Dogs yawn along with their owners, revealing a human-like ability to empathise, say scientists.
Everyone knows yawns can be contagious, but a new study shows how the irresistible impulse to yawn can even spread between humans and their dogs.
According to researchers this could be an indication of empathy - the capacity to identify with another person's emotional state - in man's best friend.
The Japanese team of scientists recruited 25 dogs and their owners for the study. Dogs watched their owner, or someone they did not know, yawn or mimic yawning mouth movements.
They were much more likely to yawn in response to their owner yawning than the actions of a stranger.
The dogs were also far more sensitive to genuine yawns, and yawned significantly less often after seeing fake movements.
One possible explanation for yawning is that it is a tension-releasing reaction to mild stress. But the fact that the dogs responded more to their owners' genuine yawns, and maintained a constant heartbeat, made this unlikely, said the researchers.
They wrote in the online journal Public Library of Science ONE: "Our findings are consistent with the view that contagious yawning.. may indicate that rudimentary forms of empathy could be present in domesticated dogs."
Two cameras were used to record dogs' responses during the testing sessions. Owners called their dogs by name and then yawned, or made a yawning movement, after making eye contact.
Four test sessions lasting five minutes were conducted for each experimental condition involving a real or fake yawn by an owner or one of the researchers.
At the same time, the dogs' heart rates were monitored using a device strapped to their chests.
Lead scientist Dr Teresa Romero, from the University of Tokyo, said: "Our study suggests that contagious yawning in dogs is emotionally connected in a way similar to humans.
"Although our study cannot determine the exact underlying mechanism operative in dogs, the subjects' physiological measures taken during the study allowed us to counter the alternative hypothesis of yawning as a distress response."
Contagious yawning is said to affect 45% to 60% of human adults, yet the causes and reasons for it remain unclear.
In non-human primates, the phenomenon has been observed in chimpanzees, bonobos, and gelada baboons. Like humans, they are more responsive to yawns from individuals they have close social bonds with.
Little evidence of contagious yawning has been seen in animals other than primates, said the scientists. But there has been a suggestion of it in one species of bird - the budgerigar.