Aug. 7, 2013 at 6:08 PM ET
When your pet dog yawns after you do, it's just one more sign that man's best friend is acutely sensitive to human feelings.
As researchers continue to debate the significance and origins of yawns in people and animals, one school of thought suggests that the "contagious yawn" — say, when one student's yawn in a slow history class sets off a chorus among his peers — is a measure of emotional connection, or empathy.
New research from a team in Japan shows that pet dogs can catch the yawning bug from people, just as we catch it from each other. But they're more sensitive to the yawns of their owners than from humans they don't know.
The researchers tested pit bulls, papillons, poodles — 25 dogs in all — and recorded their heart rates as they yawned after their owners.
"Our results show that the emotional bond between people and their dogs is reciprocal," Teresa Romero, an animal behavior researcher at the University of Tokyo who conducted the analysis, told NBC News in an email. "This attachment can shape the dog’s responses in a way similar to humans, that is, to be more sensitive to a familiar yawn than to a stranger," she added.
Back in 2008, biologists in Britain showed that yawns were contagious between humans and their pet dogs. Dogs also yawn when they are stressed.
This new study led by Romero measured the heart rates of the yawning pups and adults, and found that heart rates were stable, clarifying that the response was one of empathy rather than anxiety or distress. Romero and her colleagues present their findings in the Aug. 7 issue of PLOS ONE.
While yawning is widespread across the animal kingdom, contagious yawning — the kind that's an indicator of community and kinship — has only been seen in humans and a handful of other animals. Chimpanzee groups share collective yawns. Bonobo communities pass them around, too. In fact, one study showed that waves of yawning among bonobos are triggered more frequently by adult females, the emotionally prominent members of a group.
Contagious yawn-like behavior has also been observed in stumptail macaques — and videos were enough to set them off. When 22 primates were played video tapes of yawning adult and young monkeys, they yawned more often after a yawning video than after tape that showed stretching or biting or other mouth movements.
In humans, there's evidence that yawning is a physiological response to the environment in addition to being an unconscious emotionally derived response. One study showed that we yawn more when the weather is cooler. "Nearly half of the people in the winter session yawned, as opposed to less than a quarter of summer participants," the author of that 2011 study explained.
The question of why humans, dogs and apes, must yawn continues to intrigue researchers, but come winter or summer, rain or shine, one thing's becoming clear: When you yawn, your dog will, too.
More about yawning:
Akitsugu Konno and Toshikazu Hasegawa join Teresa Romero as co-authors of "Familiarity Bias and Physiological Responses in Contagious Yawning by Dogs Support Link to Empathy," published in PLOS ONE this week.