“Perhaps the complexities of great ape social relationships selected for large brains. But orangutans challenge this ‘social intelligence’ hypothesis: in the wild, they mostly travel about by themselves, yet they are at least as smart as chimpanzees.
Van Schaik thinks that social factors are indeed pivotal in explaining orangutan intelligence, but not in the way proposed by the social intelligence hypothesis. In a beautifully written, compelling narrative that reads like a detective story, he weaves together several threads of evidence to argue that orangutan intelligence is intimately related to technological innovations that are passed down through social learning.”
Book review of Among Orangutans: Red Apes and the Rise of Human Culture by Carel van Schaik here, by Barbara Smuts in Scientific American.