Some mysteries remain about why dogs wag their tails

When it comes to tail wagging among dogs, some questions still hound researchers.

We know that domesticated dogs (Canis familiaris) use their tails to communicate — with other dogs as well as humans — and even what various types of wags mean, researchers note in a new review of the scientific literature. But we don’t know why dogs seem to wag more than other canines or even how much of it is under their control, ethologist Silvia Leonetti and colleagues report January 17 in Biology Letters.

“Among all possible animal behavior that humans experience in everyday life, domestic dog tail wagging is one of the most common,” says Leonetti, who is now at the University of Turin in Italy. “But a lot of dog behavior remains a scientific enigma.”

So Leonetti and her colleagues pored through previous studies to figure out what elements of tail wagging are understood and which remain mysterious. They also hypothesized about the behavior’s origins: Perhaps tail wagging placates some human need for rhythm, the researchers suggest, or maybe the behavior is a genetic tagalong, a trait tied to others that humans bred into domesticated dogs.

“People think wagging tail equals happy dog. But it’s actually a lot more complicated than that,” says Emily Bray, an expert in canine cognition at the University of Arizona in Tucson who was not involved with the work. Understanding why dogs wag their tails is important partly from an animal welfare perspective, she says, as it could help dog owners read their pups’ cues better.

One main thing that researchers know about tail wagging is that it’s used predominantly for communication instead of locomotion, like a whale, or swatting away bugs, like a horse. Wagging also means different things depending on how the tail is wagged, such as its height or side-to-side movement.

For example, when the tail wags more to the right, the researchers say, it typically means the dog is interested in a stimulus or wants to approach something. But when it wags more to the left, it signals uncertainty or wanting to withdraw. When wagged low and near the legs, it’s a sign of submission or insecurity. Dogs can interpret and react differently to these varied wags (SN: 11/1/13).

But for such a ubiquitous behavior, questions abound. One gap is how much tail wagging is under dogs’ conscious control, the team reports. Several studies have also observed that dogs wag their tails more than other canines, particularly compared with their closest relative, the wolf (C. lupus) — but scientists don’t yet know why.

One idea is the domesticated rhythmic wagging hypothesis, Leonetti and colleagues suggest. Scientists know that humans’ brains respond positively to rhythm, and because tail wagging is a rhythmic behavior, the authors suggest it could be that humans consciously or unconsciously preferred dogs that wag more.

Or the domestication syndrome hypothesis could be at work. It’s an existing theory that unintended, genetically linked traits can pop up when specific traits are bred for through domestication. Perhaps, the researchers propose, the canine characteristics that humans found desirable to breed for — such as temperament — might be genetically linked to tail wagging. 

Evolutionary biologist Tom Reimchen isn’t so sure about either hypothesis. He’s skeptical that domestic dogs do wag their tails more than other canines. More research is needed to compare tail wagging between nondomestic canine species and the epigenetic influences behind  canids’ tail wagging, says Reimchen, of the University of Victoria in British Columbia.

Leonetti, who did the work while at the Max Planck Institute for Psycholinguistics in Nijmegen, Netherlands, says a multidisciplinary approach would be helpful in future tail wagging research, for example combining neurology, cognition and physiology. Because the domestication process was also an evolutionary process — one humans were involved in — studying dog behavior and their wags could reveal something about us too, she says (SN: 9/7/22). 

It “can shed light not only on our understanding of dog behavior, but also we’ll then understand something about human psychology.”