Figure 2.

(a) An adult female chimpanzee (weighing ca 42 kg) cracking Coula nuts with a wooden hammer of ca 1.5 kg, adopting a seated posture. (b) A dominant male bearded capuchin (weighing 4.2 kg) cracking a piassava nut with a 3.5 kg stone, adopting a bipedal posture. (c) An adult chimpanzee uses both hands and his mouth to collect Coula nuts. (d) An adult capuchin uses his hand to collect palm nuts. Pictures (a,c) by C. Boesch; picture (b) by L. Marino; picture (d) by G. Sirianni. (Online version in colour.)