Abstract
Use of leaves for drinking water by wild chimpanzees was studied through a field experiment. Previous observations have revealed that the chimpanzees at Bossou, Guinea, drank water by using a leaf as a spoon or a sponge. In addition to recording the leaves used in a tree hollow under natural conditions, we set up an outdoor laboratory for observing and video-recording the behavioral processes of the tool-use behavior. A water container which was utilized by local people was buried in the ground of the outdoor laboratory. We observed 5 examples by 3 chimpanzees in which leaves were used as a drinking tool. The observation revealed a third method of using leaves: The chimpanzees folded a leaf in the mouth as in paper-folding and put it into the container for capturing water. The chimpanzees preferred to use a particular leaf (Hybophrynium braunianum), a wide, soft, and hairless leaf, as a tool (21 out of 28 episodes in total). The high selectivity of leaves and the newly-found folding method shows chimpanzees' ability to find a suitable material and modifying it into a useful tool.
