When a dog wags its tail, surrounding muscles press on scent glands, releasing pheromones that signal the pooch’s age, sex, and social status. Submissive dogs may not wag their tails when frightened because they do not want to draw attention to themselves.
A high, quickly wagging tail is often a sign of playfulness and intense joy, but can also signal an aggressive dog. A horizontal, steadily moving tail suggests that your dog is closely studying something. If a dog tucks its tail between its legs, it is afraid, and attempting to communicate that it is submissive and does not pose a threat.