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 fingerprints are for people. Every time the dog wags his tail the muscles around the anus contract and press on these glands. This causes a release of some of the scent. A dominant dog who carries its tail high will release much more scent than a dog that holds his tail lower. So this so this dog's tail wags will spread his scent around announcing "I am here." Compare this to a frightened dog who holds his tail between his legs covering the anal glands so that his scent does not spread around and thus does not draw unwanted attention to himself.
It is something of a surprise to many people to learn that puppies do not wag their tails when they are very young. The youngest puppy that I ever saw systematically wagging its tail was 18 days old, and both the breeder and I agreed that this was quite an unusual case. Although there are some differences among the various breeds, the scientific data suggests that, on average, by thirty days of age about half of all puppies are tail wagging and the behavior is usually fully established by around forty-nine days of age. Why does it take so long for the puppy to start wagging its tail?
The answer comes from the fact that puppies begin tail wagging when it is needed for purposes of social communication. Until they are about three weeks of age, puppies mostly eat and sleep. They are not interacting significantly with their littermates other than when curling up together to keep warm when they sleep, or crowding together to nurse. They are physically capable of wagging their tails at this time, but they don't.
By the age of six or seven weeks (when we start to see tail wagging behaviors on a regular basis) the puppies are now socially interacting with one another. Most of the social interactions in puppies are what psychologists call play behaviors. It is through playing that puppies learn about their own abilities, how they can interact with their environment, and most importantly, how to get along with other individuals. It learns that if it bites a littermate it is apt to be bitten back, and perhaps the game that it was playing might be terminated by its now angry playmate. It is at this point that the puppy also starts to learn dog language. It is not clear to what degree these emerging social communications are pre-wired, but learning is clearly needed to refine the use and interpretation of these signals. The pups learn to connect their own signals and the signals provided by their mother and their siblings with the behaviors that come next. They also begin to learn that they can use signals to indicate their intentions, and to circumvent any conflicts. This is where and when the tail wagging behavior begins.