They aren't the symptoms of a disease, but they're contagious nonetheless: yawns. But why? Now new research out of Emory University's Yerkes National Primate Research Center has shown for the first time that humans aren't the only animals that yawn contagiously, and the discovery is already revealing important clues about why it happens, according to

It turns out that chimpanzees, our closest living biological relatives, also can't resist a good yawn when in the presence of other yawning chimpanzees. And they're not just aping around; the trait may actually hint at deep emotional and social bonds shared between them.

Scientists have long theorized that contagious yawning indicated an underlying empathy among individuals who share the yawn, much in the same way that seeing someone laugh or cry can make you feel happy or sad. Yerkes researchers Matthew Campbell and Frans de Waal have strengthened this hypothesis by observing the same effect in chimps.

"The idea is that yawns are contagious for the same reason that smiles, frowns and other facial expressions are contagious," they write. "Our results support the idea that contagious yawning can be used as a measure of empathy, because the biases we observed were similar to empathy biases previously seen in humans."

In the study, 23 adult chimps were shown several nine-second video clips of other chimpanzees yawning. Not only was the yawning contagious, but chimpanzees yawned 50 percent more often when the ape in the video was a member of their own social group, as opposed to a stranger.

The researchers believe these results indicate more than just the contagious nature of yawning. Since the behavior correlates with the closeness of their relationships, chimpanzees that share a contagious yawn also share a greater amount of empathy toward one another.

Although a similar study has yet to be performed on humans, researchers believe other research already hints at comparable results. For instance, scientists have identified certain parts of the brain that are activated both when someone experiences pain and when they see someone else experiencing pain. In these experiments, the subjects tended to show more sensitivity toward members of the same social group, much in the same way that the chimpanzees did with the yawning.

Aside from helping to solve the mystery of contagious yawning, the Yerkes research may also provide a valuable new way for primatologists to study the social and emotional connections between chimpanzees in the wild. 

"Empathy is difficult to measure directly because it is a largely internal response: mimicking the emotional response of another. Contagious yawning allows for a measurement of empathic response that is purely behavioral, and thus can be applied more widely," wrote Campbell.

At the very least, the research has provided yet another demonstration of just how deep the emotional life of our ape cousins can be, and also just how similar they are to us.

MNN homepage photo: Dan Kitwood/Getty Images

Bryan Nelson ( @@brynelson ) writes about everything from environmental problems here on Earth to big questions in space.

Chimpanzee study reveals why yawning is contagious
Humans and chimpanzees who yawn contagiously are more empathetic, according to a new study.