Your dog's tail does more than clear coffee tables; it helps him balance, spread his scent and sends a signal about what he's thinking. Tail-wagging is an important communication tool for dogs -- so important that if no other living creature is around, your dog won't bother wagging his tail. His tail's position, as well as the speed and direction it moves, provides clues about his mood.
Wagging to Communicate
Your dog had to grow into his tail-wagging ways. During his first month and a half of life, he didn't have much reason to wag his tail -- he was focused on nursing and growing. But about the time he started becoming more independent, he developed a need to communicate. His tail was one way he communicated with his mother and litter mates. For example, he may have waved his tail in surrender during some puppy play that got a little rough.
The position of your pup's tail indicates much about his emotional state. If his tail is vertical, he's sending a signal of dominance, or even warning, letting everyone know he's in charge. At a horizontal position, he's paying attention to his surroundings, whereas he's fairly relaxed if his tail is a bit lower, at a medium height. When his tail begins to droop, he's signalling submission, concern or that he's not feeling well. When he tucks his tail beneath his body, he's expressing fear and asking not to be harmed. All tails are not created equal, and tail position can vary by dog breed -- many terriers have a naturally higher vertical position than greyhounds, who tend to have low-slung tails. As you learn your dog's moods, you'll understand where his normal, relaxed level is and where the different positions are relative to his normal.
The way your dog wags means something, too. How fast his tail wags and how much ground it covers conveys his level of excitement and emotional state. A slow wag tends to be more insecure, while a faster wag is excited. Putting the height, position and movement together results in a wide variety of signals. A slow, short wag may be a bit tentative or hopeful way of saying "hi," opposed to the broad, faster wag, which is unchallenging or happy. A tail that isn't moving can still communicate something, based on where it's positioned; if it's still and higher than normal, your dog is likely on alert. If he's sending a threatening message, he may stand still and rigidly wag his stiff tail to warn someone away.
Left and Right
Your dog may not know his left from his right, but his tail does. According to a 2007 study, the different sides of a dog's brain dictate what direction his tail wags, which may indicate his level of comfort in a situation. The left side of his brain controls the right side of his body, and when his tail wags to his right (or your left as you face him), he's likely seeing something that is familiar or he's comfortable or happy with. A wag to his left, however, indicates he's scared or anxious, a result of the right side of his brain, which controls his feelings of withdrawal.
Head to Tail Mood Indicators
Look beyond your dog's tail to read his messages. His eyes and ears will offer clues to his mood, as will the stance of his body. You'll understand when he's relaxed -- his tail and ears in a neutral position -- or when he's frightened -- his tail and ears tucked in tight. His tail, hair, eyes, ears, mouth and body language all come together to give him an effective means of communication with other dogs and anyone else who pays attention.