Side biases associated with left-right asymmetries in the nervous system have been found to be widespread in the animal kingdom. The new study follows earlier work by the same Italian research team which showed that dogs wag to the right when they feel positive emotions, such as seeing their owners. Dogs were observed to wag to the left when they feel negative emotions, like when they see an unfriendly dog. The tail-wagging behavior reflected what was happening in the dogs’ brains: left-brain activation produced a wag to the right while right-brain activation produces a wag to the right.

The new research, published in the journal Current Biology, shows that the tail-wagging difference means something to other dogs.  The researchers showed dogs videos of other dogs with either left- or right-asymmetric tail wagging, and then monitored the dogs’ reactions. When dogs saw footage of another dog wagging their tail to the left, their heart rates increased and they began to appear anxious. When footage was shown to the dogs of a dog wagging its tail to the right, they stayed relaxed.

The results showed that domestic dogs could determine a communicative cue from the direction another dog was wagging its tail, as shown in both cardiac activity and behavior. A dog observing another dog wagging its tail with a bias to the right side, and thus showing left-hemisphere activation as if it was experiencing something positive, would produce relaxed responses in the observing dog. A dog that saw a dog waving its tail with a bias to the left - showing that the observed dog had right-hemisphere activation as if it was experiencing something negative - would produce anxious feelings and increased cardiac frequency in the observer dog.

Giorgio Vallortigara of the Center for Mind/Brain Sciences of the University of Trento, who was a co-author of the recent study, does not believe that the dogs are trying to communicate emotions to other dogs. Instead, he thinks the bias observed in tail wagging is likely an automatic byproduct of differential activation of the left versus the right side of the brain. 

The bias in wagging and its response may come in use for veterinarians in canine animal welfare theory and practice.