Why do dogs wag their tails when they're happy?
-- Dave, La Jolla
Uh, Dave, dogs also wag their tails when they're pissed, so we'd better clear up this mystery before some Dobie takes a chunk out of your butt. All in all, there may be no animal that does more communicating than a dog. Virtually every bit of its body is involved at some point in a message-sending task. Pack animals need those skills. Smell is the main communication tool. Since most smell-producing glands in a dog are in its back end, you can understand why they're so interested in that area, even though our hindquarters aren't as information packed as the dog might expect.
Scientific observers of dog behavior contend that a dog's wagging tail signals an emotional conflict of some kind, or a degree of excitement. Puppies generally don't wag their tails until they're about a month old, when they start asserting their individual personalities and play-fight with their littermates. In human-dog interactions, we see tail wags when the dog is happy to see its owner but apprehensive at his lower status in the dog-owner pairing. In dog-dog interactions, you will often see a tail raised high and wagging slowly as a threat before a fight. The act of tail-wagging squeezes the anal scent glands and sends a smellogram to the other dog to let him know what's what.