Solving a cooperation task without taking into account the partner's behavior: the case of capuchin monkeys (Cebus apella).

Four pairs of tufted capuchin monkeys (Cebus apella) were tested in a task requiring that both partners pull a handle simultaneously in order for both to be rewarded. The experimental design, an improved version of that of R. Chalmeau, E. Visalberghi, and A. Gallo (1997), aimed at assessing the extent to which a monkey that is pulling takes account of the behavior and spatial position of its partner, that is, whether the monkey understands what cooperation involves. Although all pairs succeeded, pulling was not affected by the partner's behavior, and it was affected only to a certain extent by the partner's spatial position. In addition, more experienced capuchins did not outperform naive individuals. The finding that capuchins were successful without understanding the role of the partner suggests that their cooperation is not cognitively grounded, as has been argued in descriptions of the hunting behavior of wild capuchins.