive mechanisms. To better understand the cognitive mechanisms associated with a particular interaction, we investigated the social information that gelada, Theropithecus gelada, bachelors use to assess rival males. Gelada bachelors may use a variety of assessment strategies, including attending to simple cues of immanent mating opportunities, vocal quality signals, and monitoring relationships between attached males and females. Our observational data indicate that bachelors preferentially eavesdrop on male–male fights (over copulations) and maintain close proximity to units that have experienced a recent take-over. As such units also experience elevated rates of fighting and extrapair copulations, bachelors appear to attend to cues of mating opportunities. Furthermore, bachelors monitor vocal contests between rival males, suggesting they are able to remember the quality displayed by competitors. Finally, we used playback experiments to simulate copulations between breeding males and ‘their’ females (control) or other females (test) and found no difference in bachelor response, suggesting a lack of knowledge about third-party relationships between males and females. In contrast to other social primates, gelada bachelors appear to use simpler information, such as outcomes of male–male competition, when assessing others. Given the size of gelada communities (>1000 individuals), the costs of monitoring third-party relationships may outweigh the benefits of obtaining this information, particularly when simpler rules of thumb are available.