Bloomsmith, M. A., Finlay, T. W., Merhalski, J. J. et al. 1990. Rigid plastic balls as enrichment devices for captive chimpanzees. Laboratory Animal Science 40(3), 319-322.

The mean percentage of ball-use time for all subjects during the study [first ten hours after initial presentation] was 7.1%. There was no sex difference in ball use. Age and housing effects were obtained, with younger animals and those housed in more barren environments exhibiting higher levels of ball use. It is concluded that the balls were worthwhile additions to the chimpanzee environments with use stabilizing at a mean of 2.5% of the subjects' time.