Why Do Dogs Have Tails?
Sooner or later everybody who lives with a dog finds himself asking the question "Why does my dog have tail?" It turns out that there are several good reasons why tails are a useful piece of apparatus for our dogs.
It is quite clear that the dog's tail was originally designed to assist the dog in his balance. When a dog is running and has to turn quickly he throws the front part of his body in the direction he wants to go. His back then bends but his forward velocity is such that his hind quarters will tend to continue in the original direction. Left unchecked, this movement might result in the dog's rear swinging widely which could greatly slow his rate of movement or even cause the dog to topple over as he tries to make a high speed turn. The dog's tail helps to prevent this. Throwing his tail in the same direction that his body is turning serves as a sort of counter weight which reduces the tendency to spin off course.
Dogs will also use their tails when walking along narrow surfaces. By deliberately swinging the tail to one side or the other in the direction opposite any tilt in his body he helps maintain his balance, much the same way that a circus tight rope walker uses his balance bar. Quite obviously, then, the tail has important uses associated with specific movements. However, the tail is not particularly important, on flat surfaces, when simply standing around or walking at normal speeds. At these times it becomes available for other uses. Evolution, seized upon this opportunity and adopted the tail for communication purposes.
For most breeds of dogs the tail is highly visible and serves as a sort of signal flag that communicates information about the animal's emotional state. Variables such as how high the dog carries his tail, how quickly the dog is moving his tail, and even whether the tail is being wagged more to the left or right side of the body can convey a lot of information about how the dog is feeling, his mood, and even his intentions. Dogs with very short tails, either because of the nature of their breed (for example the French Bulldog is born with a little stump of a tail which is about 1 inch in length) or because their tails have been docked, cannot communicate as well and such dogs often have difficulties interacting with other canines.
Although tails obviously provide an easily seen visual signal that it carries emotional information, it also has another important role in communicating. Every time your dog moves his tail it acts like a fan which spreads his unique scent around him. These smells which are designed to communicate information between animals are technically called pheromones. Some important pheromones come from the anal glands which are two sacs located under the tail. These contain a smelly liquid. The odors from the anal sacs are as unique among dogs as fingerprint