Arboreal stone tool use in wild chimpanzees

The transition from an arboreal to a terrestrial lifestyle drove significant changes in hominin evolution including the development of complex technologies. Nonhuman primates reveal a similar trend, where more terrestrial species exhibit greater technological complexity. Percussive stone tool use, such as nut cracking, is known across several primate species as a complex, terrestrial activity. However, western chimpanzees, Pan troglodytes verus , also perform arboreal nut cracking, a behaviour so far described only in the North group of the Taï Chimpanzee Project (TCP) and still understudied. Here, we investigated how chimpanzees’ efficiency at nut cracking is affected in arboreal settings in comparison to terrestrial settings, and how seasonality, sex and hammerstone weight influence Coula edulis arboreal nut-cracking likelihood. We monitored the Djouroutou community in the Taï National Park over 10 months and compared their behaviour to existing data from the nonadjacent South TCP group. Our results show that, in arboreal settings, chimpanzees across groups required more strikes to open a nut, indicating lower efficiency compared to terrestrial nut cracking. However, they did not open fewer nuts per minute in trees. As expected, this behaviour occurred mainly at the beginning of the C. edulis season when nuts are still attached to branches. Although females tended to crack more frequently in trees compared to males, chimpanzees of the same group used hammerstones of similar weight in both contexts. Cultural differences emerged between groups, as Djouroutou chimpanzees consistently selected heavier hammerstones than South TCP individuals, despite similar local stone availability. Contrasting partially with previous reports from the North TCP group, these findings suggest that arboreal nut cracking, while less efficient than terrestrial nut cracking, enables earlier access to high-value food resources, potentially offsetting the costs of transporting stone tools into trees. This study contributes to understanding the cognitive, ecological and evolutionary dimensions of arboreal tool use in wild chimpanzees and its relevance for interpreting early hominin technology.