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How Dogs Evolved to Be So Cute: More Human-Like Facial Muscles

 A labrador retriever

A labrador retriever
Photo: Simon Lees/PhotoPlus Magazine/Future (Getty Images)

New preliminary data offers insight into why we may find dogs to be so darn lovable. A study found that dogs generally have faster facial muscles than wolves—muscles that allow them to quickly react with more expression, similar to humans. These same muscles may also help explain why dogs tend to bark, while wolves usually howl instead.

That dogs can easily worm their way into our hearts is no secret. But for years, researcher Anne Burrows and her colleagues have been trying to figure out how dogs have evolved over millennia to become the adorable ragamuffins we know and love. Their earlier research has suggested, for instance, that dogs have a certain muscle largely absent in wolves that allows their eyes to open up big and wide to create that “puppy dog face” look.

Their latest research, being presented this week at the annual meeting of the American Association for Anatomy, adds more evidence to the notion that dog faces have become naturally suited to win us over.

The team observed muscle tissue taken from a range of dog breeds and the gray wolf under a microscope, looking for two different types of muscle fibers in particular: fast-twitch and slow-twitch fibers. Fast-twitch fibers can contract quickly but get exhausted sooner, while slow-twitch fibers are the opposite and allow for longer-lasting expressions. The tissues were taken from the orbicularis oris muscle (OOM), which surrounds the mouth, and the zygomaticus major muscle (ZM), located along the cheek. Both are important to creating facial expressions in dogs and humans, and the researchers were specifically looking at the ratio of slow-twitch to fast-twitch fibers in these muscles.

Gray wolves and domestic dogs both had fast-twitch fibers, the team found. But the percentage of fast-twitch fibers was substantially larger in the latter group: In dogs, anywhere from 66% to 95% of the fibers in these samples were fast-twitch, while the average in wolves was around 25%. Conversely, just under 30% of fibers were slow-twitch in wolves, while only 10% were slow-twitch in dogs.

The researchers are careful to point out that their findings are preliminary and have yet to go through the typical peer-review process. But the ratio of fast-twitch to slow-twitch muscles seen in these dogs is similar to what’s been documented in human faces, they note. And that suggests that modern dogs and their faces, following their split away from the lineage of wolves tens of thousands of years ago, have evolved to become more like us over time.

“It helps us conceptualize what humans from the Upper Paleolithic—roughly 40,000 years ago—valued when they were domesticating dogs,” Burrows told Gizmodo in an email. “Because we and dogs communicate with one another using facial expressions, Upper Paleolithic humans must have wanted, consciously or subconsciously, a dog that used facial expression in a similar fashion.”

The evolution of these faster facial muscles may not have only changed dogs’ capacity for expression but also how they verbally communicate, Burrows said. While wolves do occasionally produce short-timed barks, they more often stick to longer-lasting howls, and the reverse is generally true for dogs. The team theorizes that these behaviors are directly influenced by the facial muscles that each line of canine is now working with.

“Howling requires a sustained muscle contraction of those muscles around the mouth that turn the mouth into a funnel shape that can last while a wolf barks. Barking is a brief, staccato activity that requires little sustained muscle contraction,” Burrows explained. “So somewhere in the process of dog domestication, we selected for faces that were fast in dogs, faces that could quickly make meaningful facial expressions to communicate with us, but we also selected for a very fast face, one that could produce this new sound—barking.”

A shift from howling wolf-like dogs to barking modern-day dogs could have even been encouraged by our human ancestors, the researchers argue, since they might have preferred canine companions that could quickly let them know of danger ahead or other needed alerts.

The team plans to collect more facial muscle data before submitting their findings for peer review by the end of the summer, Burrows said. And they won’t stop there in trying to trace the origins of modern dogs. Their next goal is to study and compare the ear muscles of dogs and wolves, in order to understand how they may be involved in social communication and hearing.