'Unwilling' versus 'unable': Tonkean macaques' understanding of human goal-directed actions.

The present study investigated the understanding of goal-directed actions in Tonkean macaques (Macaca tonkeana) using the unwilling versus unable paradigm, previously used in several species. Subjects were tested in three experimental conditions that varied according to the goal-directed actions of a human actor. In the "unwilling" condition, the actor was capable of giving the subject food but unwilling to do it; in the "unable" condition, she was willing to give food but was unable to do it because of a physical barrier; and in the "distracted" condition, she was occupied by manipulating a pebble instead of food. We report for the first time that Tonkean macaques, like capuchins, chimpanzees and human infants, behaved differently across these experimental conditions. They attempted to grasp food in the actor's hand significantly more and displayed more threats in the presence of an unwilling actor rather than an unable or a distracted one. Inversely, they begged significantly more and displayed more frustration behaviors facing a distracted and unable experimenter rather than an unwilling one. These results suggest that Tonkean macaques understand human goal-directed actions by predicting whether they were likely to obtain food merely based on movements, cue and motor intentions reading and understanding of physical constraints.