Apes know that hidden objects can aVect the orientation of other objects

Four bonobos, seven gorillas, and six orangutans were presented with two small rectangu-lar boards on a platform. One of the boards had a piece of food under it so that it acquired an inclined orientation whereas the other remained Xat on the platform. Subjects preferentially selected the inclined board. In another experiment, subjects were initially presented with two inclined boards and a transformation took place in which one of the boards fell Xat to the plat-form while the other remained inclined. Subjects also preferred the board that remained inclined. Two additional experiments highlighted some of the possible limitations of their rea-soning in this task. Presented with two inclined boards, one of which was visibly supported by a piece of wood, they failed to systematically select the unsupported one whose only reason for being inclined was the presence of the reward. Another experiment presented two rewards in each trial (instead of the customary one) in one of the following two combinations: large banana vs. small carrot or small banana vs. large carrot. Prior to the test, E presented both rewards to the subject and then hid each of them under one of the boards so that both boards were diVerentially inclined due to the diVerent sizes of the rewards involved. Although subject