Why do dogs wag their tails? Humans liked the rhythm, study suggests.

Taylor Hersh was watching a YouTube video of wolves a few years ago when the animal researcher noticed something curious. The wolves hardly wagged their tails.

It was a stark difference to the frequent wagging she saw in most pet dogs, which made her curious about what had changed in the roughly 35,000 years since dogs were domesticated from wolves.

Hersh and a team of European researchers began a study in hopes of answering one question: Why do dogs wag their tails?

Last week, the researchers released their findings: Humans likely altered dogs’ tail wagging without realizing it, researchers said in the Biology Letters journal.

The findings could flip the long-held belief that dogs are wagging their tails because they’re happy. Instead, Hersh and her colleagues suggest that dog tail-wags made people happy, so humans tended to select for that trait when welcoming dog ancestors into their lives and breeding the animal.

Tail-wagging is rhythmic, and previous studies have found that rhythms — everything from music to the sound of pounding horse hoofs — trigger brain activity that helps make people feel joyful. Humans, even subconsciously, might have enjoyed the rhythm of dogs wagging their tails, researchers said.

“They look almost like a metronome — tick tick tick tick tick,” Hersh, who was a researcher at the Max Planck Institute for Psycholinguistics in the Netherlands during the study, told The Washington Post.

Since people associated dogs’ swishy tails with the animals being cheerful, it’s also possible that more tail-wagging dogs were selected as pets, researchers found. The owners might’ve assumed the movement meant the furry creature was friendly.

“We don’t have a time machine to go back and look at … what did we want out of dogs?” Hersh said. But, she added, “we can try to do the best we can with modern dogs and modern humans to try and reconstruct that evolutionary path.”

In February 2022, Hersh and her colleagues began studying dog behavior. Scientists have found that dozens of dogs’ traits and behaviors changed during domestication, including the appearance of their fur, ears, body size — and even their ability to make “puppy-dog eyes.”

But Hersh said researchers couldn’t find a clear answer from previous studies about why dogs wag the body part that extends from their spines. They pored over dozens of past studies on dogs’ evolution in an attempt to understand the movement.

One study found that dogs began wagging their tails more than wolves when they were as young as three weeks old. Another study found that dogs wag their tails faster and more often than other canines. Further research indicated that dogs use their tails to try to express their emotions to people.

Then Hersh read an American Scientist magazine article about a study observing domesticated silver foxes. The research found that foxes selected for tameness and friendliness ultimately wagged their tails more often than other foxes. The same must’ve been the case for dogs, she recalled thinking.

The authors of Wednesday’s paper said more research is needed to confirm their theories. Hersh, who’s now a researcher at Oregon State University, said she hopes to examine dogs’ brains, heart rates and other vitals to understand what the animals think and feel while wagging their tails.

The research can also shed light on what humans thought and preferred tens of thousands of years ago, according to researcher Andrea Ravignani, a co-author of Hersh’s study.

“It is a bit like finding prehistorical cave paintings from Homo sapiens or Neanderthals, which indirectly tell us that back then our ancestors enjoyed art or had symbolic reasoning,” Ravignani said in a statement to The Post. “In our case, what we know about how modern dogs wag their tails tells us that perhaps our ancestors 35,000 years ago already perceived the rhythmicity.”

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