A newly discovered feature of dog body language may surprise pet owners and dog experts. When dogs feel positive about something or someone, their tails wag more to the right side. When they have negative feelings, their tail wagging is more to the left.

A study describing the phenomenon appeared in the March 20 issue of Current Biology. The authors are Giorgio Vallortigara, a neuroscientist at the University of Trieste in Italy, and two veterinarians, Angelo Quaranta and Marcello Siniscalchi, at the University of Bari, also in Italy.

「This is an intriguing observation,」 said Richard J. Davidson, director of the Laboratory for Affective Neuroscience at the University of Wisconsin in Madison. It fits with a large body of research showing emotional asymmetry in the brain, he said.

Research has shown that in most animals, including birds, fish and frogs, the left brain specializes in behaviors involving what scientists call approach and energy enrichment. In humans, the left brain is associated with positive feelings, like love, a sense 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 feeling, are associated with feelings like fear and depression. Physiological signals include a rapid heart rate and the shutdown of the digestive system.

Because the left brain controls the right side of the body and the right brain controls the left side of the body, such asymmetries are usually manifest in opposite sides of the body. Thus many birds seek food with their right eye (left brain / nourishment) and watch for predators with their left eye (right brain / danger).

Dog tails are interesting, Dr. Davidson said, because they are in the midline of the dog's body, neither left nor right. So do they show emotional asymmetry, or not? To find out, Dt. Vallortigara and his colleagues recruited 30 family pets of mixed breed that were enrolled in an agility training program. The dogs were placed in a cage equipped with cameras that precisely tracked the angles of their tail wags.

Then they were shown four stimuli through a slat in the front of the cage: their owner; and unfamiliar human; a cat; and an unfamiliar, dominant dog.

When the dogs saw their owners, their tails 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 face with an unfamiliar human. Looking at the cat, a four-year-old male whose owners volunteered him for the experiment, the dogs' tail again wagged more to the right but in lower amplitude. When the dogs looked at an aggressive, unfamiliar dog their tails all wagged with a bias to the left side of their bodies.

Thus when dogs were attracted to something, including a benign, approachable cat, their tails wagged right, and when they were fearful, their tails went left, Dr. Vallortigara said.

Strong left and right biases are showing up in the brains of many so-called simpler creatures, said Lesley Rogers, a neuroscientist who studies brain asymmetry at the University of New England in Armidale, Australia.
Male chameleons show more aggression, reflected as changes in body color, when they look at another chameleon with their left eye. A toad is more likely to jump away when a predator is introduced to its left visual field (right brain / fear).

Brain asymmetry for approach and withdrawal seems to be an ancient trait, Dr. Rogers said. Thus it must confer some sort of survival advantage on organisms.