Brent, L., Lee, D. R., Eichberg, J. W. 1991. Evaluation of a chimpanzee enrichment enclosure . Journal of Medical Primatology 20(1), 29-34.

A large, three-part playground for captive chimpanzees was constructed and evaluated in terms of area use and behavior changes. Comparative behavioral samples were obtained on 38 subjects in the existing indoor-outdoor run and in the enclosure. The chimpanzees used the inside run, connective chute, concrete slab, and grass areas most. Activity and environmental manipulation increased in the enclosure while abnormal and self-directed behaviors decreased. The soft grass was unfamiliar to chimpanzees reared on concrete and wire, and several preferred to climb along the fence and roof during the first week. ... The swinging tires, ropes, and movable climbing structures were the last areas to be explored. The chimps used the branches, as well as the grass, as browse material and for building nests.