Bodamer, M. D. 1990. The use of raisin tubes as an enrichment device. American Journal of Primatology 20, 175 (Abstract).

Knotted plastic tubing filled with raisins. Ninety-four percent of the time that the tubes were in the cage the [single-housed] chimpanzees were interacting with them. Some animals used twigs to remove the raisins in a manner analogous to termit fishing seen in the wild.