By Tom
Metcalfe
Dogs' faces have evolved over tens of thousands of years to make them more attractive to humans, unlike the wild wolves from which they descend, a new study suggests.
Research shows that dogs' facial muscles have a much higher proportion of "fast-twitch" muscle fibers than those of wolves, and scientists believe this allows dogs to more effectively communicate their feelings to their owners.
The same researchers discovered years ago that dogs have developed a muscle above their eyes that they use to make them larger and create that endearing
“puppy dog eye” expression.
That study found that the muscle was underdeveloped in wolves, suggesting that "puppy eyes" are something dogs have developed specifically to
manipulate people.
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Taken together, the muscle changes suggest that dogs' faces have evolved anatomically to enhance their connections to people, according to biological anthropologist Anne Burrows, a professor of physical therapy at Duquesne University in Pittsburgh and leader of the project.
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“It's a pretty remarkable difference between dogs and wolves.
They just don't move their faces in the same way,” she explained.
Burrows and animal physiologist Kailey Omstead, a colleague of Duquesne's, presented the preliminary results of their research Tuesday at the Experimental Biology 2022 meeting in Philadelphia.
They found that dogs' facial muscles are between 66 and 95% fast-twitch fibers, while wolves have an average of 25%.
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The muscles of all mammals, including humans and dogs, are made up of millions of fibers of a protein called myosin.
Every muscle has a mix of fast-twitch fibers, which contract quickly but tire quickly, and slow-twitch fibers, which contract more slowly but don't tire as quickly.
Fast-twitch fibers predominate in the muscles of the human face, so we can express our thoughts on the face in an instant, but not for long.
The back muscles, however, are dominated by slow-twitch fibers that tolerate loads longer.
"If you lift 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.
The muscles of the face get tired, because fast-twitch fibers predominate in the face,” he explained.
Burroughs and Omstead's research suggests that the high proportion of fast-twitch fibers in the faces of dogs is closer to that of humans than that of wolves.
Burrows believes this could be a consequence of the
domestication process of dogs
by selecting for puppies that seemed more receptive to humans, which resulted in dogs' faces becoming "quicker" over time.
"When the Upper Palaeolithic peoples of Europe and Asia domesticated the first dogs around 40,000 years ago, they seem to have selected dogs with faces that moved very quickly," he explains.
Dogs' facial muscles may also have changed because prehistoric peoples preferred dogs that barked—an action that uses fast-twitch muscle fibers—rather than dogs that howled like wolves, which rely on slow-twitch fibers.
Barking dogs may be better at warning of danger than howling ones, he suggests: "They were selecting against that howling behavior, and selecting for these new dogs that made this new sound, this barking."
Evolutionary biologist and animal behaviorist Marc Bekoff, a professor emeritus at the University of Colorado, Boulder, cautions that the study results are preliminary and it may be that dogs' facial muscles won't turn out to be a big difference in their personalities.
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Bekoff has worked with coyotes and wolves raised by humans after being born in the wild.
Although adult animals used to not be as obedient as dogs, "hand-raised coyotes and wolves can communicate well with humans," she says.
“No one has studied whether they are as good as dogs at communicating with us, but they are social animals,” she added.
Burrows and Omstead note that the facial muscles of domesticated horses and cats do not show the same changes as those of wild horses and cats, compared to the changes between dogs and wolves.
It has also been proposed that dogs exhibit a form of "neoteny", that is, they retain various traits of juvenile wolves in adulthood, such as their less aggressive character, while wolves leave them behind, possibly because those traits they were favored by humans during the dog domestication process.
Biological anthropologist Evan MacLean, director of the University of Arizona Center for Canine Cognition, said future research could study whether the ratio of fast-twitch and slow-twitch facial muscle fibers varies with age in wolves, which could suggest that this could also be a result of neoteny.
