Stacking of irregularly shaped blocks in chimpanzees (Pan troglodytes) and young humans (Homo sapiens).

Stacking blocks provides a way to evaluate cognitive development in humans and other species using the same comparative measures. The present study used regular cubic blocks as well as cubic blocks with bumps on two sides. The bumps changed the physical properties of the blocks and increased the difficulty involved in stacking them. Subjects were required to choose the appropriate orientation for stacking the blocks. Three juvenile chimpanzees and 14 human children (aged 2-3 years) were tested under identical task settings in a face-to-face situation. The goal of a trial was to stack up four blocks (two cubic blocks and two cubic blocks with bumps). The results showed initial difficulty in stacking the blocks with bumps in both chimpanzees and humans. Experienced juvenile chimpanzees and humans older than 3 years became proficient at solving the task. Behavioral strategies adopted to succeed in the task were common to both species. The subjects spontaneously adopted a strategy of stacking as the last block of the tower a block with a bump facing upwards. The subjects also showed active change in the orientation of the blocks when necessary, although correct orientation changes were infrequent especially during the early phases of experiment. The results are discussed in the context of the underlying cognitive development in the domain of physical understanding in both species.