Figure 3. Proportion of Refusals.

The proportion of trials in which chimpanzees refused: either to exchange or to eat the offered reward. Black bars show the chimpanzees’ responses in the Baseline Phase when they had no opportunity to improve their rewards and gray bars show their responses in the Test Phase when they could obtain a high-value grape if they exchanged their token through the picture frame. Also shown are the responses by the chimpanzees when tested in the role of the partner in the Different condition for which their pay-off structure was identical to chimpanzees tested in the High-value Same condition (Table 1).