Yawn next to your dog, and she may do the same. Though it seems simple, this contagious behavior is actually quite remarkable: Only a few animals do it, and only dogs cross the species barrier. Now a new study finds that dogs yawn even when they only hear the sound of us yawning, the strongest evidence yet that canines may be able to empathize with us.

Besides people and dogs, contagious yawning has been observed in gelada baboons, stump-tail macaques, and chimpanzees. Humans tend to yawn more with friends and acquaintances, suggesting that "catching" someone's yawn may be tied to feelings of empathy. Similarly, some studies have found that dogs tend to yawn more after watching familiar people yawning. But it is unclear whether the canine behavior is linked to empathy as it is in people. One clue might be if even the mere sound of a human yawn elicited yawning in dogs.

To that end, scientists at the University of Porto in Portugal recruited 29 dogs, all of whom had lived for at least 6 months with their owners. To reduce anxiety, the study was performed in familiar rooms in the dogs' homes and in the presence of a known person but with no visual contact with their owners.

The team, led by behavioral biologist Karine Silva, recorded yawning sounds of the dogs' owners and an unfamiliar woman as well as an artificial control sound consisting of a computer-reversed yawn. (To help induce natural yawning, volunteers listened to an audio loop of prerecorded yawns over headphones.) Each dog heard all of the sounds in two sessions, each carried out 7 days apart. During the sessions, the researchers measured the number of elicited yawns in dogs in response to sounds from known and unknown people.

As the team will report in the July issue of Animal Cognition, 12 out of 29 dogs yawned during the experiment. On average, canines yawned five times more often when they heard humans they knew yawning as opposed to control sounds. "These results suggest that dogs have the capacity to empathize with humans," says Silva.

That's not surprising, she says. People first began domesticating dogs at least 15,000 years ago, and since then we've bred them to perform increasingly complex tasks, from hunting to guiding the blind. This close relationship may have fostered cross-species empathy over the millennia.

"This study tells us something new about the mechanisms underlying contagious yawning in dogs," says Evan McLean, a Ph.D. student at Duke University's Canine Cognition Center in Durham, North Carolina, who was not part of the study. "As in humans, dogs can catch this behavior using their ears alone." Still, he notes, the experiments don't tell us much about the nature of empathy in dogs. "Do they think about our emotions and internal states the way we do as humans?"

Ádám Miklósi, an ethologist at the Eötvös Loránd University in Budapest, agrees. "Using behaviors as indicators will only show some similarity in behavior," he says, "but it will never tell us whether canine empathy, whatever this is, matches human empathy." Previous work has shown, for example, that when dogs look guilty, they may not actually be feeling guilty. "Dogs can simulate very well different forms of social interest that could mislead people to think they are controlled by the same mental processes," says Miklósi, "but they may not always understand the complexity of human behavior."

ScienceNOW, the daily online news service of the journal Science