In an article published early online by the journal Current Biology, Duke University Medical Center neurobiologists Michael Platt, Robert Deaner and Amit Khera describe experiments in which they gave male rhesus macaque monkeys juice rewards for glancing at either a neutral target on a computer screen or images of other monkeys. By systematically varying the juice rewards and the images -- including a gray square, higher-ranking or lower-ranking monkeys and female hindquarters -- the researchers could precisely measure how much reward a monkey would "pay" to see which images.
The researchers found that the monkeys would forego a significant amount of reward to see an image of a higher-ranking monkey or of female hindquarters. In contrast, the monkeys had to be "paid" more juice to view lower-ranking monkeys.
The research was sponsored by The National Institute of Mental Health and the Cure Autism Now Foundation. It will be published in the March 2005 issue of Current Biology.
The aim of the study, said Platt, was to bring into a controlled laboratory setting the kinds of social judgments that monkeys were observed to make in the wild.
"Decades of studies of monkeys in the wild have indicated that they act as if they make judgments about dominance rankings and of the importance of other individuals for their own reproductive success," said Platt. "But there have been no real quantitative experimental demonstrations that monkeys actually process this information and use it in decision-making.