Published online 1 February 2005 | Nature | doi:10.1038/news050131-5

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Macaques swap juice for a glimpse of leaders' faces and females' rears.

Social animal: by keeping an eye on their fellows, macaques can find mates and avoid fights. © SPL

To a monkey, some things are worth looking at more than others. A US study has shown that rhesus macaques will pay to look at images of powerful or sexually interesting fellows.

The discovery, made by neurobiologists at Duke University in Durham, North Carolina, supports the theory that monkeys will make sacrifices to gain socially useful information, much as a human might spend money on a newspaper.

Male monkeys will 'pay' in fruit juice to look at a picture of a socially dominant monkey or a female's hindquarters. In the wild, the animals help their fitness by monitoring what their leaders are doing, and which females are sexually receptive.

Cherry paid

The researchers gave captive male rhesus macaques two options: a drink of cherry juice, or a different-sized shot of juice and the chance to look at one of a range of pictures of their troop members for just over half a second.

By varying the amounts of juice, the team worked out how much the monkeys valued each image. "Monkeys are basically juice experts; they're very sensitive to the differences," says team member Robert Deaner.

Monkeys would take a juice cut to look at powerful males' faces or the perineum of a female, Deaner and his colleagues report in Current Biology1. But to persuade the monkeys to stare at subordinate males, the researchers had to bribe them with larger drinks.

Gathering social information has costs as well as benefits, Deaner explains. "If you stare at someone too long they might attack you," he says. This goes for humans too: think about how uncomfortable it is to be stared at by a stranger for any length of time.

No surprise, then, that the juice-to-picture exchange rate was highest for images of female rears. "Virtually all monkeys will give up juice to see female hindquarters," Deaner says of his male subjects. "They really value the images."

Face to face

This is not monkey pornography, says Deaner. It's more to do with assessing sexual receptiveness, which in the wild also involves females' behaviour and smell. The monkeys do not necessarily know they are looking at a picture, he adds, so their behaviour is consistent with what they might do in a face-to-face situation.

Learning how animals weigh the value of social interactions could help in understanding human autism, suggests Deaner's colleague Michael Platt. Autistics are not motivated to look at other people, and are poor at gathering emotional information when they do.

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The monkey study might even help to explain humans' fascination with gossip magazines, Deaner suggests. The urge to keep tabs on sexy, powerful people may stem from our tribal past, when the actions of the group's movers and shakers would have influenced our own lives. "Hollywood doesn't affect our lives at all, but people still feel they get some cultural capital from knowing about it," he says.