While most dogs technically don't need their tails for balance, some rely on them to do their jobs. Working dogs are bred to use their tails, and in some breeds, that means carefully maintaining balance while performing a task. Even though most dogs don't need their tails, having the tail removed -- or "docked" -- remains a controversial procedure, particularly when performed for any reasons other than absolute necessity.
The dogs who need their tails for balance are generally working dogs, and they generally need them in a way that is directly related to their working purpose. For example, greyhounds who are bred for fast running and tight cornering in pursuit of game have long, wiry tails that they use for balance when navigating sharp turns. Other dogs may not need their tails for balance, but they still need them for another practical purpose -- for example, retrievers use their tails to help steer themselves when swimming.
Most dogs, even the ones who don't need their tails for balance, need their tails as communicative tools. Dogs communicate primarily through body language, and the tail speaks volumes. A loose, naturally-wagging tail often demonstrates happiness, while a tail tucked between the legs indicates trepidation. If a dog holds his tail stiff, he may be communicating agitation. Sometimes, the body language is breed-specific -- for example, a pug's tail is naturally curly when he's content, and straightens out when he's nervous or upset.
Tail docking is the process of surgically amputating all or part of a dog's tail. This painful procedure is generally carried out on working dogs, for whom having a tail represents an unnecessary risk. For example, guard dogs may have their tails docked because the tail is vulnerable in a fight -- for this same reason, dogs used in illegal dog fighting also commonly have their tails docked. Other working breeds, like hunting dogs, may similarly have their tails docked to avoid injury. Dogs who have their tails docked generally suffer no physical disadvantage.
Just because a dog doesn't need his tail doesn't mean that removing it is ethical. For this reason, the official policy of the American Veterinary Medicine Association is in opposition of cosmetic tail docking, and only considers the procedure justified if it is for medical reasons. Tail docking is painful and invasive, and while your dog may not need his tail for balance, he doesn't deserve to have it unnecessarily amputated.
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