Object sorting in chimpanzees (Pan troglodytes): classification based on physical identity, complementarity, and familiarity.

In 3 experiments, 5 chimpanzees (Pan troglodytes) were tested in an object-sorting task. In Exp. 1, a triad of objects and 2 trays were presented, and the subjects were trained to place 2 identical objects onto the same tray and an odd object onto the other tray. All of the subjects were able to learn this type of sorting. In Exp. 2, the 2 complementary parts of 2-part objects were presented along with 1 neutral object. The frequency of placing the complementary parts onto the same tray significantly increased after the subjects had learned to assemble the 2 parts into 1 object. In Exp. 3, test triads consisted of familiar objects and objects there were novel to the subjects. Each subject showed a significantly high frequency of placing 2 familiar objects on the same tray although all the subjects had learned to place the same objects on separate trays in Exp. 1. The object-sorting tasks may be effective for studies of classification abilities.