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You know that old phrase, "monkey see, monkey do"? Well, there might be something to it, except that chimpanzees aren't monkeys. (Sadly, "ape see, ape do" just doesn't have the same ring to it.) A new paper published today in PLoS ONE has found evidence that chimpanzees have contagious yawning - that is, they can "catch" yawns from watching other chimpanzees yawning - but (and here's the interesting part) only when the chimp that they're watching is a friend.

At first, scientists thought that contagious yawning was the result of a releasing mechanism - in other words, seeing someone yawn flips the yawning-switch in the brain, and that makes you yawn. Others proposed that yawning was a mechanism designed to keep the brain cool. But it actually turns out that there is a correlation between the susceptibility for contagious yawning and self-reported empathy. Humans who performed better at theory of mind tasks (a cognitive building block required for empathy) also yawn contagiously more often (PDF). And two conditions that are associated with a distinct lack of empathy are also associated with reduced or absent contagious yawning: schizotypy and autism.

So far, contagious yawning has been observed in five mammals: humans, chimpanzees, stumptail macaques, gelada baboons, and domesticated dogs, though the interpretation of the data has been inconsistent. There is still no consensus on the function of contagious yawning, or even whether it exists in the first place.

ResearchBlogging.orgBut now, Matthew W. Campbell and Frans de Waal of the Yerkes National Primate Research Center at Emory University have proposed a more nuanced view of contagious yawning. They wondered if social group membership could affect the transmission of a contagious yawn. After all, if empathy is indeed the thing underlying contagious yawning, then contagious yawning should show many of the same behavioral signatures that empathy itself does. For example, it is known that certain parts of the brain (the anterior cingulate and the anterior insula) activate both when people experience pain as well as when another person experiences pain (other parts of the brain only activate in response to personal pain, not to others' pain). From this data, researchers suggested that humans are able to share the emotional aspects of pain, but not the physical aspects of pain, with others. This, of course, is the basis for empathy. But additional fMRI studies have further refined these findings: activity in the anterior cingulate is greater in response to watching an in-group member experience pain than in response to the pain of an out-group member. So if contagious yawning reflects empathy, and empathy varies on the basis of social status, then it is possible that contagious yawning will vary on the basis of social status as well.

Twenty three adult chimpanzees (Pan troglodytes), ranging in age from 10 to 46 years, from two different social groups were tested. Both groups lived in large outside enclosures with indoor sleeping quarters. The two groups were completely isolated from eachother. Since chimpanzees are highly territorial and overtly aggressive towards other groups, it is certain that members of the same social group are considered part of the in-group, and strangers are automatically outsiders.

Campbell and de Waal recorded videos of chimpanzees while they were yawning to use as experimental stimuli. The videos were edited down to just nine seconds each and were shown to the chimps on an iPod touch. It was expected that they would yawn more when shown videos of group members yawning than when shown videos of strangers yawning. They were also shown videos of chimpanzees doing other things, as a control condition. In this video, Tara, an adult female, yawns while watching a video of another chimp from her social group yawning on the iPod touch.

The chimanzees indeed yawned more often after watching videos of an ingroup chimp yawning compared with the ingroup control videos. In addition, they yawned more in response to the ingroup yawn videos than the outgroup yawn videos. The response to the outgroup yawn videos was no different from the response to the outgroup control videos. Also, there were no gender differences: males yawned in response to ingroup yawn videos as often as the females did.

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It is possible that this response pattern could be the result of differences in attention, rather than underlying empathy, though. In other words, perhaps the chimps were simply paying more attention to the videos of their buddies than the videos of the strangers. If they had paid more attention to the outgroup members, the argument goes, then perhaps they would have yawned more as well when watching those videos. It turned out, however, that the chimps actually paid more attention to both types of outgroup videos. Despite that, they still yawned more in response to the ingroup yawn videos.

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Taken together, this provides strong evidence that empathy does underlie contagious yawning, and that contagious yawning is dependent on social group membership. Given that, it is therefore unclear why humans and dogs do yawn after watching strangers or outgroup members yawn. Since all members of a chimpanzee community know eachother, not only are they members of the same group, but they are all familiar with eachother. Campbell and de Waal speculate that humans, at some point in our evolution, may have evolved the ability to consider strangers, despite their unfamiliarity, as ingroup members. If this was the case, then strangers would not automatically be considered outgroup members, as they are with chimpanzees. In order to serve as successful pets, domesticated dogs must also be able to comfortably interact with human strangers as well as with other dogs. It is possible that the selection process of domestication has allowed dogs the possibility of dissociating familiarity from group membership, as we have.

Given the potential relationship between contagious yawning, empathy, and the ingroup bias, it would be interesting to extend this research to bonobos, domestic dogs, and humans. If this line of research bears out, contagious yawning could serve as a method for better investigating the social and emotional bonds among individuals. Campbell and de Waal suggest that understanding how and why chimpanzees alternate between empathy and aggression can help us understand our own human social emotions. Indeed, humans could certainly stand to have a little more empathy towards social outsiders.

Chimp photo via Flickr user Pelican.

Matthew W. Campbell, & Frans B. M. de Waal (2011). Ingroup-Outgroup Bias in Contagious Yawning by Chimpanzees Supports Link to Empathy PLoS ONE, 6 (4) : 10.1371/journal.pone.0018283

For more on contagious yawning:

Yawn yawn yawn yawn yawn! Contagious Yawn!

Yawning Together

For more on chimpanzees:

Digitizing Jane Goodall's Legacy at Duke

The Fate of the Alamogordo Chimps

A Bonobo in the Hand or Two Chimps in the Bush?

Chimpanzee Curiosity

Gratitude: Uniquely Human or Shared With Animals?