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Discrimination and categorization of pictures of natural objects by chimpanzees (Pan troglodytes)

Masayuki Tanaka, Tetsuro Matsuzawa
(Primate Research Institute, Kyoto University)

     Three experiments investigated the abilities to discriminate and categorize natural objects in four adult female chimpanzees. In experiments 1 and 2, four chimpanzees were trained to match a picture of the same item as a sample among four comparison stimuli. The stimuli were pictures of items from four categories (flower, tree, weed, grounds). The pictures of the sample and the correct comparison differed each other. The items used in experiment 1 were planted in the compound where the subjects lived (i.e., familiar with the subjects), and the items used in experiment 2 were novel to the subjects. After training, test trials were inserted among training trials. In test trials, comparison stimuli were four pictures of the items from four different categories, and involved no picture of the same item as the sample. In both experiments 1 and 2, the subjects responded to the items from the same categories as the sample significantly frequently than the chance level although the subjects had been never reinforced to respond to different items from the sample. The results suggest that the chimpanzees are able to classify items, which they could discriminate, into the same category as well as humans do. Experiment 3 investigated whether the chimpanzees could classify the items that they have never seen before. The subjects were trained to choose three pictures of flowers among 12 pictures of items from four categories. The stimuli were those used in experiments 1 and 2. All subjects became able to choose pictures of flowers almost perfectly after training. Then, the subjects were given test trials that contained all novel stimuli. The subjects chose significantly more the pictures of novel flowers than chance level. The results suggest that the chimpanzees could recognize categories of the natural objects seen for the first time.

Tanaka, M. (1997). Formation of categories based on functions in a chimpanzee (Pan troglodytes). Japanese Psychological Research 39, 212- 225.
Tanaka, M. (1996). Information integration about object-object relationships by chimpanzees (Pan troglodytes). Journal of Comparative Psychology 110, 323-335.
Tanaka, M. (1995). Object sorting in chimpanzees (Pan troglodytes): Classification based on physical identity, complementarity, and familiarity. Journal of Comparative Psychology 109, 151-161.