A wagging tail has long been seen as a sign of a happy and contented dog. Scientists have discovered, however, that dogs communicate far more information about their emotions with their tails than was previously believed.

A study shows that dogs tend to move their tails more to the right or to the left depending on how happy or sad they feel. This emotional signal can be recognised by other dogs, affecting how the animals respond to each other.

The animals' tails tend to move slightly more to the right if they are happy, the research showed. If they are experiencing negative emotions, such as feeling threatened, then their tail will move slightly more to the left.

While the subtle bias in movement can be difficult for humans to detect, it is more obvious in slowed-down video footage. Dogs, however, seem to be able to pick up on these signals.

Neuroscientists found that dogs' heart rates increased and they showed signs of anxiety when they saw a strange dog with its tail wagging slightly to the left. If the dog met a strange dog whose tail wagged slightly to the right, then the animals remained calm and showed a relaxed heart rate.

Professor Giorgio Vallortigara, who led the research at the Centre for Brain Sciences at the University of Trento in Italy, said: ''We know from neurological studies in humans that the left and right hemispheres in the brain produce different emotional responses. In dogs, single organs like the tongue or tail are controlled by both sides of the brain.

''There can be competition and dominance between these two sides. When they move their tail, it is more bias to the left or to the right, depending on which side of the brain is more dominant at the time.

''It seems dogs pick up on this when they meet other dogs.''