There's a reason we call them "puppy dog eyes"-those soulful, innocent expressions can sway even the most hardened human. That's no accident, a new study says. Centuries of domestication have radically reshaped a dog's eyebrow anatomy, making their faces-and emotions-easily readable to people. When meeting a person's gaze, dogs often raise their inner eyebrow muscle to make their eyes look larger and more appealing.

"There's no evidence that dogs move this [eyebrow] muscle intentionally, but it creates an exaggerated movement that for us means 'dog,'" says study leader Juliane Kaminski, a psychologist at the University of Portsmouth in the UK.

Eyebrow movement plays a major role in human communication, Kaminski says: "I'm doing it now when I'm speaking to you, even though I know you can't see me." The study is the latest example of how 20,000 years of cohabitation has made our pets finely tuned interpreters of human emotion-possibly more so than any other species.

In her past research, Kaminski has found dogs are uniquely skilled at understanding gestures, outperforming even non-human primates such as chimps. Several years ago, Kaminski began investigating the flip side of this relationship, looking at how people decipher dog behavior. In one experiment, published in 2013, she filmed shelter dogs to see if any of their behaviors were linked to how quickly the animal was adopted. Of all the factors Kaminski examined, only one stood out as significant: the movement of the dog's eyebrows upward and inward.

Initially, "it was a very surprising result. We didn't expect something as small as eyebrow movement to have a big effect," Kaminski says. But a question remained: Whether this eyebrow movement was unique to dogs, or if it could be found in their ancestor, the gray wolf.

The new study, published this week in Proceedings of the National Academy of Sciences, Kaminski and colleagues dissected and analyzed the facial muscles of six dogs-including a mongrel, a Labrador retriever, a bloodhound, a Siberian husky, a Chihuahua, and a German shepherd-as well as four wild gray wolves. The animals had all died natural deaths, and their bodies were donated to science. The team discovered the levator anguli oculi medialis, a large and prominent muscle, in all six dog specimens-but it was almost completely absent in wolves.

Kaminski and colleagues also found that the retractor anguli oculi lateralis muscle was smaller and more variable in size and presence in wolves than it was in dogs, with the exception of the Siberian husky, a more ancient dog breed that's closely related to the wolf.