Pestle-pounding and nut-cracking by wild chimpanzees at Kpala, Liberia

Bossou in Guinea is one of the longitudinal study sites of wild chimpanzees, and is located only a few kilometers away from the national border between Guinea and Liberia. The forests in the area spread over the national border of Guinea, and the Bossou chimpanzees have been found to use the neighboring Liberian forest. Local assistants and I started surveying these forests in Liberia, and found that additional groups of chimpanzees lived in Nimba County, Liberia. The present study reports tool use behaviors by chimpanzees living in forests of the Kpala area in Nimba County. We directly observed pestle-pounding behavior, which had been confirmed only in the Bossou group of wild chimpanzees. Moreover, we heard sounds of nut-cracking, and successfully filmed chimpanzees cracking open oil palm nuts with stones. The uniqueness of stone-tool use behaviors has been emphasized with the group of chimpanzees that have been longitudinally studied at Bossou, but the behaviors probably have a wide distribution in this area. Emigrant chimpanzees are thought to contribute to the propagation of the cultural tool-use behaviors. It is also thought that, if the distantly located groups share similar cultural behaviors, there might be genetic exchange between them. Conservation efforts should be needed not only at Bossou, but also in a wider area including nonprotected forests beyond the national border.