Figure 5.

Schematic flow of behavioural strategies in the context of nut cracking. Rectangles show observable events and behavioural strategies taken by chimpanzees. Oval-dotted rectangles are not observable cause of the events. The cause and observable events/strategies are interconnected based on the theoretical assumption and probability. The top row shows two major errors occurs in nut-cracking sequence. The second row shows the possible causes of the errors (note that these causes are not obvious and recognizable for chimpanzees). The third row shows the actual behavioural strategies observed after the errors. The fourth row shows the result of behavioural strategies taken by the old female chimpanzee during her metatool formation (discarded hammer worked as a wedge stone). (Online verison in colour.)