Owing to thousands of years of selective breeding, humans and dogs have co-evolved a special link. This is best seen in the interspecies communication between the two. Dogs can understand and follow human gestures, something that even the brainy chimps, our closest relatives, cannot do. In turn, humans can tell if their dogs are in need of food, water, a walk, or a gentle pet despite the lack of verbal communication.

Although dogs communicate with humans through a repertoire of vocalizations such as barks, growls, howls, whines and whimpers, screams, pants, and sighs, their primary mode of communication is by body language. These non-verbal cues include ear and eye position, body position and movement, facial expressions, and, last but not least, tail carriage and motion.

Why do dogs wag their tails: the unspoken language of canines

Concerning tail wagging, most people associate this motion with the dog being ‘happy’. While there is some truth to this notion, it is not entirely accurate and does not describe a dog’s true emotional state in all instances of tail motion. That being said, a dog’s tail position is instinctual and thus a pretty good gauge of your canine’s internal state — it’s just that they do it when they’re not happy as well.

Tail wagging develops between three and four weeks of age depending on the breed. When a dog is relaxed, the tail will rest in its natural position. However, this position can differ from breed to breed. For the vast majority of dogs, their relaxed tails hang down to their heels. However, a pug’s tail curls upward and, on the opposite end of the wagging spectrum, greyhounds have tails that rest slightly between their legs. You can use this tail position as a sort of reference from which you may attempt to infer your dog’s emotional communication.

According to the American Society for the Prevention of Cruelty to Animals, when a dog is anxious or overly submissive, it will hold its tail lower than its natural position. When scared or outright terrified, the tail will hang under its body. A slightly higher than usual tail position may indicate arousal and curiosity, while a vertical tail may reflect aggression.

Tail wagging typically indicates excitement. The more vigorous the wagging, the greater the excitement. In fact, the speed of the wag, depending on the tail’s position, is typically an indicator of enhanced emotional state, whether it’s positive or negative.

This can mean that the dog is happy to see its owner or to be offered a treat. However, just like humans, dogs can get excited about all sorts of things — and they’re not always necessarily positive.

A 2013 study by researchers at the University of Trento in Italy found that dog tail wagging conveys a wealth of information to other dogs, particularly in the direction of the tail. Canines that see tails wagging to the right are more relaxed, whereas they become more stressed when they see tails wagging to the left. Previously, the same team of Italian researchers found that dogs wag their tails to the right when they see something they’d like to approach. When confronted with something they want to stay away from, such as another dog that exhibits an aggressive posture, canines will wag their tails to the left.

This directional tail wagging is mediated by the activation of the left or right side of the dog’s brain. Subsequent observations showed that dogs that saw other canines wagging their tails to the left had a higher heart rate and showed more signs of stress and anxiety. When they saw other dogs’ tails wagging to the right, they were more relaxed.

The direction refers to the dog’s left or right viewed from the rear as if you are facing in the direction the dog is viewing. That means that if you are facing the dog and drew an imaginary line down the middle of his back that positive, right-sided signal would appear as tail swings mostly curving to your left. Dogs will of course wag their tails in both directions, but pay close attention to which direction the tail is more biased towards.

The researchers don’t think that the dogs are consciously responding to the direction of tail wagging. Dogs may become more stressed when seeing a left tail wag because they’re interpreting that the canine they’re facing might be more likely to attack.

Previous research focusing on the approach-avoidance behavior of other animals found that the left hemisphere is associated with the processing of positive-approach emotions while the right hemisphere is associated with negative-avoidance emotions. The brain’s left hemisphere controls the right side of the body, and vice-versa, dogs have learned to read these unintentional cues in body language.

Canine tails beyond communication

Besides assisting in communication, a dog’s tail has a wide range of functions. When they’re swimming, the tail can act as a rudder in the water while outside it helps the dog keep balance when running, allowing them to make tighter turns without falling over.

Until not too long ago, many dog owners would dock the tails and crop the ears of their pets for cosmetic reasons. If a dog has a docked tail it’s often difficult to tell how they’re feeling.

Both practices are severely detrimental to a dog’s health and ability to communicate properly. In many places in Europe, such as the UK and Germany, these practices have been banned. Unfortunately, there are no such restrictions in the United States, although New York and Vermont have considered bills to make the docking and cropping illegal.

Bottom line: wagging doesn’t always indicate the dog is happy. Pay attention to the speed and direction of the wagging tail to get a better idea of what your dog is trying to communicate.