The acquisition of stone-tool use in captive chimpanzees

In order to investigate the acquisition of stone-tool use to crack nuts, experiments were conducted with five captive chimpanzees, employing combinations of trial-and-error, and three types of observation learning of varied difficulty. Two adults and one sub-adult succeeded in acquiring this behavior in the tests where tool use was demonstrated by the experimenter, or by a successful chimpanzee. Analysis of their behavior patterns showed the adults' and sub-adult's process to success to be divided into three stages; these can be characterized as trial-and-error, understanding of the method, and learning the technique, respectively. A factor for distinguishing success from failure is whether or not a subject observes demonstrations with interest in the behavior itself. This interest, in turn, depends on the intensity of the desire for nuts. When released, the successful chimpanzees practiced the same behavior in a group of chimpanzees, and the behavior newly diffused to an infant. Unlike the adults, the infant did not go through the staged process but reached the solution through goal-directed trial-and-error.