Those tail wags are all about communication, both with humans and other canines.

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Whether your dog has a sweeping side-to-side tail wag or a high-speed wiggle butt, it always makes you feel good, right? But does it mean he's happy?

A dog's tail can tell you a lot about how he's feeling. While a person uses facial expressions to convey her emotions, a dog primarily uses his tail and other body language.

That's where the age-old question comes in: Does it mean a dog is happy when she's wagging her tail? As Lauren Novack, ABDBC, KPA-CTP, FPPE, and behavior consultant at Behavior Vets in New York City, explains, "This is a great question because many people think that dogs who wag their tails are happy, but that's not always the case. A tail's position and movement are only a part of the whole picture. If a dog's whole body is a sentence, their tail is one word in that sentence."

How to Decipher What It Means When a Dog Wags Their Tail

When attempting to interpret the wag, there are three main things to look at: 

  1. Body stiffness
  2. Tail position
  3. Tail movement

First, notice if the dog's body is stiff, meaning they are not inviting interaction, so keep your distance.

Next is the tail position. "When a dog's tail is tucked towards the belly, the dog is fearful. If the tail is hanging down, they may be nonplussed. If the tail is in line with the dog's spine and parallel to the floor, they are most likely relaxed," Novack explains. "If the tail is held high, they are in a state of arousal that can indicate fear, anxiety, or potential aggression."

To make matters more complicated, Novack adds that not all tails look the same, so "tucked" and "raised" may not look the same on two different dogs.

Lastly, look at the tail movement. The speed at which a dog wags their tail indicates arousal, which can mean stress or eustress (a positive form of stress). In other words, just because a dog's tail is wagging quickly, it doesn't necessarily communicate excitement. 

If your dog's tail is parallel to the floor and moving quickly, he's likely excited. If the tail is held high, is wagging quickly, and the body is stiff, he's uncomfortable.

But the one tail wag that always means your dog is happy is the helicopter tail—you know, when she wags her tail round and round super fast in crazy little circles?

What Does It Mean When a Dog Wags Their Tail in Their Sleep?

Every dog owner has curiously witnessed a dog wagging his tail in his sleep. It truly is comical and sometimes startling when he looks like he's sleeping soundly, and then all of a sudden, wap-wap-wap goes that tail!

Just like humans, dogs sometimes dream when they're in rapid eye movement (REM) sleep, which is a deep state of rest. Along with the tail wags, you may also observe twitching, soft whimpering or woofing, or running-in-the-air paws (also called paddling). Most likely, he's dreaming.

Wouldn't you just love to know what he's dreaming about? Is it a pleasant dream? Or a nightmare? It's really anyone's guess because the current body of canine research hasn't yet cracked that code. Maybe he's chasing squirrels, playing with his friends, or traipsing through the woods. One thing's for sure, as Novack says, it's fun for dog owners to watch their furry friends be super cute when they're dreaming … tail wags and all.

If you feel inspired to do more "research" on the subject, search YouTube for "dog tail wagging." Belly laughs are guaranteed—and your pup by your side might also enjoy seeing all of those wiggly-wagglies!

Animated illustration: YEJI KIM