The ontogeny of action comprehension: experiments with free-ranging rhesus monkeys (supplementary videos) Previous studies suggest that important aspects of our capacity to understand others’ actions are homologous, shared with nonhuman primates and other animal species. This claim is, however, tenuous as little is known about the development of this capacity in nonhuman primates, despite considerable understanding in humans. Specifically, action comprehension develops in a piecemeal manner during human development, such that infants successfully comprehend the difference between hand grasp and hand flop actions (i.e., as goal-directed and accidental, respectively) at an earlier age than non-contact actions, such as pointing and directed gazing. In the current study, we examined whether action comprehension develops along a parallel trajectory in free-ranging rhesus monkeys, under non-trained, experimental conditions. Replicating prior research with the same species and population, results indicate that adult rhesus correctly perceive both contact (e.g., hand grasp, hand flop) and non-contact (e.g., pointing, directed gazing) actions as goal-directed. In contrast, juveniles only comprehend the contact actions, failing to recognize pointing and directed gaze as goal-directed. Thus, although rhesus have evolved the capacity to comprehend that some gestures are goal-directed, the fully mature and nuanced form of this capacity develops slowly, with a piecemeal pattern that mirrors in important ways our own species’ ontogenetic trajectory. This evidence provides stronger support for the hypothesis that important aspects of action comprehension in human and nonhuman primates are homologous. Adult choice - Condition: Communicative Gaze Juvenile choice - Condition: Intentional Hand Grasp Juvenile choice - Condition: Intentional Hand Grasp