Generalized object-sorting in chimpanzees (Pan troglodytes)

Abstract

In four experiments, two adolescent chimpanzees were given an object-sorting task to assess their categorizing ability. In Exp. 1, they were presented a set of two identical objects (AA) and one odd (B) object, and trained to sort the objects in two categories, AA and B ; four kinds of objects were used in training. They learned this type of sorting easily. In Exp. 2, 3, and 4, test stimulus sets were inserted in random positions among the baseline sets of Exp. 1. In test trials, the test sets differed for each trial, and the chimpanzees always received food regardless of how they sorted. In Exp. 2, test sets were novel objects. In Exp. 3, the ebjects in each test set were identical either in color or in shape, and could be sorted based on the other dimension. In Exp. 4, the objects in each test set were different, and could be sorted based on either color or shape. In test trials of Exp. 2, 3, and 4, chimps sorted categorically and their performances were highly significant. The implications of these results are discussed.