Abstract
In three experiments, five chimpanzees were given an object-sorting task to assess their categorizing ability. In preliminary training, chimpanzees were presented a set of two identical objects (AA) and one odd object (B). They were trained to sort the objects in two categories, AA and B; four kinds of objects were used in this training. All the chimpanzees were able to learn this type of sorting easily. In Exp. 1, 2, and 3, test stimulus sets were inserted in random positions among the sets of training. In test trials, the test sets differed for each trial, and the chimpanzees always received food regardless of how they sorted. In Exp. 1, test sets were novel objects for the subjects. In Exp. 2, the objects in each test set were identical either in color or in shape, and could be sorted on the basis of the other dimension. In Exp. 3, the objects in each test set were different one another, and could be sorted on the basis of either color or shape. In test trials of the experiments, chimpanzees sorted categorically and their performance were highly significant. The implications of these results and the utility of the object-sorting task are discussed.
