Our best friends are very good at telling us what they want and how they feel. So, we assume they're also very good at telling other dogs the same things.
The dog's telltale tail
A dog's tail is a fascinating piece of work. Past research showed that when dogs wag their tail to the right (activation of the left side of the brain), for example when they see their "owner", it's an indication of a positive emotion associated with approach, and when they wag their tail to the left (right brain activation), for example when they see an image of a dominant unfamiliar dog, it's an indication of a negative emotion associated with withdrawal. Details about this study are available in this essay called "Asymmetric tail-wagging responses by dogs to different emotive stimuli". I've written about the evolution and significance of asymmetric brains, called lateralization, in a previous essay titled "Divided Brains: Fascinating Facts about Brain Asymmetries".
Another question that needed to be studied is what do dogs seeing a dog wag his or her tail make of the situation? Do they know that a dog wagging their tail to the right is feeling good and a dog wagging their tail to the left is feeling a negative emotion? Some of the same researchers have recently discovered that they do (the abstract for this study titled "Seeing Left- or Right-Asymmetric Tail Wagging Produces Different Emotional Responses in Dogs" can be seen here). A recent essay by Douglas Quenqua in the New York Times called "A Dog’s Tail Wag Can Say a Lot" noted, "When watching a tail wag to the left [of a silhouette of a dog projected on a screen], the dogs showed signs of anxiety, like a higher heart rate. When the tail went in the opposite direction, they remained calm." The authors of this study concluded, "The finding that dogs are sensitive to the asymmetric tail expressions of other dogs supports the hypothesis of a link between brain asymmetry and social behavior and may prove useful to canine animal welfare theory and practice."
So, what do the dogs really know? One of the researchers, Giorgio Vallortigara, a neuroscientist at the University of Trento in Italy, is quoted in the New York Times essay as saying, "it is unlikely that dogs are wagging their tails to communicate with one another. The mechanistic explanation is that "'It’s simply a byproduct of the asymmetry of the brain,' and dogs learn to recognize the pattern over time."
Of course, the jury is still out on this conclusion and a lot of research needs to be done, however, I find the results of the second study may just as well indicate that dogs are indeed communicating with one another and that the various responses to different tail wags aren't so easily dismissed as being a byproduct of the asymmetry of the brain. In many of my other essays (and elsewhere; see also) I've argued that mechanistic explanations are neither the most parsimonious nor necessarily correct.
Stay tuned for more on this fascinating discovery and the remarkable cognitive and emotional words of other animals.
Note: You can read an extremely important study published in 1947 by Rudolph Schenkel called “Expression Studies on Wolves” in which the researcher discusses how wolves express their emotions, including how they use their tails, here.