A two-year longitudinal study of deferred imitation of object manipulation in a juvenile chimpanzee (Pan troglodytes) and orangutan (Pongo pygmaeus).

Changes in deferred imitation of novel actions on objects were assessed over a 2-year period in two enculturated, juvenile great apes (one chimpanzee, Pan troglodytes, and one orangutan, Pongo pygmaeus). Both apes displayed deferred imitation, and both displayed improve ments in deferred imitation over the 2-year period, although the magnitude of improvement was greater for the chimpanzee. This is, to our knowledge, the first experimental demonstration of longitudinal improvements of deferred imitation in great apes. The results were interpreted as reflecting maturationally paced cognitive differences consistent with other cognitive accomplishments in these species, and as demonstrating the influence that a species-atypical rearing environment can have on cognitive abilities in juvenile great apes.