The findings were published Monday in the Proceedings of the National Academy of Sciences of the United States of America, a peer-reviewed scientific journal.
"Behavioral data show that dogs also produce the eyebrow movement significantly more often and with higher intensity than wolves do, with highest-intensity movements produced exclusively by dogs," the researchers said, adding that the inner eyebrow movement "resembles an expression humans produce when sad, so its production in dogs may trigger a nurturing response."
"We hypothesize that dogs’ expressive eyebrows are the result of selection based on humans’ preferences," the researchers said in the publication.
To determine this, researchers conducted detailed comparisons of facial movements of nine wolves from two animal parks -- one in Germany and another in the UK -- and 27 domestic dogs randomly selected from shelters. The shelter dogs consisted of seven mixed-breed dogs and 20 Staffordshire bull terriers. To study the facial muscles anatomically, researchers examined four gray wolves -- two purchased from the taxidermy industry but not killed for the study, and two obtained from the Michigan Department of Natural Resources. They also obtained six cadaver specimens of domestic dogs, which were not euthanized for the study.
The main finding was that dogs and wolves had the same facial muscle arrangements except for around the eyes. Dogs were able to widen and open their eyes, making them look droopier, bigger and cuter to humans -- the puppy-dog eyes.
For more information on the study -- including video of the puppy-dog eyes from domestic dogs -- visit PNAS.org.
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