NEW YORK The way Fido wags his tail might reveal more about him than you know. Just ask another dog.
A few years ago, researchers discovered a subtle difference in how dogs wag their tails.
When a dog sees something positive, such as its owner, it tends to wags its tail more to its right.
The wagging tends to go left when it sees something negative, like an unfamiliar dominant dog.
Now, the same Italian researchers report that other dogs pick up on that difference, and it's reflected in their behavior and even their heart rates.
Experts say the tail-wagging difference appears to be one way that dogs gauge how other dogs will respond to them.
"It's just fascinating that dogs pick up on it," said Evan MacLean, co-director of Duke University's Canine Cognition Center.
For humans, he said, "it's a difficult thing to see."
MacLean was not involved with the study, reported Thursday in the journal Current Biology.
Giorgio Vallortigara of the University of Trento in Italy, an author of the study, said Fido is not deliberately sending a message.
Instead, the tail-wagging behavior stems from how different emotional cues activate different parts of the brain, he said in an email.
For the experiment, Vallortigara and co-authors used videos of a dog or its silhouette, wagging its tail mostly to one side or the other, or not wagging at all.
They showed the videos to 43 dogs, including such breeds as Rottweilers, beagles, boxers, border collies and German shepherds as well as mongrels.
When the dog in the video wagged mostly to its left, the sign of a negative response, observer dogs tended to have faster heartbeats than when it wagged the other way or not at all.
Their behavior also indicated a higher degree of stress.
Alexandra Horowitz, who studies mental abilities of dogs at Barnard College in New York, said that the wagging difference is probably not a primary signal between Fido and Rover in daily life, but it may play a minor role.