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Ever since I was a little kid, I have wondered why dogs wag their tails. So, why do dogs wag their tails? I am looking for an answer from a credible source.

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Tail wagging can actually mean quite a few different things, depending on how the tail is held, how broad or short the strokes of the tail wagging are, and how quickly the tail is moving.

I've seen dogs wag their tails to indicate that they were happily excited, such as upon seeing a favorite toy. I've also seen dogs wag their tails in what seemed to be pitiful dejection, such as slowly wagging it back and forth while watching their human leave. Sometimes it can suggest that the dog is nervous, or that the dog has done something wrong and knows it. Sometimes it simply means that they're all wound up and have simply too much energy.

There has been a fair amount of research on the subject, and research and experts have found some surprising (and not-so-surprising) nuances to tail-wagging in canines.

A study using a remote-controlled lifelike dog replica simulated 4 different combinations of tail lengths and wagging: short/still, short/wagging, long/still, long/wagging.

Larger dogs were less cautious and more likely to approach a long/wagging tail rather than a long/still tail, but did not differ in their approach to a short/still and a short/wagging tail. Using discriminant analyses of behavioural variables, dogs responded with an elevated head and tail to a long/wagging tail model relative to the long/still tail model, but did not show any differences in response to tail motion when the model’s tail was short.

In other words, a clearly visible wagging tail made dogs more comfortable and confident in being able to safely approach the replica.

In wolves, wagging tails typically indicate an increase in energy or excitement.

There are two specific styles of tail wagging that wolves perform: rigid or fluid movement. A rigid tail (like a pendulum) wag means the wolf is excited and has dominant tendencies. A fluid, or snake-like wag typically is a signal of play or greeting toward other pack members.

The elevation and movement of each wolf's tail work together to describe the behavior of each individual. So, a wolf who is rigidly wagging a T1 tail is exhibiting intense dominance, however a wolf fluidly wagging a T3 tail is probably soliciting social play with other pack members.

One study showed that the direction of the wag (left->right or right->left) indicates different things in domestic dogs:

Dogs wag their tails to the right when they see something they want to approach, and to the left when confronted with something they want to back away from, say researchers in Italy.

Over a series of trials, they videoed each dog's response to being shown either their owner, a human stranger, a cat, or a Belgian shepherd malinois, a large dog breed similar to a German shepherd.

Shown a human or a cat, tails wagged consistently to the right. The unfamiliar person elicited less wagging than the owner, and the cat the least wagging of all — probably because the dog was so interested in giving chase that it was distracted from wagging, says Vallortigara.

Shown a large, unfamiliar and intimidating dog, the dogs wagged their tails more to the left. Dogs also wagged to the left when left on their own without anyone to look at, the researchers report in Current Biology (Quaranta A., Siniscalchi M. & Vallortigara G.. Curr. Biol., 17 . 199 - 201 (2007).)

Other research indicates that how widely a dog's tail wags, and how quickly, has meaning, too:

The speed of the wag indicates how excited the dog is .The breadth of the wag shows whether the dog's emotional state is negative or positive

Here are some combinations that Dr Coren describes-

A slight wag-with each swing of only small breadth-is usually seen during greetings as a tentative 'Hello there,' or a hopeful 'I'm here.'

A broad wag is friendly; 'I am not challenging or threatening you.' This can also mean, 'I'm pleased,' which is the closest to the popular concept of the happiness wag, especially if the tail seems to drag the hips with it.

A slow wag with tail at 'half-mast' is less social than most other tail signals. Generally speaking slow wags, with the tail in neither a particularly dominant (high) nor a submissive (low) position, are signs of insecurity.

Tiny, high-speed movements, that give the impression of the tail vibrating, are signs the dog is about to do something - usually run or fight. If the tail is held high while vibrating, it is most likely an active threat.'

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    this was one of the issues with tail docking and the removal of a dogs dominance as a visual cue, just a interesting tidbit – user6796 Oct 18 '13 at 15:04

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