What does tail wagging really mean?

It’s common knowledge that a wagging tail is a sign of a happy, friendly dog, but many pet owners don’t know that this everyday behaviour can also signal fear, insecurity or even aggression in our canine companions. Watching a dog’s body language, then, is crucial to understanding whether a wag is an invitation to play or a sign to stay away.
 
Dogs originally evolved their tails for balance. The appendage not only helped our canines’ ancestors navigate sharp turns at high speeds, but also helped them walk across narrow structures, climb and jump. Over time, however, this practical appendage evolved into a means of communication. Like any language, tail wagging has its own vocabulary that helps other dogs understand the intent behind the motion, and owners can better understand their pets by reading these signs.
 
To decipher what a wag means, note the position of your dog’s tail as well as its specific movements. Generally, a tail held high or straight up is a sign of being alert or even aggressive; a mid-set tail signals a calm or neutral pet; and a lower wag indicates your dog is feeling worried or anxious. Watching the motions accompanied by these positions is important, too, in understanding exactly what your dog is trying to say. A slight, rapid wag is often reserved for friendly greetings, while a wide, sweeping wag is your dog’s way of showing he is pleased and does not pose a threaten or challenge to you. A slower, lower wag often indicates Fido is feeling insecure, while tiny, rapid tail movements in a high position can mean your dog is issuing an active threat. Other tail actions can communicate other emotions, too. When a dog holds his tail horizontal to the ground, he is often curious about his surroundings. When he tucks it between his legs, he is either displaying submission or showing he is scared. Dogs also wag their tails to help spread their unique scents from glands located under the appendage, so more dominant dogs tend to carry their tails higher to release more scent.
 
Like any means of communication, tail wagging is a social behavior and is largely learned from other dogs. Puppies don’t start wagging their tails until they’re old enough to communicate with their littermates and mothers, and you’d be hard-pressed to catch a dog wagging his tail when no other creatures are around. Dogs don’t limit their tail talk to other dogs, however, and will use it to communicate with humans and even other pets. Knowing the difference between a cheerful wag and fearful flag, then, is key to understanding what your dog is trying to say. When in doubt, watch for other body signals such as stiffened muscles or ears pinned back to know when your pet is telling you to walk away.