Perception of chasing in squirrel monkeys (Saimiri sciureus).

Understanding the intentions of others is crucial in developing positive social relationships. Comparative human and non-human animal studies have addressed the phylogenetic origin of this ability. However, few studies have explored the importance of motion information in distinguishing others' intentions and goals in non-human primates. This study addressed whether squirrel monkeys (Saimiri sciureus) are able to perceive a goal-directed motion pattern-specifically, chasing-represented by two geometric objects. In Experiment 1, we trained squirrel monkeys to discriminate a "Chasing" sequence from a "Random" sequence. We then confirmed that this discrimination transferred to new stimuli ("Chasing" and "Random") in a probe test. To determine whether the monkeys used similarities of trajectory to discriminate chasing from random motion, we also presented a non-chasing "Clone" sequence in which the trajectories of the two figures were identical. Three of six monkeys were able to discriminate "Chasing" from the other sequences. In Experiment 2, we confirmed humans' recognition of chasing with the stimuli from Experiment 1. In Experiment 3, the three monkeys for which discrimination did not transfer to the new stimuli in Experiment 1 were trained to discriminate between "Chasing" and "Clone" sequences. At testing, all three monkeys had learned to discriminate chasing, and two transferred their learning to new stimuli. Our results suggest that squirrel monkeys use goal-directed motion patterns, rather than simply similarity of trajectory, to discriminate chasing. Further investigation is necessary to identify the motion characteristics that contribute to this discrimination.