Why do dogs wag their tails? Scientists add a new explanation
My 8-year-old came home from school and said, “Mom, do you know why dogs wag their tails? It’s because people like it.”
Her third-grade class has been reading coverage of scientific articles geared toward kids. She showed me the latest, an adaptation of a Washington Post article reporting on a study about dog tail wagging by graduate student Silvia Leonetti from the University of Turin in Italy and colleagues. Although it’s been a while since I’ve blogged, Annie’s enthusiasm for this bit of science motivated me to share it with you all and throw my two cents into the mix.
The study of dogs (that did not study dogs)
The study was not an experiment involving dogs, but rather a systematic search of previous published studies of tail wagging. There have been about a hundred to date. The authors were using previous publications to evaluate whether those studies supported either one of two hypotheses.
Idea 1: Tail wagging tagged along during domestication
The first hypothesis the authors considered was that tail wagging essentially tagged along uninvited with other traits that humans preferred. Unexpected traits have been shown to come along in the process of domestication through genetic linkages between traits; the classic example being the surprisingly short muzzles and generally cuter physiques that emerged as Russian scientists in the 1950s selectively bred for tameness in silver foxes.
Idea 2: We chose dogs that wagged their tail to a steady beat
The second hypothesis is the novel one. The authors considered that tail wagging was - either consciously or unconsciously - chosen. In other words, humans saw dogs that had some rhythmic tail wagging, we liked the rhythm, and we kept those dogs near us and set the stage for dogs that wagged often and in rhythm to have more puppies and pass the trait on (and on and on). The authors call on psychological research that shows that humans easily detect and strongly prefer regular rhythms in the world around them and that these rhythms activate reward systems in our brains. Basically, we like it.
So why do dogs wag their tails?
Although Annie came home with the conclusion that dogs wag their tails because we like it, suggesting the authors found support for the second hypothesis (that they dubbed the “domesticated rhythmic wagging hypothesis”), my read of their paper indicates that the jury is still out. The authors actually provide some support for both hypotheses. Then they call on future scientists to set up studies to test the hypotheses directly.
So yes, Annie, one of the reasons dogs wag their tails is likely because we were more likely to choose and breed the dogs that came in a cute, appealing package that included a rhythmically wagging tail. Was it because we liked the rhythm, or did that tag along? Or both? Certainly, if we had hated the wagging tail, we would have steered their rear ends in another direction. But just how much we focused on the rhythmic tail wag remains to be determined.
Why, why, why so many questions?
Just as an 8-year-old can ask “Why?” (or more often, “Why not?) a dozen times and get a dozen different answers, there are many ways to answer the question of why dogs wag their tails, and all the answers can be true. This is because there are different levels of explanations. I was excited to see that the authors touched on different levels of explanation in their article, as this was one of the most satisfying, foundational things I learned in animal behavior.
If we take the tail-wagging questions to another level, we might start asking how tail wagging works, how it develops, what function it serves, and how it evolved. When I first heard Annie’s version of the study, I thought the study was going to be about how owners unwittingly offer praise and attention to their dogs when they are wagging their tails, resulting in the dogs wagging their tails more as they experience these positive outcomes. That could also be true! So many studies (for someone else, maybe you?) to do.
Learn More
Read the original study! Leonetti, S., Cimarelli, G., Hersh, T. A., & Ravignani, A. (2024). Why do dogs wag their tails?. Biology Letters, 20(1), 20230407. Link to article.
Read more about the silver fox domestication study and the significance of that work in this article.