This is truly fascinating: Researchers from the University of Trieste in Italy have determined that dogs, which wag their tails to express both happiness and displeasure, wag their tails asymmetrically in a consistent manner to demonstrate whether their feelings are positive or negative. The New York Times reports:
When dogs feel fundamentally positive about something or someone, their tails wag more to the right side of their rumps. When they have negative feelings, their tail wagging is biased to the left. ...
Research has shown that in most animals, including birds, fish and frogs, the left brain specializes in behaviors involving what the scientists call approach and energy enrichment. In humans, that means the left brain is associated with positive feelings, like love, a sense of attachment, a feeling of safety and calm. It is also associated with physiological markers, like a slow heart rate.
At a fundamental level, the right brain specializes in behaviors involving withdrawal and energy expenditure. In humans, these behaviors, like fleeing, are associated with feelings like fear and depression. Physiological signals include a rapid heart rate and the shutdown of the digestive system. ...When the dogs saw their owners, their tails all wagged vigorously with a bias to the right side of their bodies, Dr. Vallortigara said. Their tails wagged moderately, again more to the right, when faced with an unfamiliar human. Looking at the cat, a four-year-old male whose owners volunteered him for the experiment, the dogs’ tails again wagged more to the right but in a lower amplitude.
When the dogs looked at an aggressive, unfamiliar dog — a large Belgian shepherd Malinois — their tails all wagged with a bias to the left side of their bodies.