For dog owners, few sights are more heartwarming than their pooch wagging its tail. Suggesting excitement, eagerness, or simply pure joy, the tail wag has long been emblazoned as the default symbol of canine carefreeness. But it always begs the question: Just why do dogs seem to wiggle their behinds when happy?
The answer, as it turns out, is complicated, involving a complex interplay between natural selection and humans’ artificial influence on behavior. And, dogs don’t just do it as an indicator of joy: A new study published in Biology Letters reviewed the existing body of literature to outline several theories pinpointing the mechanisms behind the infamous tail wag.
“We won’t be able to fully answer [why dogs wag their tails] until we start thinking about tail wagging as this behavior that has multiple components,” says Taylor Hersh, one of the study authors and a bioacoustician studying vocal complexity in animals at the Max Planck Institute for Psycholinguistics. “By putting all the information into one place, it helps us to find out what we still don’t know.”
For starters, it’s long been known that dogs wag their tails for a variety of reasons. After all, just as we gesture with our hands, or point our feet toward people we enjoy, tail wagging is but one mechanism by which dogs nonverbally signal to one another.
A quickly wagging tail, for example, may indicate excitement upon seeing an owner, or bashful nervousness when meeting another dog. The only time dogs do not wag their tails is when they are angry. A standoffish pooch is a still one, with its hindquarters held straight below the belly.
“At an emotional level, dogs tend to wag their tails when aroused, but this could reflect positive or negative emotions,” says Evan MacLean, director of the Arizona Canine Cognition Center, who was not involved with the study. “The idea that a dog that is wagging their tail is a happy pup is a common and dangerous misunderstanding.”
Even the directionality of the wagging plays a role. “It’s an asymmetric behavior. We’ve observed dogs whose tails are wagging more toward the right when there’s something they’re trying to approach,” says Hersh. “And then when they see something they want to withdraw from, they wag more to the left.”
Scientists have shown that dogs viewing silhouettes of other canines wagging left exhibit an increased stress response — suggesting that dogs might use tail-wagging direction to signal impending danger to others.
In the 2024 study, Hersh and co. analyzed a key discrepancy between domesticated dogs and their wild wolf counterparts, from which our beloved pets are descended: Wolves don’t wag. Or at least, not nearly to the extent that a pet dog engages in the behavior.
“What we don’t see in wolves is this huge amount of tail wagging across a variety of situations,” Hersh says. “In wolves, it’s mostly confined to a submissive individual signaling to or appeasing a dominant individual. They’ll wag their tails slowly and often pretty low.”
So, somewhere on the path to becoming a pet, wolves became integrated with human societies, and thus tail wagging took off as an even more common behavior. The research team has advanced two main theories explaining how the domestication process contributed to expressive tails.
One possible explanation is a theory known as “domestication syndrome,” and it’s well-established in the field of behavioral ecology. Humans, acting preferential toward the most palatable partners, artificially select for “good” pet behaviors. As such, a dog that’s subservient, loyal and playful is a much better housemate than an aloof, temperamental wolf.
By favoring these more domesticated canines, ancient peoples may have inadvertently bred generations of tail waggers. Decades-long breeding projects attempting to tame wild foxes demonstrated that the friendliest animals, in addition to having softer, more doglike features than their standoffish counterparts, also tended to wag their tails more.
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Scientists estimate that dogs have been partnered with humans for tens of thousands of years. Archeological evidence points to ancient Mesopotamian communities using them as hunting partners, for example, in addition to symbols of health and healing, over 3000 years ago. Over such a long time period, it’s no wonder that the artificial selection imposed on them by civilization could have major effects.
Alternatively, the scientists suggest that humans may have selected for tail-wagging, specifically, when domesticating dogs, and were more likely to pick pooches that wagged their tails more often, and more rhythmically — aptly named the “domesticated rhythmic wagging” hypothesis.
“The tail’s a very apparent visual thing for us to see,” says Hersh, who studies animal communication. “Perhaps because humans have this sort of propensity for rhythmic themes, we were actually breeding dogs that wagged more. We may not have been aware that we were doing it.”
Hersh and MacLean both advocate for continued research, positing that more studies are needed to fully understand these complex canine behaviors, as well as the exact means by which they rose to prominence.
So, too, does Emily Bray, an assistant professor who also works at the Arizona Canine Cognition Center.
“It’s really important for veterinarians and owners to just understand what dogs are telling us,” says Bray. “So a study like this will allow us to create environments that set dogs up for success. There’s lots of nuances to the behavior.”
Meanwhile, Hersh notes that there may also be certain practices, like trimming a dog’s tail at birth for aesthetic reasons, that could impair their ability to communicate.
“It has ethical implications,” says Hersh. “A third of all households worldwide have a dog. Humans love dogs, and better knowing what our pets are feeling would be really helpful.”