Tool use during display behavior in wild Cross River gorillas.

During the course of a 3-year ecological study on Cross River gorillas (Gorilla gorilla diehli) at the Kagwene Mountain in Cameroon, we observed three cases of tool use which may be unique to the gorillas of this region and possibly learned through interactions with humans. A non-habituated group of Cross River gorillas threw fistfuls of grass toward humans in display contexts. An individual gorilla was also observed to throw a detached branch toward researchers during another encounter. The third encounter occurred between the focal group and a local man, in which the man threw stones at the gorillas and they reacted by throwing fistfuls of grass back at him. We describe these observations and speculate that this sub-population of gorillas developed this display behavior as a response to particular local conditions.