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EvolutionEvolution

Canines evolved puppy dog eyes to woo human companions

Wolves lack the facial muscles required to raise their eyebrows—a feature that makes dogs especially endearing to people.

ByKatherine J. WuNOVA NextNOVA Next
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Just try to say no to these big brown eyes. Image Credit: st0l1, flickr

You know the look. Everyone does. The wide, winsome eyes; the forlorn, furrowed brow. It’s enough to melt your heart—and send your hand reaching for the treat bag.

But the next time your dog fixes you with that sorrowful stare, take comfort (or chagrin) in the knowledge that she—and the rest of the world’s pet pooches—could owe their expressive eyes entirely to us.

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According to new research published today in the journal PNAS, the allure of puppy dog eyes may be a product of domestication, acquired as a lineage of charismatic canines split off from wolves. While their wild ancestors lack the necessary facial anatomy to raise their inner eyebrows, dogs possess certain muscles that, when put to use, make their mugs resemble ours—something that may have piqued the interest of ancient peoples.

These brow-raising results hint that, on the road to becoming our closest companions, dogs may have evolved a way to hijack, or at least mimic, a core tenet of human communication: eye contact.

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It's hard to stay mad at a face like this. Image Credit: RalchevDesign, iStock

“Eye contact is central to human communicative skills,” says Aleah Bowie, a conservation psychologist at Duke University who was not involved in the study. “So it makes sense if we’re selecting for non-human animals [like dogs], we’re going to look for the same kind of traits.”

In other words, the power of puppy dog eyes is rooted not so subtly in a bit of egocentrism: Wide, expressive eyes remind us of human babies, or a person on the verge of tears—emotive extremes of ourselves.

And dogs, it seems, have reaped the benefits of this facial familiarity. There’s even evidence to suggest mutts who raise their inner eyebrows more often even get adopted more quickly from shelters.

Of course, shelters didn’t exist tens of thousands of years ago, when people first cozied up with a subset of unusually friendly wolves. But that doesn’t mean a similar process wasn’t happening in the early days of domestication: In gravitating toward friendlier canines, humans might have inadvertently reshaped an entire lineage, altering traits both physical and psychological.

If that’s the case, then wolves might be expected to show less puppy-eyed prowess—or even lack some of the anatomical accoutrements necessary to maneuver their brows.

To put this theory to the test, a team of researchers led by Juliane Kaminski and Bridget Waller, both comparative psychologists at the University of Portsmouth in the United Kingdom, compared the facial muscles of six modern mutts and four gray wolves—a stand-in for domestic dogs’ long-gone wild ancestor. Though the two species’ faces were almost identical, almost all dogs tested possessed two extra muscles around their eyes that helped them raise their brows and pull their eyelids in certain directions—both movements that are, scientifically speaking, adorable.

“To see this kind of soft tissue change in the timespan of [dog domestication] is remarkable,” Waller says. The wolves, however, weren’t a “completely blank slate,” she adds. While the wolves didn’t have fully developed muscles, a few sparse muscle fibers existed in their place—just enough, she says, for evolution to “grab hold and make bigger, more uniform, and more stable in the dogs.”

Soft tissue like muscle doesn’t fossilize well, and a face-off between two modern species is just an approximation for gazing backwards in time. But already, the study presents a tantalizing hint of how these muscles may have beefed up over time. Only one dog breed studied lacked one of the two critical eye muscles: the Siberian Husky, which is more closely related to wolves than most other dog breeds. “This is suggestive of [an intermediate stage], but we need to follow up on this with other breeds,” Waller says.

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Most dogs appear to possess two muscles that wolves lack: the levator anguli oculi medialis muscle (LAOM) and the retractor anguli oculi lateralis muscle (RAOL). The pair help dogs maneuver their eyebrows and eyelids. Image Credit: Juliane Kaminski et al., PNAS, 2019

To complement their anatomical findings, the researchers next filmed 27 shelter dogs and nine captive wolves for two minutes after a stranger approached them. As expected, dogs came out ahead in this face-off as well: On average, they raised their inner eyebrows more often, and seemed capable of producing more exaggerated movements with their eyes than their wolf cousins—findings in keeping with the team’s dissection.

“It’s really exciting to see a study exploring the connection between anatomy and behavior,” says Gitanjali Gnanadesikan, a cognitive biologist studying dog domestication at the University of Arizona who was not involved in the study. “In the process of domestication, both have changed in remarkable ways...and many of these changes are intertwined. Trying to understand how these different pieces fit together is extremely important.”

As eye-opening as the findings are, however, Gnanadesikan cautions that they raise “just as many questions as they answer...we still don’t know if these gazes contain information, or if dogs are intentionally trying to communicate with them.”

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Wolves can twitch their brows, but those subtle movements are nothing compared to the heartwrenching stares of dogs. Image Credit: Nastasic, iStock

Figuring that out, she says, will require more studies on how these heart-wrenching stares are used in different contexts. Dogged gazes may tug on our heartstrings, but it’s important not to attribute too much to our canine comrades.

Even so, the study underscores the importance of facial expressions in communication, Waller says. That seems obvious. But it’s nonetheless notable, she says, just how much humans’ penchant for emotion has shaped our interactions—with both our own species and others.

It may not have been intentional on our part, or theirs. Either way, these pooches’ expressive eyes continue to get them out of the doghouse. And for anyone who’s ever fed Fido scraps under the table, or forgiven the massacre of an unlucky shoe, it’s clear that these looks of longing have yet to lose their luster.

You sly dog, you.

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