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 Biology. 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, (researcher Robert) 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."