The findings, reported in the Cell Press journal Current Biology, suggests dogs, like humans, have asymmetrically organized brains, with the left and right sides playing different roles, the researchers said.
Earlier work by the same Italian research team had determined dogs wag to the right when they feel positive emotions -- when seeing their owners, for instance -- and to the left when they feel negative, such as when seeing an unfriendly dog.
In the new study, they said they found fellow canines can spot and respond to these subtle tail differences.
"It is very well known in humans that the left and right side of the brain are differently involved in stimuli that invokes positive or negative emotions," Georgio Vallortigara, a neuroscientist from the University of Trento, said.
In an experiment, the researchers studied dogs as the animals watched films of other dogs displaying different tail-wagging behavior, measuring the pets' heart rates and analyzing their behavior.
When the animals saw a dog wag its tail to the right, they stayed perfectly relaxed.
But when they spotted a tail veer predominantly to the left, their heart rates picked up and they looked anxious, the researchers found.
"The direction of tail wagging does in fact matter, and it matters in a way that matches hemispheric activation," Vallortigara said.
The findings could give owners, vets and trainers a better insight into their animal's emotions, they said.