Ohlinger, R., Schwandt, M., Fritz, J. et al. 2000. King Kong® rubber toys: An effective enrichment device for captive chimpanzees (Pan troglodytes). The Newsletter 11(2), 3-5.

Subjects were tested on one single occasion each when they had access to a toy for 15 minutes. High percentages of use by two chimpanzees adjacent to social groups suggest that the King Kong toy may be an especially effective enrichment device for individually-housed animals. In the case of socially-housed animals, the pattern of toy use at PFA in many cases reflected the hierarchies of the group, with lower ranking animals having to wait his or her turn to examine the toy. ... We recommend rotating the toy from group to group to help maintain its novelty.