According to a recent study reported in the journal Current Biology, dogs can pick up emotional cues from another dog by watching the direction of its wagging tail.

In a series of lab experiments, dogs became anxious when they saw an image of a dog wagging its tail to its left side. But when they saw a dog wagging its tail to its right side, they remained relaxed.

This isn’t the first time scientists noticed that the direction of a wagging tail can be linked to doggy emotions. In a previous study, Giorgio Vallortigara of the University of Trento in Italy found that dogs tended to wag to their right side when seeing something friendly, like their owners. Seeing something threatening, like a dominant unfamiliar dog, made them wag more to their left side.

That study caused scientists to wonder if dogs might actually watch another dog’s tail wagging to determine if that dog is friend or foe.

Specifically, does asymmetric tail wagging convey meaning to other dogs?

To find out, scientists had 43 dogs wear vests to monitor their heart rates. Then they had the dogs watch images of dogs where all stimuli except the wagging tail were removed.

Some of the tested dogs saw a video that showed a silhouette of a dog, while others saw a manipulated image of a real dog. In both cases, the only thing that moved in the image was the tail.

When dogs saw either a dog image or silhouette wagging its tail to its right side, they stayed relaxed. But when they saw either a dog image or silhouette wagging its tail to its left side, their hearts began to race and they looked anxious.

Tom Reimchen, a biologist at the University of Victoria in British Columbia, Canada, calls this an “elegant” study. He says a lot of work has been done showing asymmetries in animals’ bodies, but before this study there was no evidence that the wagging tail actually contained information that other dogs could interpret. That’s why “I think this study is very, very interesting and important,” Reimchen said.

Reimchen and his colleagues had previously tested how dogs would respond to asymmetrical tail-wagging by videotaping dogs as they approached a robotic dog with a remote-controlled tail.

However, they found the number of stops a dog made when approaching the robodog was higher when tail-wagging was to the right. They opined that the dogs were initially attracted to this apparently friendly tail-wagging, but then became confused by the immobile robot’s lack of other signals that might convey friendship.

Reimchen says that once people become aware of left versus right in terms of a dog communicating information, “it suddenly opens a whole new range of questions concerning the type of observations you’re making that before you completely overlooked. These animals are probably far more cognizant of those subtle differences than we would have suspected before.”

While it may be more difficult for people to notice asymmetrical tail-wagging than it is for dogs, scientists suggest there are some practical reasons for people understanding left-right differences in dogs. “These results suggest dogs have perceptual attention asymmetries,” says Vallortigara. Meaning that approaching an unknown dog on one side is more likely to evoke a friendly response; while approaching on the other side is more likely to evoke an aggressive response.

This is valuable information indeed for veterinarians, mailmen or anyone who can’t resist the temptation to pet an unknown dog. If you’re looking for a dog who wags his tail to the right every time he sees you visit the Yavapai Humane Society today.

Ed Boks is the executive director of the Yavapai Humane Society. He can be reached at moc.d1493555176aolpu1493555176ptfym1493555176.63e.1493555176f2d@s1493555176kobe1493555176 or by calling 928-445-2666, ext. 101.