Pestle pounding was first observed by professor Sugiyama in 1990 (Sugiyama, 1994).
It consists of several discrete sequential behavioural components (Yamakoshi & Sugiyama, 1995):
1. A chimpanzee will climb to the centre of the crown of an oil palm tree.
2. It usually proceeds to spread out the radiating mature leaves using its hands and feet in order to expose the base of the central young shoots.
3. These young shoots are removed by tugging to gain access to the apical meristem.
4. The petiole of these spear leaves is then often eaten and chewed.
5. A palm-leaf petiole is used as a pestle or a tool (see photo) to pound at and excavate the centre of the palm crown, resulting in a softening of the palm heart.
6. The palm heart is eaten and extracted by hand.
The Bossou community of chimpanzees is currently the only community where this type of tool use has been observed.