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Why Do Dogs Wag Their Tails?

By Matt Soniak

A wagging tail isn’t always a happy, welcome-home greeting. Sometimes it means a face full of fur for pet parents or a broken glass flung across the room by a swift swing.

Despite these occasional destructive faux pas, dogs’ tails are more than battering rams and wiggly appendages. They’re communication devices, and with some careful observation, they’ll tell you a little bit about how a dog is feeling.

Why Dogs Wag Their Tails

“The tail serves lots of functions, such as acting as a rudder in the water when the dog is swimming and acting for balance when a dog is running,” says Dr. Lisa Radosata, owner of Florida Veterinary Behavior Service. “If you watch a dog take a tight turn at high speed, you will likely see him use his tail for stability.”

The tail is also one component of the body language dogs use to communicate, along with facial expressions and body postures.

Maybe one of the most common misconceptions about dogs and their tails is that a wagging tail means a happy dog. This isn’t always the case, though.

“Tail wags are a frequent cause of misunderstanding between dogs and humans,” says Dr. Carlo Siracusa, head of the Animal Behavior Medicine Service at the University of Pennsylvania School of Veterinary Medicine. “Tail wagging is what a lot of people look at the most for gauging whether a dog is friendly or not, and even veterinarians rely on it, but this often isn’t very helpful.”

You should view a wagging tail as a sign of willingness to interact, says Radosta, but not a signal of a particular mood or state. “In other words, a tail wag means that the dog is open to interaction, not that the dog is friendly.” Here’s another way to look at it. A wagging tail simply indicates that a dog is mentally aroused and engaged with what is going on in his environment, not how he will react to whatever is going on.

Siracusa says it’s difficult to parse meaning from just a tail wag, and even other dogs, who have a better ability to read those signals than humans do, sometimes make mistakes. This is especially true when the dog doing the wagging has a docked tail, or if it’s a breed with a curly tail, like a pug, that doesn’t have a full range of motion.

Dog Tails and the Tales They Tell

The “language” of a wagging tail can be complex, and the tail doesn’t “speak” in isolation from the other components of body language.

“We tend to give more meaning to a wagging tail than it really has,” Siracusa says, and to try and figure out how a dog might feel beyond “ready to interact” you have to consider the rest of his body language.

That said, a lot of the time you can glean some info by looking at the tail, how it’s positioned, how relaxed it is and how fast it’s going:

Tail Height: A wagging tail held high, Siracusa says, often signals enthusiasm. The height of a wagging tail can also signal a dog’s level of confidence says Dr. Karen London, a Certified Applied Animal Behaviorist and Certified Pet Dog Trainer in Flagstaff, Ariz. A higher tail usually means a dog is confident, while a lower tail indicates a more timid or nervous dog.

It’s important to keep a close eye on a tail held high or over the dog’s back, Radosta says. “A tail wag which just involves the tip, with the tail high above the back, or less than 90 degrees from the back indicates high arousal. This dog may want to interact, but not necessarily in a controlled or friendly manner. I would avoid a dog who was wagging his tail this way or if I was the owner, I would work to help him calm down.”

Tail Stiffness: Generally, a loose, flowing wag is good, London says, but a stiff wag can communicate tension or hostility.

Tail speed: A fast wag is generally good, London says, but a slow wag usually signals that a dog will not be friendly.

Then there’s the “full body wag,” where the butt or whole body is relaxed and moves back and forth.

“A big tail wag where the butt is moving and the tail is making wide sweeping motions” generally indicates a friendly dog who’s ready for a friendly interaction with a person or other dog, Radosta says. London agrees, and says that the closer to the front of the body the back-and-forth rocking starts, the friendlier the dog.

Keep in mind that these wags can have different meanings depending on the situation, and you should always look at the other signals that a dog is giving. “Wags can mean different things in different contexts, just like words,” Siracusa says. “It’s like if I say, ‘One of these days I’m going to kill you.’ That can be read as a joke or an actual threat, depending on the situation.”

Wags and their meanings can also differ from dog to dog. “It [wagging] is a pretty universal behavior. What isn’t universal is temperament between and within breeds,” Radosta says. “One individual dog may wag his tail a little lower or a little higher or a little faster than another individual. It is important for owners to get to know their dog’s body language.”