Development of object manipulation and tool use in chimpanzees (Pan troglodytes) and bonobos (Pan paniscus)

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In human infants, combinatory manipulation seems important as a precursor of tool use. Previous studies have suggested developmental similarities in object manipulation in humans and chimpanzees during the 1st year of life but delayed combination of detached objects for chimpanzees from the 2nd year onward. Systematic studies on the development of combinatory manipulation in chimpanzees, however, have only recently started. These studies compared both the products of combination and the combinatory processes adopted and how these are shared by chmpanzees and humans. Such comparisons of actions generated by both species can illuminate how features of cognition may arise from bodily interactions with the environment and provide a powerful methodology to clarify origins of human behavior and cognition. The present study focused on combinatory manipulation and tool use by chimpanzees and their closest relative, bonobos, aiming to clarify the similarities with and the differences from that by human infants. Several experiments were conducted and common motor characteristics were observed across the tasks between both species, namely "repetition" of actions, "adjustment" of actions, "reversal" of actions, and "shifts" of attention. Humans, chimpanzees and bonobos share these actions when manipulating multiple objects to complete tasks which include tool use. Repetition, adjustment, and reversal of actions and shifts of attention underlie higher levels of cognition common to both species. However, subassembly patterns of tool use were hardly seen in both chimpanzees and bonobos. In addition, subjects did not reproduce the model behaviors, a subassembly pattern of tool use, by a human demonstrator. The results could be discussed in terms of generation of hierarchical actions and evolution of language.