As any dog owner knows, there are a few surefire ways to cue a pup’s wagging tail: arriving home from work, picking up their favorite toy or uttering the word “outside.” But why do dogs wag their tail? Is it only because they are happy? It turns out that the answer is complex.
Tail-wagging is “clearly a communication mechanism,” says animal behaviorist Nicholas Dodman, a professor emeritus at Tufts University and head of the Center for Canine Behavior Studies. In most cases, “a wagging tail is akin to waving a white flag of surrender—that is, ‘I’m happy to see you and present no threat,’” he says.
But dogs use their tail to communicate more than happiness—both to humans and other dogs. An upright tail can imply dominance, a horizontal tail can hint at neutrality and a low tail can mean submission, Dodman says. Frantic wagging implies excitement, whereas slow wagging suggests ambivalence. Dogs can also have “helicopter tail,” or “circle wag,” which is when their tail goes around like a helicopter blade—a sign of extreme joy, he says.
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Dogs have only very limited vocalizations—growls, whines and barks—so they communicate with body language, Dodman notes. In addition to their tail, dogs use other body parts to send signals. For example, they may retract their lips, pull back their ears, take a hunched or erect body posture, or roll over in submission, he says.
Veterinary physiologist Federica Pirrone at Italy’s University of Milan suggests that tail-wagging in dogs is similar to gesturing during human speech, “something I, being Italian, am especially attuned to,” she says.
Wagging tails are visible at a distance, which allows dogs to communicate with other dogs while keeping enough space to minimize conflicts—an idea supported by studies indicating that dogs’ eyes may focus better on objects that are a foot or two away than ones that are more close-up and see moving objects better than static ones. This can be useful when they want to signal to other dogs that they are happy or wary.
Humans, too, respond to movement, and tend to read a great deal into a dog’s wagging tail, “even though we may occasionally misinterpret these signals,” Pirrone says. In fact, our responsiveness to tail-wagging may be part of the reason dogs have adapted this behavior over tens of thousands of years of domesticity (scientists now think the ancestors of dogs may have started to be domesticated as early as 35,000 years ago, possibly without intentional effort from humans). Studies show that wolves don’t wag their tail as often as dogs and that dog puppies wag at an earlier age than wolf pups. The behavior would have been especially necessary in the early phases of domestication, when the animals’ capacity for interaction with humans was foundational to their success as a species, she says.
The evolution of tail-wagging may also be a genetic fluke. Scientists have suggested that more frequent tail-wagging may be a by-product of dog domestication, perhaps because of a genetic link between tail anatomy and tameness. In a famous long-term experiment in Russia, geneticists who domesticated silver foxes over generations found that the domesticated foxes regularly wagged their tail and acted more like dogs than their wild counterparts.
But still, human preferences would have likely played a role. A recent review of the science of tail-wagging led by biologist Silvia Leonetti of Italy’s University of Turin suggests that people may have selectively bred dogs to wag their tail because humans responded to its rhythmic nature like they do to beats in music.
“We attribute a lot of meaning to this—we think that a tail-wagging dog is a happy dog, for example,” Leonetti says. “So we need to understand this behavior and all its complexity.”
Other clues about tail-wagging’s origins could come from canine brains. One study found that dogs wag their tail with a “bias” to the left or right side, depending on whether they are experiencing positive or negative emotions. This shows a “lateralization” of tail-wagging in a dog’s brain, she says, which could reveal more about the behavior.
In their review, Leonetti and her colleagues proposed ideas for future studies that could reveal more about wagging, one of which involves looking at scans of a dog’s brain while monitoring its tail. Dogs are one of the few animals for which noninvasive brain scans have been developed, and neuroimaging will help pinpoint the parts of a dog’s brain that govern the behavior, the authors wrote.
Pirrone is skeptical, however, that our affinity for tail-wagging comes down to its rhythm, mainly because it can’t be heard, unlike many other rhythms that humans respond to. And experiments to find out more will be challenging, she says, because of the complexities of defining rhythmic behavior and the limited scientific understanding of the cognitive architectures that underpin rhythms.
Nevertheless, such investigations are worth making, she says, because they promise “to disclose new scientific revelations about the complex dynamics behind our profound bond with dogs.”