Why Puppy Dog Eyes Are Irresistible To People

What is it that makes puppy dog eyes so irresistible to humans? It may have something to do with the muscles they use to form facial expressions, a new study has found.

Dog owners are all too familiar with the puppy dog eyes that can make them give another treat to their pets or perhaps even dissipate a bit of their anger if their dog has done something quite naughty. In a new study, presented at the American Association for Anatomy's Annual Meeting at Experimental Biology, a team of researchers had a closer look at what makes dogs' faces so endearing to people compared to wolf faces.

For their work, the researchers focused on mimetic muscles, or the small muscles that are used to form facial expressions, Experimental Biology noted in a news release. The mimetic muscles in human beings are "dominated" by "fast-twitch" myosin fibers.

These fibers can contract fast, allowing people to have "greater facial mobility" and make fast and small movements such as raising an eyebrow. However, they also get tired faster, which is why people can't quite hold facial expressions for long. On the other hand, slow-twitch fibers don't get tired as quickly, so they're better for "long, controlled movements," the researchers explained.

The team compared these muscle fibers in dogs and wolves, two species that are actually related but have diverged genetically when humans began breeding wolves. They found that both dogs and wolves have fast-twitch myosin fibers, but wolves have more slow-twitch fibers. As such, dogs are more similar to humans in terms of facial expressions than wolves.

"These differences suggest that having faster muscle fibers contributes to a dog's ability to communicate effectively with people," said study senior author and presenter Anne Burrows, of the Duquesne University in Pittsburgh. "Throughout the domestication process, humans may have bred dogs selectively based on facial expressions that were similar to their own, and over time dog muscles could have evolved to become 'faster,' further benefiting communication between dogs and humans."

The researchers noted that further studies are needed to confirm the findings and understand more about dog-human interactions. In 2019, researchers found that dogs' facial anatomy had evolved over thousands of years to help them communicate with humans better. In fact, they have a small muscle above the eye that lets them raise their eyebrows, triggering a "nurture response" in humans because it makes them look more infant-like. Wolves don't have this muscle.

"Dogs are unique from other mammals in their reciprocated bond with humans which can be demonstrated through mutual gaze, something we do not observe between humans and other domesticated mammals such as horses or cats," Burrows said. "Our preliminary findings provide a deeper understanding of the role facial expressions play in dog-human interactions and communication."