We may have helped give our canine pals ‘puppy dog eyes’

Dog owners know the look: Your pooch stares up at you, eyes wide, and you can’t resist giving them a hug or favorite treat. A new study of dog facial anatomy suggests we may have helped create this expression by favoring canines with “puppy dog eyes” over the course of thousands of years of dog evolution.

To conduct the work, researchers dissected the remains of four wolves and six dogs, focusing on their faces. They spotted two striking differences: The levator anguli oculi medialis muscle, which raises the eyebrows, was highly developed in all of the dogs but barely there in wolves. And all dogs except a Siberian husky—an ancient breed—sported a robust retractor anguli oculi lateralis muscle, which widens the eyes by pulling the eyelids towards the ears. This muscle was mostly absent in the wolves.

Combined, the two muscles allow dogs to express the big, sad eyes that melt our hearts, the team reports today in the Proceedings of the National Academy of Sciences . And indeed, when the researchers asked strangers to approach a number of shelter dogs and tame wolves, the dogs produced the sad eye look—known scientifically as “the AU101 movement”—on average five times more often and with far more intensity than the wolves did.

The team suspects that early in dog evolution humans were more likely to care for canines with this look, perhaps because it reminded them of the big eyes of human infants. Those dogs had more pups, and so the muscles that power big eyes spread through dog populations. Even today, shelter dogs that rock the look are more likely to find a home. The next question: whether other domestic animals like cats have hit on the same strategy.