Most people think that dogs wag their tails only when they're happy. But it's a little more complicated than that.

Canine tail wagging is a form of communication (dogs don't usually wag their tails when they're alone), but it can relate a variety of emotional states, as anyone who's been bitten by a tail-wagging dog knows full well. The wag can convey good spirits, fear, aggravation, dominance, or submission. The well-known zoologist Desmond Morris contends that a dog's wagging tail expresses a state of conflict - the simultaneous need to advance and retreat.

Some have suggested that tail wagging is simply a physiological means of getting rid of surplus energy. P. Dwight Tapp, who's conducted research at the University of Toronto in the cognitive functions and brain structures of dogs, points out that wagging also spreads pheromones by causing the muscles around the anus to contract, pressing on glands that release a scent. This scent communicates information about sex, age, and social status to other dogs.

There's another compelling question about canine behavior: Why don't dogs, whose loathsome, irresponsible owners leave them tethered to parking meters in dangerously frigid temperatures and then spend several hours leisurely browsing in Walgreens, rip their faces off (or at least viciously maul them) when they finally come out, instead of enthusiastically wagging their tails?

By Mark Leyner and Billy Goldberg, M.D. Copyright 2006