Thanks to Linode Cloud Computing for supporting this episode of SciShow.

Head to linode.com/scishow to learn more and get a $100 60-day credit on a new Linode account. If you have a dog in your life, you’ve probably found it nearly impossible to stay mad about a chewed-up shoe once they give you those irresistible puppy eyes.

Instant forgiveness! And it turns out, their ability to tug at our heartstrings is no coincidence, but actually a trait we selected for during doggo domestication. The future of canine companionship began around 33,000 years ago with the start of the domestication process, eventually resulting in the dogs we have today.

Domestication of dogs has transformed their facial muscle anatomy, which has significantly raised the bar for how we communicate with them. One of those muscles is the levator anguli oculi medialis, and it dramatically lifts a dog’s inner eyebrow. And while that muscle shows up across virtually all the dog breeds, it isn’t present in their wolfy relatives.

Domesticated dogs also move their eyebrows more often and intensely than wolves do. They’ve mastered the inner eyebrow raise, a movement that makes their eyes look bigger, resembling what people think of as looking sad. And their soulful gaze can trigger a nurturing response in us.

Eye contact between a person and their pooch seems to release the hormone oxytocin in both the dog and the human. This hormone promotes bonding between individuals, so it appears dogs are able to exploit its release to their benefit, improving the emotional connection with their humans. But it’s not simply the presence of these eyebrow-raising muscles that tugs at our heartstrings.

It turns out how they use their facial muscles, in general, helps the modern dog better relate to their human pals. We, humans, form facial expressions using our mimetic muscles, the majority of which are considered fast-twitch fibers. This means that the muscles contract quickly allowing us to change our expressions dramatically and rapidly, but this also means we can’t hold our expressions for very long.

On the other hand, slow-twitch muscle fibers are used in slow, steady movements. We humans do have some of this type concentrated around our lips and tongues to help us with the careful, controlled motions required for speech. In a new study presented at the Experimental Biology 2022 annual meeting, researchers found that while dogs and wolves do share similar facial muscles, wolves have more slow-twitch fibers, which are needed for sustained behaviors like howling.

And dogs have around three times as many fast-twitch fibers compared to wolves, allowing for more rapid facial movements which are used for barking. Plus, these fibers are responsible for many of their characteristic facial expressions, from animating their expressive eyebrows to breaking out into what looks to us like a full-on grin. And the range of these fast-twitch fibers found in dog’s faces was similar to those found in humans.

While further study is needed to fully understand the facial differences between dogs and wolves, these findings strongly suggest that fast-twitch muscles allow dogs to better communicate with humans, thanks to selective breeding. Over many years, we gravitated towards dogs with facial expressions that were more like our own, and drove the evolution of today’s puppy eyes that we cherish so dearly. And now, we can not only interpret and react to the facial cues of our dogs, but they also do the same for ours, forever strengthening the bonds between us and our furry besties.

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