A specific muscle causes the inner-eyebrow raise that makes dogs’ eyes appear bigger and more infant-like and produces a look similar to one humans make when sad. The researchers found that while six deceased dogs they dissected uniformly possessed this muscle, four dissected wolves either did not have it or barely did. They also found that 27 shelter dogs in the United Kingdom did the eyebrow-raise far more often and intensely when interacting with strangers than did nine wolves at two U.K. wildlife parks.
This suggests ancient canines with expressive eyebrows might have elicited nurturing from humans, the authors write, and that care would have given the animals a selection advantage that allowed them to pass on puppy-dog eyes to their descendants. The study was published in the journal Proceedings of the National Academies of Sciences.
“What is so provocative about this finding is the likelihood that our unconscious biases shaped the evolution of the dog’s eye musculature,” said Brian Hare, a Duke University evolutionary anthropologist and canine cognition expert, who edited the study but was not involved in the research. “The presence of these anatomical differences between wolves and dogs is a smoking gun for the role of our desire to cooperate and communicate with dogs being a driving force in dog evolution.”
The new research built on the authors’ previous work examining this muscle movement in dogs. One study found shelter dogs that performed the inner-eyebrow raise were adopted more quickly than those that did not, in a sort of modern-day demonstration of puppy-dog eyes’ power over humans. Another study found that dogs make puppy-dog eyes more when people are looking.