Why Dogs Wag Their Tails

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The question in the title is simple and straightforward, and many people—researchers and non-researchers including dog lovers—all have opinions on the matter. Everyone seems to know why dogs wag their tails, but do they really?

While we have some pretty good ideas, as I've written about in my book, about why dogs wag their tails in different contexts, social and non-social, we still don't know much about why and how it evolved. I just read an open-access essay by Silvia Leonetti and colleagues titled "Why do dogs wag their tails?" that attempts to answer these questions. I found it very interesting and think it's worth sharing widely.

Why do dogs wag their tails in different social and non-social situations?

This question is concerned with why dogs wag their tails in different situations that occur before, during, or after a given moment. Biologists call this a proximate, or immediate view of this behavior that is concerned with what is happening—what a dog is thinking or feeling at the moment, or shortly before or afterward. Of course, a dog might remember some past situations and wag their tail when they experience them again, and it's not obvious to a person why they're doing it.

In his book Canine Terminology, Harold Spira writes about more than twenty-five types of tails. Dogs use their tails to communicate with other dogs and humans, and we’ve learned to recognize their signals and understand them ourselves. A happy dog usually wags their tail, while a frightened dog often tucks it between their legs. In general, puppies do not wag their tails until they are about thirty to fifty days old, but I’ve seen what I’d call tail wagging earlier than that as a prelude to play wrestling or what seemed to be a “hello.”

Interpreting what a tail wag or tail position means depends on context—who the dogs are, their social relationship, and how what the tail is doing fits in with other components of composite signals that combine information from different body parts or senses including ear positions, facial expressions, body postures, vocalizations, odors, and gait, You've got to look at the whole dog. A loose wag is likely friendly, whereas a stiff wag likely signals assertiveness or aggression. However, there are no hard-and-fast rules, and a wagging tail does not always indicate friendliness or a happy dog.

We also know that tail wagging with a bias to the right tends to indicate that a dog is happy and relaxed, whereas left-bias wagging may indicate anxiety. In one study, dogs seeing their owners were more likely to show high-amplitude wagging with a bias to the right side (showing left-brain activation), while dogs seeing dominant, unfamiliar dogs tended to wag to the left (showing right-brain activation). These findings are consistent with the hypothesis that dogs have a left-hemisphere specialization for approach behavior and a right-hemisphere specialization for withdrawal behavior.

What if a dog loses their tail?

Stanley Coren tells a story about a dog whose tail had to be amputated after an unfortunate dog-motorcycle collision. Other dogs seemed unable to understand what this dog was trying to communicate.

My friend Marisa Ware told me the story of her dog Echo, who lost her tail in an accident. After the loss, Echo changed the way she communicated with dogs and people by using her body and ears to compensate for the loss of her tail. Tailless Echo relied more heavily on her ears to express her feelings. When she was excited to see someone, she put her ears very far back and almost wiggled them.

She also developed a kind of “hop-wiggle,” taking a little hop and wiggling her butt very quickly if she was excited to see someone. Echo never did the “hop-wiggle” before losing her tail.

Why and how did tail wagging evolve?

This question focuses on why dogs wag their tails from an evolutionary point of view, also called an ultimate explanation—how did it come about in the first place and why has it persisted? This is where Leonetti and her colleagues offer some very interesting and important perspectives.

To answer this question they focus on Nobel-prize-winning ethologist Nike Tinbergen's four questions: How does it work, mechanistically? How does it develop? What is it for? How did it evolve?¹ They offer two hypotheses to explain tail wagging's increased occurrence and frequency in dogs when compared to other canids. They write, "During the domestication process, enhanced rhythmic tail wagging behaviour could have (i) arisen as a by-product of selection for other traits, such as docility and tameness, or (ii) been directly selected by humans, due to our proclivity for rhythmic stimuli."

Where to from here? What's rhythm got to do with it?

Simply put, we need more research to come to a better understanding of why tail wagging evolved. The researchers say it best: "Dog tail wagging is a conspicuous yet scientifically elusive behaviour. Its uniqueness, complexity, and ubiquity have the potential to be associated with multiple functions, but its mechanisms and ontogeny are still poorly understood. These knowledge gaps prevent us from fully understanding the evolutionary history of modern tail-wagging behaviour and what role humans played in the process."

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They also write, "A more systematic and thorough investigation of tail wagging will not only better map this iconic dog behavioural display, but also provide indirect evidence into the evolution of human traits, such as the perception and production of rhythmic stimuli."

What I really like about this inquiry is that the authors make clear that it's not always easy to figure out why different behavior patterns have evolved, although we know the more immediate functions they may serve.

I hope to see more research on this simple yet elusive question. We may also gain more insights into why dogs wag their tails in different contexts and if there is a more robust reason that's also tied into the rhythm of an ongoing situation or the beat of certain other dogs.

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