Paquette, D., Prescott, J. 1988. Use of novel objects to enhance environments of captive chimpanzees. Zoo Biology 7, 15-23.

Following their familiarization with the novel objects [rubber or plastic toys for small children], the [group-housed] chimpanzees' manipulation frequency decreased whereas self-grooming and abnormal behaviors were increased. The importance of a periodical substitution of the objects was suggested to enhance their usefulness.