Can Monkeys Mislead?

2nd July 2010

This is one of the only studies which has actually [used] an experimental paradigm to look at tactical deception," said primatologist Katie Slocombe of the University of York, UK, who was not involved in the work. In this case, producing false alarm calls allows animals lower in the social hierarchy "to get hold of food that they would not be able to access otherwise."

In the forest of Iguazá National Park, Argentina, primatologist Brandon Wheeler of Stony Brook University in New York observed a well-studied population of approximately 25 capuchin monkeys giving "hiccup" calls -- two-syllable cooing sounds commonly uttered in response to danger (play audio for an example) -- outside of their usual predator-alert context. "There [was] no apparent reason to give these calls other than to chase the other individuals off the food platform," Wheeler said.


Struck by this curious observation, Wheeler set out to determine if these so-called resource-related deceptive alarm (RRDA) calls were indeed deceptive signals. By placing highly-valued banana pieces on small feeding platforms 3 to 10 meters above the ground, he was not only able to elicit these calls at 10 times the rate they occur in a natural context, he was also able to correlate this behavior with individual rank, food distribution, and proximity to the food resource.

With one exception, RRDAs were produced exclusively by subordinate individuals. "There would be no reason for dominants to [call] because they could take the food anyway," Slocombe reasoned. Furthermore, 12 of the 14 individuals observed producing RRDAs were within two meters of a feeding platform, and these calls elicited escape reactions in at least one nearby monkey 40% of the time. The caller was therefore able to feed more freely, either by gaining access to a platform occupied by another animal or by scaring off competitors from the platform on which it was currently feeding.

Additionally, the monkeys produced RRDAs more often when the bananas were distributed across just one or two platforms, when competition for access was greater, as compared with three or more. There was no difference in call rate as a result of total food availability, however.

Overall, "it does look like tactical deception," Slocombe said. But "we've got no way of knowing whether they actually intend to deceive."

The difference between intentional deception and what researchers refer to as "functional deception," Wheeler explained, is whether or not "the individual that's acting deceptively understands that they are creating false belief. If the monkeys are intentionally being deceptive then they understand that they are making the other group members believe there is a predator present."

Alternatively, capuchin monkeys may produce RRDAs in response to stress. "Every context that I've seen them give these calls is a context where they can be assumed to be under some degree of stress," Wheeler said. "In the context of feeding, there's lots and lots of aggression."

If the stress of a competitive feeding situation elicits RRDAs and these calls successfully scare off other feeding monkeys, individuals may then learn to associate the calls with access to food. In this way, while the calls are indeed deceptive, their behavior "would not be intentional in any sense of the word," Wheeler said.

In other words, "deception could be a mere 'spandrel,' a by-product of a signal that is naturally given to a broad range of events," comparative psychologist Klaus Zuberbühler of the University of St. Andrews in Scotland wrote in an email. "It forces us to rethink the notion of alarm calls and what might have caused the evolution of these signals."