Abstract
This study aimed to examine object manipulation in 2 to 4-year-old hand-reared infant African great apes, two bonobos and three gorillas. Humans and the great apes differ from other primate species in terms of their ability to use tools. Orienting manipulation (or combinatory manipulation), especially object-to-object manipulation, is considered to be a precursor of tool use. Although developmental changes in object manipulation that lead toward tool use have been investigated in chimpanzees, studies concerning bonobos and gorillas are extremely scarce. We observed free play in ape infants manipulating three kinds of objects: wooden blocks, nesting cups, and rings and a stalk. The size of the behavioral repertoire was measured by focusing on four variables: body parts used, number of objects manipulated, motor patterns, and types of orienting manipulation. Both species showed orienting manipulation (or combinatory manipulation) comparable to chimpanzee infants of similar ages. The gorillas' repertoire included fewer behaviors involving the use of the foot than the bonobos'. They showed the tendency of manipulating single object not in a single motor pattern but in multiple motor patterns.
