You might think a dog with a wagging tail is always a happy dog, but it might not actually be that simple. New research suggests that dogs react differently to other dogs’ tail wags, depending on if the tail flips primarily to the right or the left side.

What seems like a subtle difference, as shown in the video below, may end up carrying a lot of meaning:

A new study from a group of Italian researchers, published on Thursday in the journal Current Biology, builds upon the authors’ previous work on tail-wagging. In a 2007 study, the researchers examined the difference between a dog’s tail-wagging responses when confronted with his or her owner, an unfamiliar person, a strange and dominant dog, and a cat. They noticed that the dogs tended to swish their tails more to the right when interacting with people, or a cat; when interacting with a strange dog, they tended to wag to the left. The researchers suspected that dogs wag to the left when experiencing negative emotions (anxiety at being confronted with a dominant dog) and wag to the right when experiencing more-positive sensations (excitement at seeing their owner or a cat to potentially chase).

To test their theory, the researchers recruited 43 healthy domestic dogs for a new set of experiments. Some dogs were shown a video of another dog wagging its tail with a rightward or leftward bias; others were shown a video of the same dog, but shown in silhouette. The scientists monitored the dogs’ reactions by observing behavior and monitoring heart rates.

What they found seemed to validate the hypothesis.

"The direction of tail wagging does in fact matter, and it matters in a way that matches hemispheric activation," author and University of Trento researcher Giorgio Vallortigara says in a statement. "In other words, a dog looking to a dog wagging with a bias to the right side -- and thus showing left-hemisphere activation as if it was experiencing some sort of positive/approach response -- would also produce relaxed responses.”

But when a dog sees another dog’s tail wagging primarily leftward, indicating activation in the right hemisphere of the brain and a negative response, the observing canine responds with anxious signals and increased heart rate, Vallortigara said.

“It is always terrifically interesting to see the subtleties of dog behavior which we humans usually miss,” Barnard College psychologist Alexandra Horowitz, who runs a dog-cognition research lab in New York City, wrote in an email. “This is another example of that.”

But what the dogs are communicating to each other, and whether they are intentionally communicating it, is another question altogether, Horowitz said. The authors are also uncertain as to whether the tail wagging is intentional emotional communication. Plus, the book isn’t fully closed on what both left-hand and right-hand wags actually signify. A different group of researchers found in a 2011 study that dogs were more likely to approach a robot dog that was wagging its tail to the left than one that wagged to the right -- seemingly the opposite from what you’d expect to happen if the Italian researchers are right about left-ward wags signifying negative emotions.

Horowitz points out that videos of tail wags can be slowed down for better scrutiny. Real-world tail-wagging is much more subtle and less obviously directed than the wags used in the new paper, she says.

“The stimulus that the subjects in this study looked at had quite a profound left/right wag bias -- so while their results look genuine (seeing different heart rate for left bias, primarily), it is unlikely that they would often experience actually seeing a left/right tail bias on another dog,” Horowitz says. “Thus, this shows an interesting theoretical response to a tail wag bias -- but it is not at all clear that dogs are seeing or using this information in any of their dog-dog interactions.”

Evan MacLean, the co-director of the Duke Canine Cognition Center in Durham, N.C., praised the authors' decision to use both the full video of a dog wagging its tail and a silhouetted version in their experiments.

"If only full resolution video was used we might be left with the question of whether it was really tail wagging the dogs were tuning in to," MacLean wrote in an email. "For example, were there other cues from the face that dogs were using? In the silhouette condition we can clearly see that its the tail wagging that drives dogs' responses."

Scientists have known for some time that brain activity in different hemispheres can influence animal behavior, and Vallortigara and colleagues have shown that tail wagging seems to follow this principle, MacLean says. Now they have provided the first evidence suggesting that dogs can pick up on these subtle cues derived from the left or the right side of the brain.

"What's really interesting is that dogs have similar biases in how they process human faces, looking more to one side or the other, depending on the emotional state of the person," MacLean says. "This paper suggests that dogs may have been predisposed to understanding humans this way based on cues they use to understand other dogs."

SOURCE: Siniscalchi et al. “Seeing Left- or Right-Asymmetric Tail Wagging Produces Different Emotional Responses in Dogs.” Current Biology published online 31 October 2013.