The direction in which a dog wags its tail tells other canines what it's thinking, new research has revealed.

Scientists have confirmed previously that tail wagging isn't just a back and forth affair: the wag can be biased towards either the right or left.

Dogs watching the approach of - and greeting - their owners, or other dogs they are friendly with, tend to wag more towards the right. On the other hand, a cautious dog, considering doing a runner because it's seen a potential threat in the form of a larger, unfamiliar animal, will tend to wag more to the left.

Whether dogs themselves are sensitive to the wagging signals of their canine counterparts wasn't known, however, until now.

Writing in Current Biology, University of Bari Aldo Moro researcher Marcello Siniscalchi and his colleagues showed 43 domestic dogs, of both sexes and from a range of breeds, video sequences of unfamiliar dogs describing either left or right-biased tail wags.

The team monitored the dogs' behaviours and heart rates during the process to gauge their degree of stress.

Watching tail wags to the left significantly elevated the animals' stress levels with their heart rates increasing to almost 200 beats per minute.

A non-moving / wagging picture elicited a smaller increase - to a heartrate of about 150, and rightwards wags elicited an increase to about 140.

The team also repeated the experiment with dog-shaped silhouttes to avoid any non-wagging related bias and obtained the same result.

This shows, they say, that domestic dogs actively extract communication cues from the tail wags of their peers.

The reason it happens, they suggest, is because the brain's left hemisphere - which controls movements on the right side of the body - contains a region specialised for "approach" or friendly behaviour, while the right hemisphere contains a region specialised for withdrawal or retreat behaviours.

"The finding that dogs are sensitive to the asymmetric tail expressions of other dogs supports the hypothesis of a major role of social behavior in the evolution of brain asymmetries," the team speculate.

"It also opens a window to the objective investigation of the emotional lives of animals."