Leaf-pile pulling: an unusual play pattern in wild chimpanzees.

Play in nonhuman animals has generally been viewed as being uniform among study sites. No studies have examined whether there are local variations in play. In this work we report an apparently locality-specific form of play that is basically solo locomotor play, but also has aspects of object play and social play. We describe this unusual "leaf-pile pulling" (LPL) pattern based on video footage of the chimpanzees of Mahale, Tanzania. Typically, when a party of chimpanzees moves in a procession down a slope in the dry season, a youngster will turn around and walk backward while raking many dry leaves with both hands. This activity accumulates many dry leaves while producing a lot of sound. After the player walks 1-15 m, he/she either turns around and walks forward or moves in a somersaulting fashion. The performer usually faces an individual that is immediately following him/her in the procession. The age of the performers ranges from 2 to 22 years, but 3-10 years are most typical. Compared to younger (< 8 years) individuals, older (> or = 8 years) individuals tend to cover longer distances during play, and to be more likely to play only on sloped surfaces and during travel. One of the authors (W.W.) has only seen the behavior exhibited by five different individuals on 10-15 occasions at Gombe. The behavior is exhibited less often at Gombe than at Mahale. To date, LPL has not been reported elsewhere. The absence of LPL at other sites may or may not be explained by environmental differences, such as differences in the availability of dead leaves and sloped terrain. Although the possibility that LPL is acquired by individual learning cannot be completely ruled out, the hypothesis that it is a tradition of the Mahale study group is more likely. This suggests that play may warrant more careful consideration in studies of nonhuman primate culture.