Cultural differences in army ant predation by West African chimpanzees? A comparative study of microecological variables

Behavioural diversity in the predation pattern of army ants (Dorylus spp.) by different populations of wild chimpanzees, Pan troglodytes, has been proposed to reflect different cultural traditions. Chimpanzees use either stick tools (known as ant dipping) or simply their hands to prey on two groups of army ants (epigaeic and intermediate species). A recent analysis has shown that, contrary to the cultural hypothesis, the tool length and associated harvesting technique used by chimpanzees in different populations is to a large extent influenced by characteristics of the ants themselves. However, in line with the cultural hypothesis, chimpanzees at two long-term study sites in West Africa (Bossou, Guinea, and Tai National Park, Cote d'Ivoire) prey on the same five army ant species but adopt different strategies to do so. We conducted controlled human simulations of ant dipping and an ant survey at these two sites to evaluate alternative ecological explanations related to ant behaviour and ecology that could account for the observed differences in chimpanzee predation behaviour. Ant speed explained differences in tool length within Bossou but not between Bossou and Taï. Our results do not support an ecological basis underlying the lack of dipping at ant trails in Taï chimpanzees. Finally, neither ant aggressiveness (measured as speed and persistence) nor yield when using tools could explain why, unlike Bossou chimpanzees, Taï chimpanzees do not use tools to harvest epigaeic species. We conclude that an interaction of cultural and ecological factors shapes the differences in army ant predation between Tai and Bossou chimpanzees.