That suggests dogs are "emotionally connected" to people, study leader Teresa Romero of the University of Tokyo said in a statement.

Scientists already knew that dogs sometimes yawn when they see people yawn, but it was unclear if that was considered a form of empathy or mild stress, as yawning can be caused by anxiety.

So Romero and her team set up an experiment in which 25 pet dogs watched both their owners and strangers yawn or pretend to yawn. (Read about why people yawn.)

The team ruled out stress when researchers saw no significant differences in the dogs' heartbeats during the experiments, according to the study, published August 7 in the journal PloS ONE.

Not only did the dogs in the study yawn more in response to their owners' yawns, they also yawned less when they saw fake yawns from their owners or from strangers, suggesting they were exhibiting true contagious yawning.