Tonkean macaques do not prefer the helper or the hinderer in the hill paradigm.

Evaluating others' prosocial tendencies can benefit individuals by allowing them to interact with prosocial individuals and avoid antisocial ones. The ontogeny of humans' strong prosocial preference has been widely investigated using the hill paradigm. Infants' preference for helper over hinderer agents was measured after they watched a scene in which the helper agent pushed a climber up a hill while the hinderer agent pushed the climber down the hill. Bonobos tested with the hill paradigm preferred the hinderer over the helper, contrasting previous findings for other nonhuman primates. In this study, we explored whether another primate species would exhibit a hinderer preference using the same procedure as the one used with bonobos. Tonkean macaques (Macaca tonkeana) did not prefer the helper over the hinderer (or vice versa). While the small sample size (n = 12) and low attentional level observed in our subjects limit interpretation, this finding contributes to a broader critical thinking on the relevance of the hill paradigm to investigate prosocial preferences and on methodological limitations when testing nonhuman animals. Studies using various experimental paradigms with conspecifics or human actors as social agents are needed to further investigate the social evaluation of prosocial behaviours in primates.