Grief, L., Fritz, J., Maki, S. 1992. Alternative forage types for captive chimpanzees. Laboratory Primate Newsletter 31(2), 11-13.

Chicken scratch, because it is small and harder to find [in the litter], elicited the most foraging of the three foods ['sweet feed', popcorn]. It is very encouraging to see the chimpanzees still foraging late in the day for these small kernels. In addition, for subjects such as our blind animal, who had one of the highest foraging scores in our study, this [inexpensive] enrichment cannot be overemphasized.