Ant-dipping among the chimpanzees of Bossou, Guinea, and some comparisons with other sites

We present a detailed study of ant-dipping among the wild chimpanzees (Pan troglodytes verus) of Bossou, in southeastern Guinea, West Africa. Observations suggest a strong influence of prey (Dorylus spp.) characteristics, including aggressiveness and/or gregariousness, on tool length and technique employed by the chimpanzees. Bossou chimpanzees exhibit two ant-dipping techniques: 1) direct mouthing, and 2) pull-through. In addition, they were observed dipping for several species of Dorylus ants, classed into two categories: Red and Black. Tool length was longer when dipping in higher-risk contexts, i.e., at the ants' nest site or on Black ants. The pull-through technique was almost exclusively associated with dipping at the nest site. This latter technique was associated with tools over 50 cm long, whereas direct mouthing was the only technique observed with tools <50 cm long. Our experimental findings, together with our observations on the behavior of the chimpanzees, suggest that at the nest, the pull-through technique was a more efficient technique than direct mouthing. We review our results in the context of ant-dipping observed at two other long-term chimpanzee study sites, i.e., Gombe (Tanzania) and Taï (Côte d'Ivoire), where differences in tool length, technique used, and focal Dorylus ant species have been reported. Finally, we urge similar detailed studies of this tool-use behavior in both Gombe and Taï to shed further light upon our results and their implications.