If you wash yourself with raw onion, you might expect some aggression from your friends. Now it seems the same holds true of some primates – but for rather different reasons.
For capuchin monkeys, rubbing themselves with pungent-smelling plants is normally a communal and perhaps even a therapeutic activity.
Wild capuchins are known to get together and rub their fur with plants like citrus and peppers that have antifungal or antiseptic properties.
Some biologists think that the behaviour is medicinal, and that the monkeys are ridding themselves of parasites with their plant rubs. But until now no-one had looked to see what happens after the communal rubs.
Annika Paukner and Stephen Suomi at the National Institute of Child Health and Human Development in Poolesville, Maryland, observed 15 captive capuchins who liked to rub themselves with yellow onions – which also contain high levels of antifungal or antiseptic compounds.
They watched what happened after giving the monkeys either onion or apple as a control, five times a week for five months.
The researchers found that while the capuchins were very social with one another during the onion washing, this polite behavior vanished afterwards, and levels of aggression increased.
Capuchins are thought to signal their relative ranking in the colony by urinating on their hands and feet, so the smell of the onion might be overpowering that signal, says Paukner.
"We think the scent of onions may make detecting the scent of urine difficult or even impossible, which may cross wires in the capuchin social circles and explain the increased aggression," she says.