4 things your dog is trying to tell you with the movement of their tail

• Dogs wag their tails for a number of reasons, not just because they're happy.
• If your dog has a stiff tail with only the tip moving, it can signal aggressiveness or unrest.
• If your dog puts their whole body into wagging their tails, they're probably happy or excited.

If you're a dog owner, you're likely familiar with the tail wag of your furry companion when you come home from work or when your dog spots another animal companion. While tail wagging is most commonly associated with happiness and excitement, dogs wag their tails for a number of reasons.

Humans often communicate using facial expressions, like smiling, to let other people know they are friendly and approachable. Because dogs can't smile, they use body movements, like tail wagging, to indicate feelings they want other dogs to know, Dr. Rebecca Greenstein, the chief veterinarian and practice owner at Kleinburg Veterinary Hospital, said.

Here's a look at four reasons dogs wag their tails and what their tail position may indicate about what they're feeling.

1. A whole-body wag: happiness

The most common feeling associated with dogs' tail wagging is happiness.

"The best type of wag is when the dog is using a loose, soft tail wag or a whole-body wiggle where it seems almost as if the body and tail are moving back and forth together," said Elizabeth Dimit, a dog trainer with Dogtopia, a dog-day-care and -boarding service. "That is a true happiness wag."

A 2018 review found a happy tail wag was a way for dogs to express energetic enthusiasm both to other dogs and to humans.

You can even get clues about how your dog is feeling by the direction of their tail wagging. A 2018 review suggested that:

• When a dog sees something positive, like the sight of its owner, its tail moves more toward the right side.
• But when they see something that makes them uncomfortable, like an unfamiliar dog, your dog may wag their tail more to the left.

"The direction of tail wagging isn't random, it's actually a very real form of canine communication," Greenstein said.

2. A tucked tail: nervousness

A wagging tail does not always indicate happiness, Dimit said. Dogs use tail wagging to communicate many complex emotions. For example, a tucked tail with fast wagging can indicate anxiety or uneasiness.

According to a 2018 report by veterinarian-medicine researchers, dogs likely tuck their tails to avoid conflict by reducing the appearance of their size to show other dogs they are not a threat.

"While humans often mistakenly assume that all tail wagging always equates to a welcoming smile, dogs understand that even something as subtle as how high or low they carry their tails or how fast they wag tells them a lot about whether that gesture is more dominant or submissive, inviting or threatening," Greenstein said.

3. A stiff tail: alertness

Sometimes, a dog's tail position may not be expressing happiness or fear but simply sending a signal that they saw or heard something they were interested in, Dimit said.

A tall, stiff tail shows that your dog is alert and may soon run after something, Dimit said, like a squirrel they spot in your yard or another dog they see at the park. A tail raised high can also indicate a dog's confidence and intention to positively approach another human, dog, or animal.

4. A rigid tail with a slow-moving tip: aggressiveness

Sometimes, you'll see a dog with a stiff tail with the tip slowly moving back and forth. This is a movement called "flagging," which can indicate aggression.

Those who don't understand what different tail wags and positions mean may try to approach a dog who is "flagging," thinking they are showing a happiness wag when the dog is actually not in a calm state of mind.

Even if you think you see a friendly dog wagging its tail, always approach with caution and ask the owner if you can pet their dog. Other signs that a dog may feel aggressive include an arched back, a still body, and a low growl.

What about tailless dogs?

Some dog breeds like Boston terriers and French bulldogs do not have tails, but they still use body language to communicate, including their ear positions, facial expressions, and body stances, Greenstein said. They also may approach other dogs, animals, or humans with more caution than dogs with tails in an attempt to avoid miscommunication.

Insider's takeaway

Dogs wag their tails to express complex emotions, like happiness, fear, anxiety, alertness, or aggression. Paying attention to your dog's tail position, as well as the speed and direction of their wag, can help you determine what your dog may be feeling.

A wagging tail does not always indicate happiness or friendliness and, in some cases, can even be a sign of aggression. Always approach a dog you don't know slowly and ask the owner if you can pet them.