Why do dogs have tails? First, why do any animals have tails? There are a number of evolutionary reasons why animals have developed tails. Some of the reasons:

Prehensile Tails are used to grab and hold things and act as a fifth limb. Monkeys are an example of this. Human ancestors are thought to have prehensile tails. As discussed in a previous IFOD, some humans are still born with tails: Vestigial Structures

Swatting. The main purpose of some tails is to swat away annoying insects like flies. Examples are cattle, horses, elephants.

Balance and Coordination. For most animals, tails assist with movement and act as a rudder and counterbalance. According to Dr. Stanley Coren of the University of British Columbia: “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.”

Dogs and Communication: Many animals, including cats and horses use tails for communication, but dogs may have the most developed tail communication system. Dr. Coren again: “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.”

Interestingly, a study found that dogs without tails (whether snipped due to fashion or injury/disease) have more difficulty communicating with other dogs and tend to get in more fights.

How a dog holds his/her tail communicates about the dog’s mental state.

• A tail held middle height suggests the dog is relaxed.
• A lowered tail (or between legs) communicates submission.
• Holding a tail high means that the dog is becoming more aggressive and the tail is being held up to make the dog appear larger.

Wagging has nuances as well:

• A slight wag is a tentative greeting
• A broad wag means I am pleased and/or I am not challenging you and I am very friendly
• A wag with the tail held low means that the dog is feeling insecure or anxious
• A very fast shallow wag usually means the dog about to take action, like run or possibly fight (especially if held high while vibrating)

Another way that wagging aids communication is through smell. From the book “Being a Dog” By Alexandra Horowitz: “[Tail wagging] is spreading scent. Whether intentional or not, when a dog wags her tail. all the very fascinating odors (to a dog from her anal sacs spread in a bloom around her body.  She is not only telling other dogs how she’s feeling; she’s scenting who she is . . . . So though a dog wagging his tail looks simply like a dog wagging his tail happily, he is also saying Come smell the wafty odor that is me.“