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Never Trust a Hungry Monkey
3 June 2009 (All day)
Do you covet your neighbor's candy? Create a diversion, and that chocolate bar could be yours. Capuchin monkeys may know this trick as well: New observations reveal that they falsely make predator alarm calls and, in so doing, grab food abandoned by those they've scared off. If this deceptive behavior is deliberate, the monkeys have evolved a highly complex mode of thinking.
Tufted capuchin monkeys (Cebus apella nigritus) are relatively small primates living in South America. They feed mainly on fruits and insects, and live in groups of about seven to 40 animals. When feeling threatened by a snake or a predator cat such as an ocelot, the monkeys give off warning calls that sound like hiccups to alert other members of the group, who then respond by fleeing or becoming more alert.
But these calls may not always serve as a real warning. Brandon Wheeler, a biological anthropologist at Stony Brook University in New York state was studying a group of capuchins eating food left on platforms constructed in trees, when he noticed some of the monkeys made the calls when predators weren't around. "They were giving the same calls that they give for cats extremely frequently," he says. "When they do, other individuals often run out off the platform, which potentially leaves [the platform] available for whoever called to jump in to get some food."
Researchers have noted deceptive behavior in primates and other species, but Wheeler pursued his anecdotal observations more systematically. In a series of experiments, he used the treetop platforms to offer food such as bananas to the monkeys in their natural habitat in Iguazú National Park in Argentina. Capuchin monkey society is hierarchical, with high-ranking individuals bullying those below them for food. But Wheeler found that the low-ranking monkeys seem to even the score by crying wolf: When offered bananas, low rankers who would otherwise risk missing out on the treat tended to give false alarm calls, especially when they were close to the platform. In 40% of the cases, the dominants reacted to the calls by running off while the low rankers stole a banana or two, Wheeler reports online today in the Proceedings of the Royal Society B.
If these false alarms are deliberate, says Wheeler, they suggest that capuchin monkeys have evolved the ability to predict how others respond to stimuli. "There is a hypothesis called the Machiavellian Intelligence hypothesis, which argues that increased cognitive abilities evolved so that individuals could better compete in the kinds of competitive interactions that inevitably result from living in a group," he says. "One of the predictions of the hypothesis is that deception should be a common behavior."
Comparative psychologist Klaus Zuberbühler of the University of St. Andrews in the United Kingdom, an expert on primate vocalizations, praises Wheeler's study as "excellent empirical work." But he's skeptical that the data offer evidence of intentional deceit. He notes that the low-ranking monkeys may make more false alarms simply because they're stressed by being around the bullies; that would make the deceit an accidental byproduct.