Do dogs really wag their tail because they are happy?

A dog's tail is a part of a dog's body language and allows them to communicate.

The answer is yes; dogs wag their tails when they are happy but also when they are feeling "alert" or "agitated".

A dog's tail is part of a complex system of body language that the domestic canine uses (along with "verbal" cues such as barking, growling or whining) to communicate. A wagging tail indicates excitement or agitation - but whether he means it as an invitation to play, or to warn another dog or person to stay back, depends on other body language.

A wagging tail that curves down and backs up into a "U" usually indicates a relaxed, playful dog. If his ears are erect and pointing forward, and he is in the classic "play bow" position, he's inviting you to play.

A dog will hold its tail high, whether wagging or not, to indicate dominance and/or increased interest. If the end of the tail is arched over the back, and is twitching back and forth, you may be faced with an aggressive dog.

The tail is a purely social indicator for other living things. A dog doesn't usually wag his tail when alone.  You can watch key parts of a dog’s body to determine how he's feeling and reacting to the world around him. A dog's body language is very interesting.

Dog Language  

Face Although the dog's facial muscles are not as refined as a human's, he can wrinkle or straighten his forehead to show confusion or determination. If your dog wants you to give him further direction, he may raise his eyelids quizzically and tilt his head to one side.

Eyes A dog's eyes brighten when he looks at a creature he considers friendly and when he wants to play. If he is afraid, his pupils dilate and he shows the whites of his eyes. He averts his eyes to avoid confrontation. But if he is angry or ready to defend himself, his eyes narrow and follow your every move. At this point, it's particularly important not to look the dog in the eye because he sees that as a challenge to defend his position.

Lips, teeth and tongue A relaxed dog in normal posture may let his tongue loll out of his mouth. If he wants something from you, if he is happy or wants to play, he may pull his lips back in what appears to be a smile and show his teeth, an expression, by the way, dogs show only to humans and not to other dogs. But beware the dog that bares his clenched teeth and wrinkles his nose. He is ready to attack.

Ears The dog's sense of hearing is much more acute than ours and even dogs with floppy ears have the ability to move and turn them to follow sounds. If a dog's ears are raised, he is relaxed, listening, or showing acceptance. If they are back, he may be signaling submission and deference or may be frankly fearful.

Tail A dog wags his tail when he is happy or wants to play. It is really an energy indicator. When he is submissive, he tucks it between his legs. A taut tail, held down rigidly behind him, may show that he is prepared to spring since he uses his tail for balance when jumping.

Voice Dogs are vocal animals. They yip, bark, whimper, howl, and growl. The pitch or volume of their sounds can increase with their level of emotion. A bark may be playful or aggressive. Unlike body signals, dog noises can mean different things from different dogs.

When two dogs meet they carry out a series of actions that looks like a choreographed dance. With their bodies tense and tails taut, they circle and sniff each other, silently gathering and exchanging information, ready to defend themselves at any moment if necessary. They hold their ears back and the hair on their back may stand on end. They often avoid direct eye contact at first, sizing each other up to determine if the stranger is strong or weak, male or female, hostile or non-hostile. One dog may place his head on the nape of the other's neck or nip at his nose. It seems they are getting ready to fight and then, one lies down. Soon, they may separate and urinate. At this point they have agreed on which dog is dominant.

Dogs learn body language from their mothers during the first 8 weeks of their lives and they test out this form of communication with their littermates. If a dog misses out on such training, he will have trouble communicating with other dogs throughout life.

Normal posture The dog appears alert with head held high. His tail moves freely. His jaw is relaxed.

Invitation to play The dog happily signals his desire to play by wagging his tail and dipping down into a "play bow." His front legs are in a crouch and his backbone swoops up, leaving his rear haunches high. His head is held up expectantly to capture your attention. He may raise a front leg or lean to one side with his head.

Submission The dog crouches down further and still appears relaxed. He may lift a front foot as in a play invitation, but his ears are back and his tail is down. He may yawn, scratch, or sneeze, which is meant to calm him and the dogs or people confronting him.

Fearful aggression A dog that is afraid tenses his body and holds his tail rigid, though it may be wagging. His rear legs are ready to run or spring. He bares his teeth, draws back his ears and the hair on his back stands on end. He growls or snarls constantly to warn off the subject of his fear.

Dominance aggression Teeth bared, this dog stares you down and advances confidently with his tail wagging slowly and his ears in the forward (alert) position.

Total submission The dog drops his tail and curls it between his legs. He drops his head to avoid eye contact. He rolls over on his side and bares his belly, with one hind leg raised and urinates. If he isn't afraid, he'll tilt his head up a bit and raise his ears to show trust.

Every dog knows the same language. You and your dog probably pick up on each other's signals without thinking much about it. But if your dog begins to behave differently, if you are getting to know a new dog, or if you encounter a dog you don't know, it helps to be able to read the universal body language of dogs.