Come with me: experimental evidence for intentional recruitment in Tonkean macaques

Recruitment is a process by which animals can initiate collective movements: the action of an individual prompts conspecifics to follow. Although it has been hypothesized that animals may be able to intentionally recruit others, there is no experimental evidence of this to date. We tested this hypothesis in two pairs of Tonkean macaques in a situation requiring the subjects to find a food site in a 2800 m² area, and approach the site together to release rewards. Each subject was informed of the location of either highly or little-valued rewards. We recorded attention-action sequences in which an individual checked that his partner was attending to him before moving, and also simple departures (i.e., not preceded by eye contact). Analyses showed that sequences were more often followed by recruitment and leading the partner to a baited site than simple departures were. Moreover, subjects used attention-action sequences more frequently when informed of the location of the highly valued reward. This may be explained by the fact that the more motivated they were by the expected rewards, the more likely they were to actively recruit their partner. No such effect was found when subjects performed simple departures. We conclude that Tonkean macaques are capable of intentional recruitment because the subjects voluntarily behaved with the goal of influencing their partner's movement: they checked that the partner was paying attention to them and prompted him to follow by moving. Such performances can be accounted for either by associative learning or by intentional communication.