The other day, my brother was telling me about playing mandolin on songs he didn't know, with a band he'd just met. Then there are people who can play several chess games at once or write crossword puzzles for the New York Times. How did humans get so smart? Over most of our evolutionary history, mandolin players and crossword writers probably had a hard time making a living.
This week's paper tests the hypothesis that natural selection favored intelligence largely because it was important in social interactions. If that's true, then species that interact with more different individuals should evolve greater intelligence. Federica Amici and colleagues tested this hypothesis using apes and monkeys and reported their results in Current Biology. Their paper is titled "Fission-fusion dynamics, behavioral flexibility, and inhibitory control in primates."
Chimps, gorillas, orangutans, bonobos, macaques, and two species of monkey were given the same battery of tests, including the "delayed reward" test, where they could get more food by waiting. When each species' performance was compared with that of related species (chimps vs. gorillas or spider monkey vs. capuchin monkey, for example), the species that did best was the one with the most complex social interactions in the wild. Chimps, which live in fission-fusion bands that get together in ever-changing combinations, did much better than gorillas, which live in small stable families. When they developed a "family tree" based on test results, rather than known relatedness, those with more complex social interactions were grouped together (bonobos with spider monkeys, for example). Orangutans were more similar to chimps than gorillas, which seemed a bit inconsistent with the overall pattern, as they are mostly solitary. Or so I thought.
They propose testing other species, including sperm whales. I'd like to see how they do on the delayed gratification test.