If you are wondering "why do dogs wag their tail?" rest assured, you are not alone. Countless dog lovers at one point or another wonder why dog tails are so mobile and what's up with dogs wagging their tails. 

One of the most common answers to the question "why do dogs wag their tail?" is "because they are happy!" Turns out though, this is a too simplistic answer.  

Tail wagging isn't always a sign of happiness, and therefore, there's more to the behavior than we may think.  

So let's delve deeper into the real function of tail wagging. 

A Lesson in Anatomy

OK, everybody knows that dogs have tails and that they are often wagged. Dogs can wag their tails in large sweeping motions, in small wags restricted only to the tip and sometimes even in full circles (when dogs do this it's called "helicopter tail" or even "circle wag" by the way). Intrigued? Discover more about circle wags in dogs and what they mean. 

However, many people do not know exactly what hides under the tail. Under a dog's tail we obviously find the dog's behind which is obviously meant to expel feces, but it doesn't "end" here.

Around the dog's rectum are found two paired reservoirs, one on each side of the rectum, which are known as anal sacs or anal glands. To be more specific, these sacs are found around the 4 o'clock and 8 o'clock position.

A dog's anal sacs are each equipped with a duct which discharges fluid every time the dog has a bowel movement that is solid enough to express them.

Interestingly, according to research by Natynczuk, Bradshaw and McDonald, the anal gland compounds excreted are different between one dog and another.

The secretions also differed on a day-to-day basis. This helps explain why dogs are so interested in sniffing each other's' behinds and why they are so intrigued by each others' feces which are often covered with some of these secretions.

Turns out, dogs can learn a lot about each other (possibly sex, age, reproductive status, etc.) by simply sniffing these secretions.

Tail wagging isn't always a sign of happiness, and therefore, there's more to the behavior than we may think.

Tail wagging isn't always a sign of happiness, and therefore, there's more to the behavior than we may think.

So Why Do Dogs Wag Their Tails?

Now that we have a deeper knowledge of what hides under the hood, we can better understand the real reason why dogs wag their tails. 

Tail wagging is meant to spread personal information courtesy of those anal gland secretions which contain pheromones for identification purposes. 

With those wide, sweeping motions, dogs are spreading their personal information with others, sort of like we do when we hand out our business cards. 

At a closer insight, here's what more precisely happens: every time the tail is wagged, the muscles around the dog's rectum contract and end up pressing on the anal glands triggering the release of scent.

More than "smelling" pheromones though it would be more accurate to say that dogs detect them, courtesy of a specialized organ known as the "Jacobson organ" also known as vomeronasal organ. 

The Jacobson organ is located within the dog's nasal septum and connects to the nasal and/or oral cavities courtesy of a narrow duct. Its primary role is to detect the small, volatile molecules associated with pheromones.

 Of course, dogs don't know that our noses are not as sensitive as theirs, so they still use their tail wagging with us because that's just how they have learned to communicate.

A Form of Communication

Tails in dogs carry out several functions. They help dogs balance and they can be used as a rudder when swimming. 

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Tails also come handy in keeping things away from a dog's behind: watch how quickly dogs tuck their tails in when your veterinarian tries to insert a thermometer!

 On top of these functions, tail movement is used for social interactions. Tail wagging in dogs is ultimately a form of communication. 

Indeed, tail wags don't typically take place when dogs are alone, but rather they seem to come to life when dogs are around people, other animals or other dogs.

The fact tails are used to communicate is further proven by the fact that newborn puppies don't wag their tails up until they are around their third week of life. 

The reason for this is that, prior to this time, most of the puppy's life is spent nursing and sleeping. It's not that at such a young age tail wags are not possible. It's just that puppy tail wags come only handy once pups are ready for more involved social interactions.

 Generally, tail wags become fully established once the puppies are around 49 days old, explains Stanley Coren in the book "How Dogs Think: Understanding the Canine Mind."

Not All Tail Wags Are Created Equal

The sweeping, broad and fast-paced wag of a happy dog's tail upon greeting the owner is quite easy to interpret, but as mentioned, not all tail wags convey friendly communication.

 Tails may wag to communicate various emotions such as excitement, arousal, and also concern which, given the right circumstance, may even morph into aggression.

A tail that is held high and vibrating in sharp, short bursts may be coming from a dog who is the opposite of friendly; indeed he may be warning you off because you're coming too close to his perceived territory. With the tail high, more scent is expelled, so to communicate "I live here."

A tail wag that is coming from an insecure or fearful dog, will likely occur with the dog's tail being held low, and possibly, in the more extreme cases, tucked between the legs. These dogs aren't much eager to spread any chemicals around as they may want become as small and invisible as possible. More on this is explained here: why do dogs tuck their tail between their legs. 

At worse, if they are really frightened, they may secrete "alarm pheromones" under the form of stinky, anal gland secretions that may smell like rotten fish. 

This scent left behind is meant to warn other dogs of danger. It's sometimes released by frightened dogs at the vet and it is picked up by other dogs. This possibly explains why vet offices are often perceived by dogs as scary places.

 Fortunately, more and more vet offices are now aiming to make their places less and less frightening for dogs. 

As seen, tail wags can mean many different things. When in doubt about what a tail wag is trying to convey, it's a good idea to keep an eye on the rest of the body. Watch for a stiffened body, dilated pupils and tense facial muscles.

While tails can be a big piece of a puzzle they don't tell the whole story. And of course, if you're not sure whether a dog is greeting you or telling you to back off, best to play it safe and not approach.

Not all tail wags convey friendly communication.

Not all tail wags convey friendly communication.

Did you know? According to research, a dog wagging its tail to the right is conveying positive emotions, while a dog wagging its tail to the left is conveying negative emotions.

 Interestingly, for those wondering whether dogs have the cognitive capabilities of understanding asymmetric tail wagging, according to a study, it appears that they actually do, and therefore, right-wagging tails relax dogs, while left-wagging tails make them preoccupied.

References:

  • The Domestic Dog: Its Evolution, Behaviour and Interactions with People. edited by James Serpell, Priscilla Barrett
  • Tail Talk, Sophie Collins, foreword by Dr. Karen Overall
  • K.A. Artelle, L.K. Dumoulin, T.E. Reimchen Behavioural responses of dogs to asymmetrical tail wagging of a robotic dog replica Laterality, 16 (2011), pp. 129-135

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