If eyes are truly windows to the soul, then maybe your dog is trying to tell you something.

An international research team has found that "puppy dog eyes" evolved over thousands of years to help dogs better communicate with humans.

Their study, published today in Proceedings of the National Academy of Sciences, is the first detailed analysis comparing the facial anatomy and movement behavior of dogs and wolves. The researchers discovered that the facial muscles of both species were similar, except above the eyes. Dogs have a small muscle, which allows them to raise their inner eyebrow, which wolves do not.

The team, which includes Duquesne University professor Dr. Anne Burrows, suggests that raising the inner eyebrow triggers a nurturing response in humans because it makes the dogs' eyes appear larger, more infant-like and resembles a movement humans produce when they are sad.

The study found that when exposed to a human for two minutes, dogs raised their inner eyebrows more and at higher intensities than wolves.

"The raised inner eyebrow movement in dogs is driven by a muscle which doesn't consistently exist in their closest living relative, the wolf," said Burrows, a biological anthropologist in the Department of Physical Therapy at Duquesne and an expert in the evolution of primate facial expressions.

"This is a striking difference for species separated only 33,000 years ago and we think that the remarkably fast facial muscular changes can be directly linked to dogs' enhanced social interaction with humans," she said.

The only dog species in the study that did not have the muscle was the Siberian husky, which is among more ancient dog breeds.

The study noted that an alternative reason for the human-dog bond could be that humans have a preference for other individuals which have whites in the eye and that raising the eyebrow exposes the white part of a dog's eyes.

It is not known why or precisely when humans first brought wolves in from the cold and the evolution from wolf to dog began, but this research helps us understand some of the likely mechanisms underlying dog domestication, the study said.

In addition to Burrows, the team included behavioral and anatomical experts from the University of Portsmouth in the U.K., Howard University and North Carolina State University. The research team was led by comparative psychologist Dr. Juliane Kaminski at the University of Portsmouth.

Duquesne University

Founded in 1878, Duquesne is consistently ranked among the nation's top Catholic universities for its award-winning faculty and horizon-expanding education. A campus of nearly 8,000 graduate and undergraduate students, Duquesne prepares students by having them work alongside faculty to discover and reach their goals. The University's academic programs, community service and commitment to equity and opportunity in the Pittsburgh region have earned national acclaim.

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June 17, 2019