Barnett, C. 1997. A gravity roulette feeder for chimpanzees. The Shape of Enrichment 6(2), 11.

A few chimps will use sticks or branches to knock the enrichment items down the hole in the platform and then climb down and get the treats at the openings. Sometimes it works - sometimes it doesn't, because other chimps see and hear the enrichment treats falling and move to the dispensing holes.