An old joke about wagging tails goes like this: A young boy is afraid to pet a dog. An adult says, “He’s friendly look, he’s even wagging his tail.” The boy responds, “Yeah, but he’s barking and growling I don’t know which end to believe!”
This poor excuse for a joke contains a lot of truth, because a wagging tail does not necessarily mean a dog is friendly. So, if a wagging tail does not always indicate friendliness, what does it mean?
A dog’s tail position and motion is incorporated as a component of a complex system of body language that domestic dogs use, along with “verbal” cues such as barking, growling or whining, in order to communicate. A wagging tail indicates excitement or agitation. But whether the dog means it as an invitation to play, or to warn another dog or person to stay back, depends on other body language.
A slowly wagging tail that curves down and back 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 tail that is held higher, whether wagging or not, indicates dominance and/or increased interest in something. If the end of the tail arches over the back, and is twitching, you may be faced with an aggressive dog.