Why Do Dogs Wag Their Tails? Scientists Examine the Endearing Behavior

For pet parents, perhaps nothing is more heartwarming than coming home to a dog excitedly wagging its tail in greeting.

But, for all the daily tail-wagging that goes on in homes around the world, scientists still don’t understand exactly why dogs do it. In a new paper published Wednesday in the journal Biology Letters, researchers outline a few theories. They hope other scientists will take up the mantle and test these hypotheses to shed more light on this endearing habit of man’s best friend.

“People think wagging tail equals happy dog,” says Emily Bray, a canine cognition expert at the University of Arizona who was not involved with the new research, to Science News’ Jude Coleman. “But it’s actually a lot more complicated than that.”

One possible explanation, known as the “domestication syndrome” hypothesis, relates to the human domestication of dogs, which began as early as 35,000 years ago. Perhaps tail-wagging was a behavior that humans unintentionally selected for, because it was linked with other preferable traits, like tameness or friendliness toward people. Tail-wagging may have simply been a byproduct of other specifically targeted characteristics.

Another theory, called the “domesticated rhythmic wagging” hypothesis, suggests that humans consciously or unconsciously selected for tail-wagging during domestication, because they are drawn to rhythmic stimuli.

Of course, these are just theories—and not everyone agrees with them. Holly Root-Gutteridge, an animal cognition researcher at the University of Lincoln in England, tells the Guardian’s Nicola Davis the rhythm hypothesis is an “interesting thought,” but adds that she’s skeptical about how much humans are really responding to the rhythmic beats of a dog’s tail.

More broadly, the new paper adds to the big-picture understanding of what scientists do and don’t know about tail-wagging. The authors reviewed more than 100 studies about the behavior and summarized their findings.

For example, past research has shown that dogs primarily wag their tails to communicate, which stands in contrast to how other animals use their tails. Some, like whales, use the appendage for swimming, while others, like horses, use them to keep away flies. Kangaroos use their tails like a powerful fifth limb to help propel themselves forward.

Dogs also seem to wag their tails in different ways to convey different messages. For instance, scientists believe wagging more to the right means a dog is curious and wants to approach, while wagging to the left is correlated with uncertainty. Low tail wagging—where dogs pin their tails down against their back legs—is also linked with insecurity and submission.

“Perhaps the coolest thing is that dogs can perceive those asymmetries in other dogs,” study co-author Taylor Hersh, a bioacoustician at Oregon State University, says to Science’s Sara Reardon. “There’s a really neat study where they showed dogs video silhouettes of a dog either wagging to the left side of its body or to the right. They found that they responded differently [with dogs acting more anxious when the silhouette dog was wagging to the left].”

The researchers also identify gaps in the scientific literature. A big one: How much are dogs consciously controlling their tails, and how much of their wagging is unconscious? Past research also indicates that dogs tend to wag more than their other canine relatives, like wolves—but, again, scientists don’t know why.

Moving forward, they suggest researchers study tail-wagging in a more systematic way, such as by analyzing videos. They suggest exposing dogs to a variety of stimuli and taking measurements to see if any patterns emerge in their heart rates or hormone levels while wagging their tails. In addition, they propose more studies that involve neuroimaging techniques to study the animals’ brains during this behavior.

Learning more about tail-wagging may have animal welfare implications, reports Newsweek’s Pandora Dewan—for example, does docking a dog’s tail affect its ability to communicate? But it could also help us learn more about ourselves. Since humans domesticated dogs, any evolutionary insights into a ubiquitous behavior like tail wagging could reveal more about early humans, too.

“By looking at how dogs are today, we get to travel back in time and see what the human domestication process has done in terms of dog behavior,” says study co-author Andrea Ravignani, a comparative bioacoustics researcher at the Max Planck Institute for Psycholinguistics, to Science. “So modern dog behavior, to some extent, brings a fingerprint of early humans.”

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Sarah Kuta is a writer and editor based in Longmont, Colorado. She covers history, science, travel, food and beverage, sustainability, economics and other topics.