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.
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 same researchers were involved a few years ago in the discovery that dogs have developed a muscle above their eyes that they use to make their eyes look larger and create that endearing “puppy dog eyes” expression.
That study found that the muscle was undeveloped in wolves, which suggests that “puppy dog eyes” is something dogs have evolved specifically to manipulate people.
Taken together, the muscle changes suggest 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, presenting preliminary findings of their research Tuesday at the Experimental Biology 2022 in Philadelphia.
They found that the muscles in dogs’ faces are 66 percent to 95 percent fast-twitch fibers, while wolves average about 25 percent.
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.”
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.”
Evolutionary biologist and animal behaviorist Marc Bekoff, professor emeritus at the University of Colorado, Boulder, cautions that the results of the study are preliminary and it may be that the facial muscles of dogs don’t turn out to make a lot of difference to their personalities.
Bekoff has worked with coyotes and wolves that were reared by humans after being born in the wild. While the adult animals were generally not as obedient as dogs, “hand-reared coyotes and wolves can communicate well with humans,” he said. “No one’s ever studied whether they are as good as dogs at communicating with us, but they are social animals.”
Burrows and Omstead note that the facial muscles of domesticated horses and cats don’t show the same changes from wild horses and wild cats, compared to the changes between dogs and wolves.
It’s also been proposed that dogs exhibit a form of “neoteny” — that is, they retain several features of juvenile wolves in adulthood, such as their less-aggressive characters, while wolves grow out of them, possibly because such traits were favored by humans during the dog domestication process.
Biological anthropologist Evan MacLean, the director of the University of Arizona’s Canine Cognition Center, said future research could study whether the proportion of fast- and slow-twitch facial muscle fibers varies with age in wolves, which might suggest that this, too, could be a result of neoteny.