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(Eric Jaffe)

Animal Insight

Recent studies illustrate which traits humans and apes have in common—and which they don't

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(Continued from page 1)

On the opposite end of the behavioral spectrum, Keith Jensen, also at Max Planck, recently found that chimps are likely to exact revenge as well. Given the chance, chimpanzees retaliated against thieves by collapsing the bandit's table, thereby ruining the stolen meal, Jensen reports in the Aug. 7 Proceedings of the National Academy of Sciences. The idea is vengeance acting as a deterrent. In other words, you steal from me, I punish you enough to make you think twice before taking my banana the next time.

This type of vengeance, even if it takes the ugly form of punishment, is healthy in that it discourages freeloaders. So even if vengeance is considered bad, it can often serve the greater good.

Spite, however, doesn't appear to have any such obvious perks, which might explain why chimpanzees didn't exhibit it in Jensen's experiments. When an adult person took food away from one chimpanzee and gave it to another, the first chimpanzee didn't collapse the second chimpanzee's table, the researchers found.

"I'm not very surprised that we don't see a lot of spiteful behavior in the chimps," says Joan Silk, an anthropologist at the University of California, Los Angeles who was not affiliated with the study. "In some sense it's a little bit irrational, because you hurt yourself to hurt someone else more."

Still if the chimpanzees don't display spite, then why do humans? Spite, which Jensen describes as "altruism's evil twin," might help motivate behaviors related to a sense of fairness, he says. "In the absence of punishment, at least in studies that have been done on human adults, cooperation falls apart, because it only takes a few selfish individuals … to ruin everything for everybody," Jensen says. "But if you give people the opportunity to punish free riders, they stop cheating."

Other differences between human and chimpanzee behavior have been teased apart by testing infants, chimpanzees and orangutans in identical conditions. Esther Herrmann of Max Planck recently found that apes and two-and-a-half-year-old children performed similarly on tasks that tested their understanding of the physical world, such as space and quantities.

For example, chimpanzees were better than infants at detecting added quantities of food or toys, a rudimentary math skill. Their spatial relationships were similarly developed; both extracted food and toys from difficult places.

However, the similarities in their cognitive skills broke down when it came to Herrmann's social learning tests, she reports in the Sept. 7 issue of Science. Herrmann says these social cognition skills, which people display more often than chimpanzees, are the same skills that give us the leg up to perpetuate our culture and society.

"Human children have much more sophisticated skills dealing with the social world like dealing with imitating another's solution to a problem, communicating non-verbally and reading the intentions [of] others," Herrmann says. "These skills enable them to participate in the cultural world, and by doing so children become members of this cultural world."

Still, such studies cannot replicate one major linchpin of our evolutionary story, even if they can guess at it. For traits to evolve, they must be inheritable, and for them to persist, they must bestow reproductive success or increased survival to the individual.

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