Tail wagging means a lot more to a dog than "I'm pleased to see you", research has shown.
Most dog owners may not notice if their best friend's tail wags more to the left or the right. But to another dog, the distinction makes a world of difference, scientists have found.
Put simply, a wag to the right signifies happiness, and a wag to the left indicates fear.
The behaviour reflects what is happening in the dog's brain. Earlier research showed that left brain activation produces bigger wags to the right, and vice-versa.
Now scientists have learned that dogs deliver signals to other dogs via their tails that are hidden from humans.
The Italian team showed dogs videos of other dogs whose tail wagging was more pronounced either towards the left or the right.
When dogs saw another dog wagging more towards the left, their heart rates picked up and they began to look anxious. Dogs shown wagging biased to the right stayed perfectly relaxed.
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The findings are reported in the journal Current Biology. Study leader Dr Giorgio Vallortigara, from the Centre for Mind/Brain Sciences at the University of Trento, said: "The direction of tail wagging does in fact matter, and it matters in a way that matches hemispheric activation.
"In other words, a dog looking to a dog wagging with a bias to the right side - and thus showing left-hemisphere activation as if it was experiencing some sort of positive/approach response - would also produce relaxed responses.
"In contrast, a dog looking to a dog wagging with a bias to the left - and thus showing right-hemisphere activation as if it was experiencing some sort of negative/withdrawal response - would also produce anxious and targeting responses as well as increased cardiac frequency. That is amazing, I think."
The study involved 43 healthy dogs of various breeds. Each dog was shown videos of another dog wagging its tail.
Some clips were shown as darkened silhouettes to obscure features besides tail wagging that might have had an influence, such as facial expression.
The experiments were conducted in a large room with the videos presented on a large screen.
The scientists wrote in their paper: "The results show that domestic dogs could extract communicative cues from tail-wagging direction.
"Dogs facing stimuli... of a dog wagging its tail with a bias to the left side revealed a greater emotional reaction than those facing similar stimuli wagging its tail to the right side. This was apparent in both cardiac activity and behaviour."
Dr Vallortigara believes the research could be helpful to dog owners and vets.
"It could be that left/right directions of approach could be effectively used by vets during visits of the animals or that dummies could be used to exploit asymmetries of emotional responses," he said.