Glassman, D. M., Buttler, S. J., Willis, F. B. et al. 1987. An attempt at behavioral enrichment for a captive male lowland gorilla. American Journal of Primatology 12, 344 (Abstract).

An attempt was made to overcome the lethargic behavior of a singly housed male gorilla. The animal was provided with a plastic ball and burlap bags, and his food was intermixed with hay in order to encourage foraging behavior. Behavioral data collected before and during the study indicated an increase in the male's activity level and in his use of the cage space.