Since dogs haven't yet figured out a way to talk to their owners and their other animal cohorts, they have to use their bodies to communicate. The eyes, ears and body positions are a few clear indicators of how they feel. But wagging their tails is one of the most visible and well-known ways canines use to clue people and other animals in on exactly what's going on with them.

A dog's tail is a very important feature of her body. The original purpose of this furry mass of bones was for balance while walking on narrow surfaces. Dogs also use their tails to help them make turns while running or to guide them while swimming. But for generations of pups, tail-wagging has additionally been an effective tool to let their owners and other animals know if they are happy, sad, anxious, scared or even feeling aggressive. So, is tail-wagging involuntary, or voluntary?

"We can't talk to dogs so we don't know if they think about the tail wag and then do it, or if it just occurs due to the neurochemical effects of a certain state of mind," says Lisa Radosta D.V.M. and owner of Florida Veterinary Behavior Service. "However, you can watch dogs in a dog park and know that the dogs have an intent when they hold their tail a certain way or wag it a certain way."

The America Society for the Prevention of Cruelty to Animals (ASPCA) expert Dr. Stephen Zawistowski believes it can vary. On the ASPCA Web site, he advises to "think of tail wagging like a smile. Sometimes you smile without knowing, and sometimes you can make yourself smile. I think dogs are the same with their tails. Sometimes it just happens, and other times it seems like they 'think' about it."

A 2007 study on dog body language that appeared in the journal Current Biology found that the different sides of the dog's brain dictate the direction of the tail wag. A dog will wag his tail to the right side of his derri?re when he is feeling happy, positive or confident about approaching something. The left brain, which controls the right side of his body, specializes in behaviors that scientists refer to as "approach" and "energy enrichment."

On the other hand, the dog will wag his tail to the left of his derri?re if he feels scared or wants to bolt from the situation. The right side of a dog's brain controls feelings of withdrawal and energy expenditure and also controls the left side of his body.

So, wagging is both innate and learned, says Dr. Radosta. If a dog learns that a certain body signal, such as tucking her tail, keeps her safe from a threat, she is likely to use that signal again. She reminds us of the story of Pavlov's dog, a classic example of conditioning. When Pavlov rang the bell, the dog salivated. He didn't have to think about salivating, he just did it. It was an innate behavior which was under the control of a stimulus (a bell).

Dr. Radosta says the changes in tail wag and tail carriage depend on the stimulus and can occur very quickly. As a result, experts believe that the tail is also under voluntary control. You don't need to go any further than your dog's veterinarian's office to prove this fact.

"Any veterinary technician who has tried to take a dog's temperature rectally will tell you that the tail can be tucked quickly to avoid a rectal thermometer," says Dr. Radosta.