Burrows and his team discovered that domestic dogs have a muscle in the eyebrow region that gray wolves do not: the levator anguli oculi medialis. “It allows dogs to deal with those puppy eyes,” says Burrows. “And wolves just don’t make that face.”
They also studied two muscles around the mouth: the orbicularis oris and the zygomaticus major. Dogs and wolves have these muscles. In dogs, however, they are mostly made up of fast-twitch fibers. In wolves, they are mostly slow-twitch.
To understand what that means, Burrows says to think about human runners. “If you’re a sprinter, you’re going to run very fast but only for a short distance. Your leg muscles are likely dominated by fast-twitch fibers, as these can contract quickly.
“If, however, you’re a marathon runner, it might take you a while to get up to speed, but you’re going to last a long time. Thus, marathon runners probably have leg muscles dominated by slow-twitch fibers. These move more gradually, but do not tire immediately.
A dog with mostly fast twitch fibers can therefore quickly make facial expressions. (Humans are also fast-twitch face makers.) These fibers also mean they’re excellent at barking, “which is a very fast movement of the lips.”
On the other hand, wolf muscles are great for prolonged movements, like howls. “They kind of turn their mouth into a funnel, and they hold that contraction for, you know, 30 seconds maybe,” Burrows says.
But why are the muscles and behaviors of dogs and wolves different? One possibility is that when humans first started domesticating wolves — which would eventually become what we now call “dogs” — “they were choosing this animal to bark instead of howl,” Burrows says. Around 40,000 years ago, people decided to hang out with dogs which were good at creating alarm calls, such as indicating when a stranger was outside. It turns out that these dogs — the ones with more fast-twitch fibers than slow-twitch ones — could also make the sweetest faces.
Then again, maybe those ancient humans felt a stronger connection to dogs that could look cute and the barking trait was a bonus. After all, looking into a dog’s eyes is known to release a hormone called oxytocin (ox-see-TOH-sin) in both humans and dogs. “It’s thought to be a love hormone, a bonding hormone,” Burrows says.
As for pet cats, they have similar facial muscles to dogs, but don’t usually use them to stare at us enviously. Instead, they are primarily used to control their whiskers, which helps them navigate their environment. These movements generally do not receive an emotional response from humans.
“I love my cats,” Burrows says, “but in a different way.”