Comprehension of cause-effect relations in a tool-using task by chimpanzees (Pan troglodytes).

Five chimpanzees (Pan troglodytes) were tested to assess their understanding of causality in a tool task. The task consisted of a transparent tube with a trap-hole drilled in its middle. A reward was randomly placed on either side of the hole. Depending on which side the chimpanzee inserted the stick into, the candy was either pushed out of the tube or into the trap. In Experiment 1, the success rate of 2 chimpanzees rose highly above chance, but that of the other subjects did not. Results show that the 2 successful chimpanzees selected the correct side for insertion beforehand. Experiment 2 ruled out the possibility that their success was due to a distance-based associative rule, and the results favor an alternative hypothesis that relates success to an understanding of the causal relation between the tool-using action and its outcome.