Same/different Concept Learning by Capuchin Monkeys (Cebus apella)

Scarce evidence for same/different abstract concept learning comes from studies in monkey species using matching-to-sample (MTS) tasks. This study aimed at evaluating the capacity of tufted capuchin monkeys (Cebus apella) to acquire abstract same and different concepts and use them to solve MTS tasks involving relations of increasing level of abstraction. Six capuchins had to choose which stimulus between two comparison ones matched the stimulus presented as sample. In Phase 1, Identity Matching-to-Sample (Id-MTS) tasks were used to evaluate the capuchins' ability to discriminate between individual items on the basis of the physical features of the stimuli. In Phase 2, Relational Matching-to-Sample (RMTS) tasks were used to assess the extent to which capuchins were able to judge the relation (either same or different) between the items in a sample display and select the comparison display in which the items had the same relation. The acquisition of the rule based on perceptual similarity (Id-MTS) and the acquisition of the rule based on relational similarity (RMTS) was inferred from the subject's ability to solve transfer tests with novel stimuli. Results demonstrated that capuchins' ability to judge physical equivalence and to transfer to novel stimuli is significantly increased by the number of stimuli used during training. Moreover, capuchins' ability to judge relational similarity was positively affected by the increase of both the stimulus-set size and the number of items featuring the stimuli. These findings clarify the role of specific factors on the acquisition of abstract same and different concepts in monkeys and suggest strong similarities between New World and Old World monkeys.