New Study Reveals How Dog Facial Expressions Evolved to Strengthen Human‑Dog Bonds

Most dog owners know when their pup wants something or is vying for their attention-without letting out so much as a whine or bark. The animals are known for "puppy eyes," which convey a sense of emotion and allow them to better communicate with humans. The facial expressions canines make is more than just a way to get an extra treat or pat on the head, however. It's the results of thousands of years of domestication, according to a new study presented by researchers at the Experimental Biology 2022 meeting in Philadelphia. The study authors found that the faces of dogs have evolved over time to make them more appealing to humans, a trait that sets them apart from the wild wolves they descended from.

The researchers behind the study tested the hypothesis that dog facial muscle physiology is more similar to humans than to wolves in terms of myosin fiber type distribution. The hunch is based on the understanding that domesticated dogs and humans are able to comprehend on another's facial expressions and movement around the eyes. "This unique, mutual ability to accurately process facial expressions is part of the dog-human bond," the research states.

To obtain their findings, the researchers used samples from two muscles from a range of domestic dog breeds and gray wolves. They found that the facial muscles in dogs' faces are 66 percent to 95 percent fast-twitch fibers, while wolves average about 25 percent. It's believed by scientists that this trait lets dogs effectively communicate their feelings to their owners.

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These findings come just two years after the same researchers found that dogs have developed a muscle above their eyes that they use to make their eyes look more endearing. The research, which was published in the journal Proceedings of the National Academy of Sciences, revealed that dogs evolved to have the facial muscle that enables them to mimic what researchers call an "infant-like" expression that often prompts a "nurturing response" from humans around them.

According to the study, dogs use their eyebrow muscles to create what's known as "expressive eyebrows" and, combined with rounded eyes, use the expression to knowingly influence anyone around them. "When dogs make the movement, it seems to elicit a strong desire in humans to look after them," the study reports. This particular eyebrow muscle has become stronger over time to allow dogs' eyes to "appear larger, more infant-like, and also resembles a movement humans produce when they are sad."

The group of researchers behind the study examined the eye muscles in six different deceased dogs and two different wolves-the wolves didn't have the same eyebrow muscle that the dogs did. The scientists also recorded 27 different dogs and nine wolves as they interacted with nearby humans; the pups frequently flexed their eyebrow muscles to make sad expressions, whereas the wolves in the experiment rarely conveyed the same expression.

The study suggests that the evolutionary trait has been passed down to modern dogs because owners chose to breed pups who had it in the first place; an "unconscious preference" to protect and breed dogs who used puppy eyes over the years. Dogs may have developed the trait soon after they were domesticated from wolves, but researchers say their work has suggested that facial change has occurred over the thousands of years that dogs have lived alongside humans.