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Nuts are high in energetic and nutritional value, but the kernel inside is difficult to access. In the Taï forest, it is estimated that chimpanzees (Pan troglodytes verus) can eat 6–10 times as many nuts with the help of a tool as to when they crack them with their own teeth. However, sympatric-living terrestrial monkeys never crack nuts using tools. So this begs the question, why not? In this chapter, we report on the foraging behaviour of the sooty mangabeys (Cercocebus atys atys). A quick synopsis goes as follows: they observe nut-cracking chimpanzees at a distance of 5–10 metres, relish in the leftovers of the freshly cracked nuts, and then continue to follow the chimpanzees to different nut-cracking sites. With this information, we go on to consider the underlying reasons for the absence of nut-cracking in sooty mangabeys, with a particular focus on cognitive limitations, and then discuss the implications of field observations for studies on imitation in the laboratory.
Despite appealing support for theories that argue that social complexity is the main force driving primate brain-size evolution, it is still unclear how great apes were able to afford the evolution of larger and more expensive brains than sympatric species. Comparative phylogenetic studies suggest that the costs of evolutionary brain enlargement were overcome by a permanent increase in net energy intake, renewing interest in the role of ecological complexity in primate brain-size evolution. As relatively larger-brained primates, like chimpanzees, show less seasonality in their net energy intake than smaller-brained species, larger brains are proposed to provide a ‘cognitive behavioural flexibility’ that facilitates the consumption of nutritious foods during periods of food scarcity (cognitive buffer hypothesis). To date, it remains unclear what this cognitive flexibility entails. In this chapter, I will provide evidence for a variety of mechanisms of temporal cognition that chimpanzees employ to gain first access to newly ripened, energy-rich fruit in a competitive and complex rainforest environment.
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