A Dog's Wagging Tail – Not a Guarantee of Friendliness

Analysis by Dr. Karen Shaw Becker

dog tail wag direction meaning

Story at-a-glance -

  • Most people assume a dog with a wagging tail is happy and friendly, but that’s not always the case
  • Tail wagging may have evolved as a canine communication tool, since it appears dogs send messages to each other through tail wags
  • A 2013 Italian study concluded that dogs who see another dog wagging to the left experience anxiety and elevated heart rates; dogs who see another dog wagging to the right remain relaxed
  • Generally speaking, these results suggest that tail wags to the right signal openness and wags to the left indicate wariness
  • Other tail wagging signals includes full body wags, the “circle wag,” tip of the tail wags and stiff or rigid wags

A wagging tail is pretty much the universal sign for a happy, friendly, approachable dog … or at least that’s what most people assume. So, you can imagine how shocking and upsetting it is when a dog with a wagging tail becomes aggressive, even to the point of biting. And it happens more often than you think.

To understand more precisely when and why dogs wag their tails, we have to expand our thinking beyond the “happy dog” hypothesis:

“The best way to think of a dog’s tail and its side-to-side motion is as an energy indicator,” writes world-renowned animal behaviorist and author Dr. Nicholas Dodman. “When a dog’s energy level is up or when he’s excited, his tail will wag fast. When he’s interested but not fully engaged, it might wag slowly. Then, as he becomes progressively more riveted or excited, his tail will wag progressively faster.

Think of the tail as you would a car’s tachometer. It indicates how fast the animal is revving internally. Now, that can be happy revving or frightened revving or conflict revving. In other words, fast and furious tail wagging could mean the dog is “locked and loaded” and ready to charge. The wagging has to be interpreted circumstantially.”1

Tail Wagging Evolved as a Dog-to-Dog Communication Tool

A 2013 study published in the journal Current Biology2 suggests dogs send messages to each other through tail wags. And according to study co-authors, the direction of a wag is quite significant. What looks like just another friendly wag to you or me is actually communicating important information to other dogs.

As it turns out, when dogs are feeling stressed, they tend to wag their tails to the left. The reason for this, according to lead author, Giorgio Vallortigara of the University of Trento in Italy, is that tail wagging is a reflection of what’s going on in a dog’s brain. Activation of the left-brain causes the tail to wag to the right, and activation of the right-brain produces wagging to the left.

For the study, Vallortigara and his team used videos of a dog or dog silhouette wagging its tail mostly to one side or the other, or not at all. The only thing moving in the wagging videos was the tail.

The video was shown to 43 pet dogs, including 18 mixed breeds, 2 Rottweilers, 3 Boxers, 3 Basset Hounds, 1 Bulldog, 4 German Shepherds, 3 Border Collies, 2 Jack Russell Terriers, 2 Siberian Huskies, 3 Beagles, and 2 Labrador Retrievers. The group consisted of 25 females and 18 males ranging in age from 1 to 11 years. Each dog wore a heart rate monitor.

When the video dog wagged primarily to its left, indicating a negative response, the dogs in the study tended to have faster heartbeats than when the video dog wagged to the right or not at all. The dogs’ response also suggested a higher degree of stress.

Left-brain activity in dogs resulting in tail wagging to the right means they are having a positive response that invites another dog to approach. Right-brain activation suggests a negative withdrawal response. The study concluded that dogs who see another dog wagging to the left experience anxiety and elevated heart rates, whereas dogs who see another dog wagging to the right remain relaxed.

The researchers don’t believe the dogs are intentionally sending signals with their tails, but rather the tail wagging is a consequence of the inner workings of the canine brain. Tail-wagging behavior results from the way in which different emotional signals activate different parts of a dog’s brain.

"These results suggest that dogs have perceptual and attentional asymmetries," says Vallortigara. "So for example, if you are going to visit a dog, if you are vet, there will be probably a side which is better with respect to the probability to evoke a more friendship response or to evoke a more aggressive response."3

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Wags to the Right = Openness, Wags to the Left = Wariness

In an earlier study, Vallortigara and his team demonstrated that dogs wag to the right side when they encounter something pleasant (like their owners).4 When they see something threatening, for example, a strange dog exhibiting dominant behaviors, they wag more to the left side.

When the 30 dogs in the study caught sight of their owners, all of them wagged their tails to the right side. After a 90-second break, the dogs viewed a stranger, and while their tails still wagged to the right, the angle was more moderate than when they spotted their owners.

When a cat came into view, again the dogs wagged to the right, but more subtly than when they viewed the stranger. Finally, the sight of a large, unfamiliar dog caused all 30 dogs to wag more to the left side. These results raised the question of whether dogs notice another dog’s tail wagging and use the information to decide whether the dog with the wagging tail is friend or foe.

The study authors concluded that when dogs feel positive or curious (as in the cat sighting), they wag their tails to the right. When they’re feeling negative or wary, they wag to the left. This makes sense when you consider that the left brain controls the right side of the body, so the right-side muscles of the tail transmit positive feelings, whereas the left-side tail muscles controlled by the right brain respond when a dog is feeling cautious or concerned.

Other Tips for Reading a Dog’s Tail Language

Certified applied animal behaviorist and professional dog trainer Karen B. London isn’t so much focused on the left-right angle as other observable features of tail wagging.

“Generally, the more the wag encompasses the whole body, the friendlier the dog’s intentions,” London writes in a recent Bark blog post. “The full body wag that extends from the shoulders through the belly to the hips and the tail is the classic friendly tail wag.

An active tail wag that gets the hips swinging is also a likely sign of high sociability. (Bonus — these wags can even be recognized in dogs who have no tails to speak of!) When just the tail moves, the wag may or may not have anything to do with being sociable. When only the tip of the tail moves, the dog is most likely not friendly.”5

According to London, the speed of the wag is also informative:

“Fast wags that move through a big arc are associated with friendliness, while a slow wag that doesn’t move the tail much is a sign of an absence of friendliness,” she writes. “Sometimes, dogs who are a little nervous, or perhaps reserved, wag slowly, which simply shows some hesitancy about interacting. In other cases, a slow tail wag can signify aggressive tendencies.”

London also recommends observing the tension in the tail. Generally speaking, the more relaxed the tail is while wagging, the more likely it is the dog is friendly. A stiff, rigid wag typically signals the opposite. And then there are “circle wags” in which the tail moves in a rotating motion starting at the base, with the tip tracing a big circle in the air.

The circle wag, writes London, “is one of the positive signals that experienced dog handlers attend to when evaluating dogs because it is so strongly associated with dogs who are in a friendly mood. Also called the ‘propeller wag’ or ‘helicopter tail,’ this tail motion is particularly common in dogs who are greeting a close friend, especially after a long absence.”

Takeaway Message

Wags that indicate the dog is probably friendly:

  • Wags to the right
  • Full body wags
  • Fast wags that move in a big arc
  • Relaxed tail while wagging
  • The circle wag aka propeller wag aka helicopter tail

Wags that indicate the dog may not be feeling friendly:

  • Wags to the left
  • Just the tail is wagging, and especially, just the tip of the tail
  • Slow wags that don’t move the tail much
  • A stiff, rigid wag