If you can’t resist those large, liquid puppy-dog eyes from your pooch begging for just about anything, it’s only natural and humans are the reason they have those eyes.
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A new study found humans are the culprit behind dogs having those irresistible eyes, according to a report from CNN. Humans bread those doleful eyes into today’s domesticated dogs starting around 33,000 years ago, according to the Duquesne University study.
“Dogs are unique from other mammals in their reciprocated bond with humans which can be demonstrated though mutual gaze, something we do not observe between humans and other domesticated mammals such as horses or cats,” senior author Anne Burrows, a professor in the department of physical therapy at Rangos School of Health Sciences at Duquesne University in Pittsburgh, said in a statement to CNN.
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“Throughout the domestication process, humans may have bred dogs selectively based on facial expressions that were similar to their own,” Burrows said.
Research found that dogs, when compared with their genetic cousin the wolf, have more “fast-twitch” facial muscles, Burrows told CNN. That allows dogs to more closely mimic our expressions, or at least look at us in ways that are irresistible to humans.
“Over time dog muscles could have evolved to become ‘faster,’ further benefiting communication between dogs and humans,” Burrows said.
One of the reasons helping this is the presence of fast-twitch fibers, which are more prevalent in domesticated dogs than wolves.
Fast-twitch fibers in humans are all throughout the body and help people make sudden, powerful movements such as erupting off a starting block in a foot race, CNN reports. But those muscles tire quickly and can’t keep high-intensity very long.
Tiny “mimetic” muscles help form facial expressions in mammals. In people, those muscles contain a good many fast-twitch fibers, which enable us to form facial expressions quickly and easily, CNN reports.
A study presented Tuesday at the American Association for Anatomy examined fibers in facial muscle samples from both wolves and domesticated dogs. The study found wolves had a lower percentage of fast-twitch fibers. 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, CNN reports.
“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, according to CNN. The study found dogs have a muscle called the levator anguli oculi medialis, which can raise their inner “eyebrow,” and makes the eye look larger and more infant-like.
“This eyebrow movement creates the ‘puppy-dog eyes’ expression, resembling facial expressions humans make when we are sad, making them irresistible and resulting in a nurturing response from humans,” co-author Madisen Omstead, laboratory manager for the Rangos School of Health Sciences department of physical therapy said to CNN.
Another muscle, called the retractor anguli oculi lateralis muscle, pulls the outer corners of the eyelids toward the ears, which 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 the rule is the Siberian husky, which is more closely related to wolves than many other breeds, CNN reports, citing researchers.
Researchers also found that looking into the eyes of domesticated dogs appears to create an “oxytocin feedback loop” between humans and our dogs, which is similar to the one that exists between human mothers and their infants.
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