The technique has three parts: an anvil, usually an exposed root or a flat stone; a hammer chosen for weight and hardness; and a graded strike that breaks the shell without pulping the kernel. Panda oleosa nuts need several times more force than the softer Coula edulis, and chimpanzees switch to heavier stone hammers for them.
Infants begin handling hammers and discarded shells at about two years old. First successful cracks are usually recorded around three to four years, and adult-level efficiency, measured in nuts opened per minute, arrives closer to ten. Mothers occasionally leave a good hammer on the anvil, which is among the strongest field evidence for anything resembling teaching in a wild ape.
The puzzle is the boundary. Nut-cracking stops abruptly at the Sassandra-N'Zo river and is unknown across East Africa, in forests where suitable nuts and suitable stones both occur. Whether the gap reflects invention and transmission, or an unmeasured ecological difference, remains unsettled.