If you're happy and you know it, wag your tail. Click here for a video of a pup wagging left and right.Getty

Dogs wag their tails to the right when they see something they want to approach, and to the left when confronted with something they want to back away from, say researchers in Italy. The finding provides another example of how the right and left halves of the brain do different jobs in controlling emotions.

Unfortunately, because dogs move about so much, the bias can only be detected using video analysis. It's not obvious enough for you to tell whether the next dog you encounter is going to lick your face or turn tail.

"After discovering this, I look at every dog I meet, but my impression is that this is difficult to check outside the lab," says psychologist Giorgio Vallortigara of the University of Trieste. But it could be used in animal welfare, he suggests, to help gauge an animal's state of mind.

Vallortigara and his colleagues tested 30 pet dogs of varying breeds, recruited through an obedience school at the University of Bari's veterinary faculty.

Over a series of trials, they videoed each dog's response to being shown either their owner, a human stranger, a cat, or a Belgian shepherd malinois, a large dog breed similar to a German shepherd.

Shown a human or a cat, tails wagged consistently to the right. The unfamiliar person elicited less wagging than the owner, and the cat the least wagging of all — probably because the dog was so interested in giving chase that it was distracted from wagging, says Vallortigara.

Shown a large, unfamiliar and intimidating dog, the dogs wagged their tails more to the left. Dogs also wagged to the left when left on their own without anyone to look at, the researchers report in Current Biology¹.

Divided brain

Previous studies have shown that, in humans, strong activity in the brain's left hemisphere (which controls the right side of the body) is associated generally with a sunny disposition. Human studies have also linked left-brain activity with approach behaviour, and right-brain activity with retreat.

Dogs are already known to favour one paw over the other — most male dogs are left-pawed, whereas females show a weaker tendency to right-pawedness. But what they do with their tails may be a better guide to how their brains work, says Vallortigara.

"The use of forelimbs is not so important in animals other than humans," he says. "But tail wagging is an important emotional response."

"This is a fascinating way to measure lateralization," says neuroscientist Lesley Rogers of the University of New England in Armidale, Australia. "It will be valuable in a range of tests, not only in dogs, but in other species with tails."

Biased behaviour

Biases for right- or left-'handed' behaviours have been seen in fish, amphibians, reptiles, birds, reptiles and mammals. "The evidence is that brain asymmetry is quite ancient," says Vallortigara. "It seems to have started early in the vertebrates."

Parcelling out tasks to one side of the brain or the other avoids duplication, and may help decision-making by reducing conflict between brain regions.

More puzzling, says Rogers, is why such biases also arise in behaviours such as escape. Toads and chicks, for example, are both more likely to leap away from something seen in the left eye — suggesting that a predator could learn to sneak up from the right. Why such biases do not vary at random from animal to animal is still uncertain.

Vallortigara's team next aims to see whether a dog's retreat or approach response depends on which eye or nostril is stimulated by a friend or foe.

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• References

  1. Quaranta A., Siniscalchi M. & Vallortigara G.. Curr. Biol., 17 . 199 - 201 (2007).