Chimpanzees using stones to crack open oil palm nuts in Liberia

The use of stone-tools to open palm nuts (Elaeis guineensis) was studied in a group of 16 chimpanzees (Pan troglodytes) released from captivity to a natural island setting in Liberia. The behaviour was started by one female of the group; it then spread to 12 others over periods varying from a few seconds to a few weeks. Nut-cracking soon spread to three other sites, both spontaneously and with human encouragement. Both nuts and tools were carried distances of several hundred meters. Social interaction at cracking sites was rich and varied, ranging from fights over possession of tools to unsolicited sharing of nuts. Nut-crackers showed selectivity in schoosing “good” nuts and varied their methods according to the qualities of the nuts. Individual differences in technique emerged. These are the first detailed behavioural data on palm-nut-cracking, and they show many parallels with nut-cracking of other species by chimpanzees of the Tai Forest, Ivory Coast. This is another example of hammer-stone use from a limited region of West Africa: southeastern Guinea, eastern Liberia, and western Ivory Coast. This suggests limited cultural diffusion of the custom.