Do l'Hoest's monkeys show sensitivity to conspecifics' informational state?

A series of field studies in chimpanzees have shown that the examination of communicative behaviour (e.g. alarm calling) could be particularly fruitful to help uncover mind-reading abilities in non-human animals. In this study, we sought to extend communication-based protocols to one species of Cercopithecids. Specifically, we looked at whether social gazing behaviour of L9Hoest9s monkeys (Allochrocebus lhoesti) is sensitive to the informational state of conspecifics using an original experimental design, thereby focusing on their ability to represent conspecifics9 rather than human experimenters9 mental states. We presented a group of 11 zoo-housed monkeys with a box that contained either an appetitive (mealworms), aversive (raptor stuffed toy) or neutral stimulus (wood bark chips). The discoverer (the first individual that inspected the content of the box) emitted more social gazes in the aversive condition than in the neutral and appetitive conditions. Besides, social gazing was related to the informational state of the conspecifics in the appetitive and neutral conditions, with more social gazes directed towards informed individuals (who had inspected the box) rather than uninformed ones (who had not inspected the box yet). Additional examination of the behaviour of the discoverers revealed that they were likely addressing social gazes to individuals who were in close proximity to them, suggesting that they preferentially gaze at informed conspecifics because the latter remained in proximity. Our study calls for a more widespread assessment of mind-reading capacities in primates, to further reveal the evolutionary history of traits that were thought to be uniquely human until fairly recently.