Chimpanzees have at least one thing in common with the late US president Richard Nixon: tracking those who do them favours and putting those who don't come up to scratch on an "enemies list".
Nearly 3000 hours of observations of wild chimpanzees show that they keep tabs on which of the troop has groomed them the most - returning the favour to frequent groomers, while freezing out the selfish ones.
In a way, grooming works like currency in chimpanzees, says Cristina Gomes, a behavioural ecologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. "If you don't have a set price, then you're susceptible to being cheated and cooperation would probably break down."
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