Dogs have a secret weapon when it comes to getting what they want or getting out of trouble: puppy dog eyes. When our canine companions raise their eyebrows, making their eyes look wider, more helpless and baby-like, it seems the facial expression was designed to manipulate human emotions. And it turns out, that’s likely true, according to a new study in the journal Proceedings of the National Academy of Sciences.
In previous studies, researchers had already identified the muscle movement that raises the dogs’ inner eyebrow and produces those adorable droopy eyes. “This movement makes a dogs’ eyes appear larger, giving them a childlike appearance,” co-author Bridget Waller, a psychologist at the University of Portsmouth, says in a press release. “It could also mimic the facial movement humans make when they’re sad.”
Dogs likely evolved these special muscles after the breed split with wolves, the closest living wild relatives of domestic canines, and started cozying up to our human ancestors. That’s why researchers decided to look more closely at the facial anatomy of dogs and wolves to better understand the origins of the eyebrow lift. Ian Sample at The Guardian reports that the team analyzed the facial structures of both wolf and dog cadavers from taxidermists, a state wildlife organization and several museum specimen. (No dogs were killed for this research.) They also conducted behavioral studies of wolves at wildlife parks and dogs in rescue shelters in Germany and the United Kingdom.
They found that the dog breeds—including a chihuahua, a labrador, a bloodhound, a German shepherd, a Siberian husky and a mutt—all had a small, specialized muscle called the levator anguli oculi medialis, or LAOM, which they use to produce their wide-eyed, doleful stare. In the four wolves that were looked at, the muscle did not exist—save for a few stray fibers. Another muscle, called the retractor anguli oculi lateralis or RAOM, which pulls the eyelids toward the ears, was well developed in all the dogs except the husky, a breed with an ancient lineage. It was also less prevalent in the wolves.
The researchers suggest that in the 20,000 years or so since humans and dogs began hanging out, evolutionary pressures have caused the LAOM muscle to develop in canines in order to communicate with their human companions. “They are very powerful animals in how they capture our hearts,” Waller tells The Guardian’s Sample. “We pay a lot of attention to faces, they are meaningful to us, and this expression makes dogs look juvenile and sad. It induces a nurturing response. It’s a cute factor.”
In the behavioral portion of the study, the researchers also had a team of experts observed humans interact with the shelter dogs as well as the wolves in captivity. The team recorded the number of times the dogs and wolves made the puppy-dog eye expression, scoring the intensity on a scale of 1 to 5. While wolves occasionally made the expression with “low intensity,” dogs made it much more frequently and with more intensity, suggesting it's an important part of human-dog interaction.
It’s unlikely that doggos developed the puppy-dog eye routine on purpose. Instead, humans probably bred the look into their four-legged companions. “The findings suggest that expressive eyebrows in dogs may be a result of humans unconscious preferences that influenced selection during domestication,” lead author Juliane Kaminski, also of the University of Portsmouth, says in the release. “When dogs make the movement, it seems to elicit a strong desire in humans to look after them. This would give dogs, that move their eyebrows more, a selection advantage over others and reinforce the ‘puppy dog eyes’ trait for future generations.”
That’s not just idle speculation. In a 2013 study, Kaminski and her team found that dogs with big, droopy puppy eyes were adopted more quickly from animal shelters. Haley Weiss at The Atlantic reports that in a 2017 study, Kaminski found that dogs moved their eyebrows more when humans looked at them and kept the brow-raising to minimum when they were ignored or denied food, suggesting that some of the puppy-eyeing is voluntary.
The development of the LAOM muscle is surprising because it actually developed quite rapidly in the long scale of evolution. “These muscles are so thin that you can literally see through them—and yet the movement that they allow seems to have such a powerful effect that it appears to have been under substantial evolutionary pressure,” co-author and anatomist Adam Hartstone-Rose of North Carolina State University says in the press release. “It is really remarkable that these simple differences in facial expression may have helped define the relationship between early dogs and humans.”
There’s also an alternative explanation. The muscles might help expose the whites of a dogs eyes, which is appealing to humans, the only creatures whose eye whites are constantly exposed. Whatever the case, it’s likely that the LAOM muscle makes dogs look more human and more expressive, which makes us feel more bonded with the furry beasts—even if those wide, doleful eyes are just an evolutionary trick.