It seems like all dogs wag their tails. It's not just when they're happy, though; it's also when they're agitated. A dog's tail language is robust and complex, and it probably has been around since the first dogs. It's an instinct for movement, albeit one they can control.
Your dog's tail may seem to have a mind of its own, but the muscles that control it don't. Just like the rest of his body, she can control her tail muscles by thinking in order to manipulate movement. She can raise, lower or wag her tail at will, and she can stop it mid-motion, too. A dog's tail helps her balance, so it's necessary to have voluntary control over its muscles, lest she couldn't swing it one direction to compensate for a lean in the other.
Your dog's tail is kind of like an emotional barometer. While it's easy to associate a wagging tail with a happy dog, the angle and nature of the wag could mean agitation or anger, too. Tail wagging is a learned behavior -- puppies don't wag their tales immediately after they're born. Different tail wags probably signify different emotions, although not all sources agree on the meaning of the same behavior. Puppies, for instance, wag their tales when they approach their mother to nurse, so the wag could be signal submission, but the mother typically wags her tail, too, which presumably means that's not the case.
Dogs can control their tails and their tail wags, but it appears they often start wagging out of instinct, not conscious thought. It's kind of like a human frowning. You might begin frowning as a response to, say, an inappropriate joke, but you can return your mouth to resting, smiling or a deeper frown at will. As such, tail wagging appears to be a response to stimuli that can be manipulated by conscious thought. That makes it part involuntary and part voluntary.
If you're confused why your dog's wagging his tail, look at her for other signals. If she looks tensed, she may be upset. If she looks relaxed, she may be happy. Tail movements alone are sometimes hard to read. There are no hard and fast rules, but a languid wave is probably a good wag and a stiff wave is probably a bad one. Dogs lower their tails when they're submissive, so a low tail wag probably fits into that arena. You can make your own personal dog-tail-wagging dictionary if you want to get a better understanding of your canine friend.
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