Sucking up to win the support of the boss dates back to our furry ancestors. The motivation, for monkeys, is life and death.

Rather than grabbing coffee for the CEO, monkeys have for eons picked dead skin and bugs from the fur of higher-ranking monkeys. They do it in exchange for backing in fights.

The finding, detailed in a recent issue of the journal Behavioral Ecology, resolves a historical question about primate grooming.

“Scientists have wondered for decades, 'Why should a monkey spend time cleaning the fur of another,'” said study author Gabriele Schino of the National Research Council in Rome. In the late 70s and 80s, psychologist Robert Seyfarth hypothesized that monkeys exchange grooming for support. Ensuing investigations into the matter turned up inconsistent results, leading scientists to criticize this theory.

But those studies didn’t use large enough samples of individuals to tease out other factors. So Schino ran statistical tests on 36 relevant studies carried out on 14 different primate species, a technique called meta-analysis.

Like other social animals, primates live in societies where individuals often come to blows over food or access to mates, as well as trying to climb the ladder to alpha-hood. When witnessing a conflict between two group-mates, a third party must choose which contestant to back.

Schino found this choice is at least partially determined by past grooming received from each mate, with frequent groomers receiving more frequent support from those served.

The study also showed this reciprocal altruism is generally more frequent in some primates than others. Old World monkeys, such as macaques and baboons found in Asia and Africa, practice groom-for-support behaviors more than New World monkeys such as capuchins and marmosets found in Mexico and Central and South America.

“Nobody understands why,” Schino said.

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