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Here's something to contemplate on a Friday. When dogs wag their tails, they sometimes wag more on the right side of their bodies, and other times more on the left. It turns out which side the tail is wagging is actually a clue to the dog's emotional state. And a new study shows that other dogs react to the direction of the wag, as NPR's Nell Greenfieldboyce reports.

NELL GREENFIELDBOYCE, BYLINE: Giorgio Vallortigara is a biologist at the University of Trento in Italy. And he says the point of his experiments is not just to understand dogs.

GIORGIO VALLORTIGARA: I have a dog, actually. And I love dogs. But in this particular case, the reason was scientific interest.

GREENFIELDBOYCE: Vallortigara studies how asymmetries in the brain lead to left-right differences in how animals behave. Years ago, his research team did an experiment that showed one example in dogs. Dogs that saw friendly things, like their owners, wagged their tails more to the right side. Dogs that saw threatening things - like an aggressive, unfamiliar dog - had more wagging to the left.

Vallortigara says this seems to be related to how those different stimuli got processed in the dogs' asymmetrical brains, but it made him wonder.

VALLORTIGARA: The question was, at this point: Does asymmetric tail wagging convey meaning to other dogs?

GREENFIELDBOYCE: To find out, the researchers had 43 dogs come to a lab. The dogs wore some little vests that monitored their heart rates. And these dogs watched videos. The videos showed either a natural-looking dog, or just a silhouette of a dog, with the tail wagging to the right or to the left.

It turns out, the dogs did respond to the direction of the wagging. When the video dog wags to its left - the kind of wagging that's linked to negative emotions - the watching dogs got anxious, and their hearts began to race. If the video dog wagged to its right, the watching dogs stayed relaxed. The results are reported in the journal "Current Biology."

TOM REIMCHEN: I really felt that their paper was elegant.

GREENFIELDBOYCE: Tom Reimchen is a biologist at the University of Victoria, in Canada. He once tested whether dogs paid attention to the direction of another dog's tail wagging, by going to a park and setting up a robotic dog with a remote-controlled tail.

REIMCHEN: Our robo-dog is so realistic, it does, actually, trick many, many dogs until they finally get to it, and they realize that they've been duped.

GREENFIELDBOYCE: His group found that when the robo-dog wagged to its right, other dogs would stop and pause more often as they approached. It's not clear exactly why, since right wagging should be a more positive signal. But Reimchen says the important thing is that, like this new study, it shows that animals seem to get information about each other by watching for left versus right.

Nell Greenfieldboyce, NPR News.

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