Poignant puppy-dog eyes are a product of domestication, researchers say

Researchers say dogs acquired eyebrow muscles as humans domesticated them.
(Dave Olson / Columbian)

What’s behind those hard-to-resist puppy dog eyes?

New research suggests that over thousands of years of dog domestication, people preferred pups that could pull off that appealing, sad look. And that encouraged the development of the facial muscle that creates it.

Today, pooches use the muscle to raise their eyebrows and make the baby-like expression. That muscle is virtually absent in their ancestors, the wolves.

“You don’t typically see such muscle differences in species that are that closely related,” said Anne Burrows of Duquesne University in Pittsburgh, an author of the study published Monday in the Proceedings of the National Academy of Sciences.


Dogs differ from wolves in many ways — they have shorter snouts, smaller sizes and more expressive faces. And unlike wolves, dogs heavily rely on human eye contact, whether it’s to know when someone’s talking to them or to get help with a problem like hopping a fence or getting out the door.

Burrows and her colleagues examined the eye muscles in the cadavers of six dogs and two wolves. The dogs had a meaty eye muscle to lift their eyebrows and make puppy dog eyes. But in wolves, the same muscle was stringy or missing.

An illustration of the facial musculature in the dog and wolf. The anatomical differences are highlighted in red.
(Tim Smith)

The scientists also recorded 27 dogs and nine wolves as each stared at a person. Pet pooches frequently and intensely pulled back their eyebrows to make sad expressions, while the wolves rarely made these faces, and never with great intensity.

The researchers believe dogs, over their relatively short 33,000 years of domestication, used this eye muscle to communicate, possibly goading people to feed or care for them — or at least take them out to play. And people, perhaps unwittingly, obliged.

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Dog experts who weren’t involved in the research said they were impressed.

“The implications are quite profound,” said Brian Hare of Duke University, who edited the study. These muscles almost certainly developed because they gave dogs an advantage when interacting with people, and people have been unaware of it.


“The proof has been in their puppy dog eyes all this time!” he said.

Evan MacLean at the University of Arizona called the findings fascinating, but cautioned that the muscle difference could be an indirect effect of other changes rather than a specific response to human influence.

Clive Wynne of the Canine Science Collaboratory at Arizona State University noted that the study included dogs from only five breeds, and that the videos the researchers examined were mainly of Staffordshire bull terriers.

He also said it would have been helpful to have some background information about each dog. (Burrows said she planned to conduct follow-up studies with more breeds.)


But Wynne praised the study for shedding light on how dogs came to be man’s best friends.

“Kudos to the researchers for thinking of a cool way to investigate an important aspect of dogs’ success” with humans, he said.

Rehm writes for the Associated Press.