DEVELOPMENT OF STONE TOOL USE BY WILD CHIMPANZEES (PAN-TROGLODYTES)

At the age of 3.5 years, wild chimpanzees at Bossou, Guinea, begin to use hammer and anvil stones to crack oil-palm nuts to get the kernels. To clarify the developmental processes, the authors did a field exper iment in which stones and oil-palm nuts were provided. Infant chimpanz ees' stone-nut manipulation was observed and video recorded. Data were collected from 3 infants younger than 4 years old from 1992 to 1995. The authors analyzed 692 episodes of infants' stone-nut manipulation a nd 150 episodes of infants' observation of nut cracking performed by a dults. Infants observed other chimpanzees' nut cracking and got the ke rnels from them. The stone-nut manipulation developed from a single ac tion on a single object to multiple actions on multiple objects. Altho ugh infant chimpanzees at the age of 2.5 years already acquired basic actions necessary for nut cracking, they did not combine the actions i n an appropriate sequence to perform actual nut cracking.