By: Stephanie Pappas, LiveScience Senior Writer
Published: 06/08/2012 10:04 AM EDT on LiveScience

Plenty of pet owners are comforted by a pair of puppy-dog eyes or a swipe of the tongue when their dog catches them crying. Now, new research suggests that dogs really do respond uniquely to tears. But whether pets have empathy for human pain is less clear.

In a study published online May 30 in the journal Animal Cognition, University of London researchers found that dogs were more likely to approach a crying person than someone who was humming or talking, and that they normally responded to weeping with submissive behaviors. The results are what you might expect if dogs understand our pain, the researchers wrote, but it's not proof that they do.

"The humming was designed to be a relatively novel behavior, which might be likely to pique the dogs' curiosity," study researcher and psychologist Deborah Custance said in a statement. "The fact that the dogs differentiated between crying and humming indicates that their response to crying was not purely driven by curiosity. Rather, the crying carried greater emotional meaning for the dogs and provoked a stronger overall response than either humming or talking."

Humans domesticated dogs at least 15,000 years ago, and many a pet owner has a tale of their canine offering comfort in tough times. Studies have shown that dogs are experts at human communication, but scientists haven't been able to show conclusively that dogs feel empathy or truly understand the pain of others. In one 2006 study, researchers had owners fake heart attacks or pretend to be pinned beneath furniture, and learned that pet dogs failed to go for help (so much for Lassie saving Timmy from the well).

But seeking out assistance is a complex task, and Custance and her colleague Jennifer Mayer wanted to keep it simple. They recruited 18 pet dogs and their owners to test whether dogs would respond to crying with empathetic behaviors. The dogs included a mix of mutts, Labrador retrievers, golden retrievers and a few other common breeds. [What Your Dog's Breed Says About You]

The experiment took place in the owners' living rooms. Mayer would arrive and ignore the dog so that it would have little interest in her. Then she and the owner would take turns talking, fake-crying and humming.

Of the 18 dogs in the study, 15 approached their owner or Mayer during crying fits, while only six approached during humming. That suggests that it's emotional content, not curiosity, that brings the dogs running. Likewise, the dogs always approached the crying person, never the quiet person, as one might expect if the dog was seeking (rather than trying to provide) comfort.

"The dogs approached whoever was crying regardless of their identity. Thus they were responding to the person's emotion, not their own needs, which is suggestive of empathic-like comfort-offering behavior," Mayer said in a statement.

Of the 15 dogs that approached a crying owner or stranger, 13 did so with submissive body language, such as tucked tails and bowed heads, another behavior consistent with empathy (the other two were alert or playful). Still, the researchers aren't dog whisperers, and they can't prove conclusively what the dogs were thinking. It's possible that dogs learn to approach crying people because their owners give them affection when they do, the researchers wrote.

"We in no way claim that the present study provides definitive answers to the question of empathy in dogs," Mayer and Custance wrote. Nevertheless, they said, their experiment opens the door for more study of dogs' emotional lives, from whether different breeds respond to emotional owners differently to whether dogs understand the difference between laughter and tears.

Follow Stephanie Pappas on Twitter @sipappas or LiveScience @livescience. We're also on Facebook & Google+

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