Whenever I come home to my 14-year-old Shih Tzu, as most dogs do [1], she greets me by pawing excitedly at my legs and wagging her little tail vigourously enough to cause a tornado. But I've noticed she wags her tail all the time, even in situations that are considered threatening according to dog standards, like the mailman arriving [2], the neighbour's cat peeking through our backyard fence, and seeing her own shadow. Even if my dog is barking or growling, she is usually wagging her tail as well.

For the longest time, I mindlessly assumed that she was just an excessively happy-go-lucky dog, because I was sure that tail wags indicated happiness. For the most part that's true, but after seeking out more information from three veterinarians — one of whom specialises in veterinary behaviourism — I learned it's not always the case.

Why Do Dogs Wag Their Tails?

It turns out that tail wags coincide with a whole range of emotions, not just happiness and excitement. According to Lynn Buzhardt, DVM, a veterinary consultant for VCA Animal Hospitals [3], dogs will often wag their tails if they feel threatened, anxious, insecure, interested, curious, or friendly. Determining your dog's exact emotion depends on three factors: wag position, wag speed, and most importantly, context.

What Does Wag Position Tell Us?

A 2013 study on canine body language that was performed by the Department of Veterinary Medicine at University of Bari, Italy confirms that the position of a dog's tail wag [4], corresponding to the left and right hemispheres of the brain, is what determines the types of emotions they're experiencing. A strong right-sided wag (so when your dog's tail wags more to the right than the left) reflects positive emotions, while a strong left-sided wag (when your dog's tail wags more to the left than the right) reflects negative ones. A tail in the up position establishes dominance and assertion, while a tail in the down position establishes submission.

What Does Wag Speed Tell Us?

Certified dog trainer Cesar Millan reminds us on an episode of Dog Whisperer [5] that it's helpful to acknowledge the speed of the tail wag, too. This determines the energy level of the dog. In other words, the faster the wag, the higher the level of positive or negative emotion.

What Does It Mean When a Dog Wags Its Tail?

Tail wagging is also a social indicator [6], which means that a dog won't wag their tail unless they're around other dogs or humans. Even though "tail talk," as Dr. Buzhardt calls it, is a form of communication, dogs don't talk to themselves, so you won't see one wagging their tail if they're alone. "Pups aren't born knowing what a wagging tail means any more than a newborn baby understands words," Dr. Buzhardt explains to POPSUGAR. They start wagging their tails at one month old, and because they're born in litters, they learn how to do so from each other. Puppies [7] figure out they can use their tail movements to communicate with their mother if they're hungry [8], or with their littermates if they want to play. Their communication skills develop from then on.

Is a Dog Always Happy When They Wag Their Tail?

Not always, as I learned when I put my "tail talk" interpretation skills to work on my Shih Tzu and my boyfriend's Jack Russell terrier, both frequent tail waggers. While each dog was busy chewing on a toy [9], I playfully snatched it away (then returned it immediately, of course). As I expected, my Shih Tzu growled, then wagged her tail. My boyfriend's Jack Russell terrier on the other hand, growled, then bared her teeth.

The reason behind these contrasting reactions was explained to POPSUGAR by veterinary behaviourist E'Lise Christensen, DVM and veterinary surgeon Massimo Orioles, DVM. It's not that Shih Tzus have happier temperaments than Jack Russel terriers; it's that both these individual dogs express their possessiveness in a different way. Although they both growled, one used her teeth and the other used her tail to communicate that she felt threatened by me stealing her toy.

"It is so important for people to realise that a wagging tail does not equal a dog that is friendly or wants to be petted," Dr. Christensen says. "It can, but you are much better off looking at the entire dog. If there are stiffened muscles, dilated pupils, tense facial muscles, or ears pinned forward or back, these are signs that you should back off." Dr. Orioles points out that in the same way a tail isn't always a reliable indicator of happiness, growling isn't always a reliable indicator of aggression. "Dogs can growl when playing, when frustrated, when experiencing pleasure," he says.

This combined with tail wagging is one of the reasons that children are often unable to differentiate between a friendly dog and a non-friendly or aggressive dog. [10] Considering this particular context and the dogs' reactions, we can clearly infer how my Shih Tzu and her Jack Russell friend were feeling. But in situations where it's not as obvious, we can still apply the same logic, and tail wagging definitely helps clue us in.

Dogs have complex feelings just like we do, and learning to understand them is an important part of having one in your life, as Dr. Buzhardt, Dr. Christensen, and Dr. Orioles spelled out for us. For as much happiness that dogs bring to our lives, the least we can do to make theirs more comfortable and enjoyable is to try our very best to tune into how they feel and how they interact with their social environment. Dogs surprise us every day, and getting to know their quirks, habits, and tail wagging tendencies is part of what makes loving a dog so gratifying.