Dogs have more “fast-twitch” muscle fibres around their eyes and mouths than wolves do, which allows them to make more facial expressions, likely to communicate with humans better.
The faces of dogs have evolved over tens of thousands of years to make them more appealing to humans, unlike the wild wolves they descended from, a new study suggests.
Dogs make faces similar humans
The research shows that the facial muscles of dogs have a much higher proportion of “fast-twitch” muscle fibers than wolves, and scientists think this lets dogs more effectively communicate their feelings to their owners. The study, presented Tuesday at the American Association for Anatomy annual meeting in Philadelphia, examined fibers in facial muscle samples from both wolves and domesticated dogs.
Dogs’ faces have evolved anatomically to improve their connections with people, said biological anthropologist Anne Burrows, a professor of physical therapy at Duquesne University in Pittsburgh and the leader of the project.
“It’s quite a remarkable difference between dogs and wolves,” she said. “They just don’t move their faces in the same way.”
Burrows and animal physiologist Kailey Omstead, a colleague at Duquesne, presented preliminary findings of their research Tuesday April 5th, 20203 at the Experimental Biology 2022 meeting in Philadelphia.
Fast vs. slow twitch fibers
The muscles of all mammals, humans and dogs included, are made of millions of fibers of a protein called myosin. Each muscle has a mix of fast-twitch fibers that contract quickly but are fast to fatigue, and slow-twitch fibers that are slower to contract but don’t tire as fast.
The muscles in human faces are dominated by fast-twitch fibers, so we can express thoughts on our faces in an instant, but not for long. The muscles in our backs, however, are dominated by slow-twitch fibers that tolerate loads for longer.
“If you pick up a 10-pound weight, you can hold it for a full minute,” Burrows said. “But if you try to hold a smile in the mirror for a full minute, you can’t do it. Your face muscles get tired, because your face is dominated by fast-twitch fibers.”
Results showed wolves have a lower percentage of fast- verus slow-twitch fibers compared with today’s domesticated dogs. Having slow-twitch muscles around the eyes and face would be helpful to wolves as they howl, the researchers said, while having more fast-twitch muscles would help dogs get their owners’ attention with short, fast barks and more varied expressions.
“These differences suggest that having faster muscle fibers contributes to a dog’s ability to communicate effectively with people,” Burrows said.
Wolves also lack another ability that most dogs have, according to a previous 2019 study by Burrows and her team. That study found dogs have a muscle called the levator anguli oculi medialis, which can raise their inner “eyebrow,” thus making the eye look larger and more infantlike.
Puppy Dog Eyes
Another muscle, called the retractor anguli oculi lateralis muscle, pulls the outer corners of the eyelids toward the ears, in effect producing what humans would call an “eye smile.” The 2019 study found that while wolves had a bit of this muscle fiber, most domesticated dogs had a more fully developed muscle and used it frequently.
The exception to that rule is the Siberian husky, which is more closely related to wolves than many other breeds, the researchers said.
If muscles that allow your dog to smile and look sweet aren’t enough, looking into the eyes of our “best friends” also appears to trigger an “oxytocin feedback loop” between humans and our dogs – much like the one that exists between human mothers and their infants, according to researchers.
Evolving to communicate
The research by Burroughs and Omstead suggests that the high proportion of fast-twitch fibers in the faces of dogs is now closer to that of humans than that of wolves.
Burrows said this could be a consequence of the process of domesticating dogs by selecting puppies that seemed most responsive to humans, resulting in dog faces becoming “faster” over time.
“When Upper Paleolithic people across Europe and Asia were domesticating the first dogs about 40,000 years ago, they seem to have selected dogs that had faces that moved very quickly,” she said.
Dogs’ facial muscles may also have changed because prehistoric people preferred dogs that barked — an action that uses fast-twitch muscle fibers — rather than dogs that howled like wolves, which relies on slow-twitch fibers.
Barking dogs may have been better at alerting danger than howling dogs, she suggests: “They were selecting against that howling behavior, and selecting for these new dogs that made this new sound, this bark.”