Study Reveals Science Behind Those Irresistible Puppy Eyes

A wild gray wolf (left) and a domesticated Bernese mountain dog (right), highlighting some common facial differences between the wolf and domestic dogs. Most domestic dog breeds have ears that lie flat, display a range of fur patterns and colors, and have shortened muzzles. Red arrows point to the levator anguli occuli medialis muscle, a muscle not found in gray wolves that supports ocular communication between dogs and humans. Credit: Anne Burrows, Duquesne University; Image copyright left Defenders of Wildlife, Washington, DC.

Can’t resist your pup’s adorable expression when he asks for a treat? A new study reveals key anatomical features that could explain what makes dogs’ faces so attractive. The findings also suggest that humans have contributed to dogs’ ability to form facial expressions over thousands of years of selective breeding.

“Dogs are unique from other mammals in their reciprocal bond with humans which can be demonstrated by mutual gaze, something we do not observe between humans and other domesticated mammals such as horses or cats,” said Anne Burrows, Ph.D., professor. in the physical therapy department of the Rangos School of Health Sciences at Duquesne University in Pittsburgh, the study’s senior author. “Our preliminary results provide insight into the role that facial expressions play in dog-human interactions and communication.”

Burrows will present the research at the annual meeting of the American Association for Anatomy at the Experimental Biology (EB) 2022 meeting, held in Philadelphia April 2-5. The research team also included Madisen Omstead, laboratory manager in the Department of Physical Therapy at the Rangos School of Health Sciences.

Dogs and wolves are closely related. Although the exact timing is unclear, scientists believe the two species diverged genetically around 33,000 years ago when humans began selectively breeding wolves, the first species to be domesticated.







A barking dog. Fast-twitch muscle fibers are important for fast contractions such as those used for barking. In contrast, slow-twitch fibers are important for long, controlled movements such as those used in howling (as shown in the video at https://www.youtube.com/watch?v=KhQGo2IdA9s, for example). Credit: Anne Burrows, Duquesne University

The new study focuses on the anatomy of tiny muscles used to form facial expressions, called mimetic muscles. In humans, these muscles are dominated by “fast-twitch” myosin fibers that contract quickly but also tire quickly, which is why we can form facial expressions quickly but not hold them for long. Muscle cells with more “slow twitch” fibers are more efficient at long, controlled movements and don’t tire as quickly.

For the study, the researchers compared myosin fibers in facial muscle samples from wolves and domestic dogs. The results revealed that, like humans, dogs and wolves have facial muscles dominated by fast-twitch fibers, but wolves have a higher percentage of slow-twitch fibers compared to dogs.

“These differences suggest that having faster muscle fibers contributes to a dog’s ability to communicate effectively with people,” Burrows said. “Throughout the process of domestication, humans may have selectively bred dogs based on facial expressions similar to their own, and over time dogs’ muscles may have evolved to become ‘faster’. , which further benefits the communication between dogs and humans.”

Faces of a wild gray wolf, a Golden Retriever domestic dog, and a human, along with orbicularis oris muscle tissue samples for each species. In the photos, the dog and the human are actively using the zygomatic and orbicularis muscles (note the upturned lip of the dog, which mimics a smile). The stained muscle samples reveal similarities in muscle content between dogs and people that likely contribute to their facial flexibility compared to wolves. Credit: Anne Burrows, Duquesne University, images from top copyright iStock.

Having more fast-twitch fibers allows for greater facial mobility and faster muscle movements, allowing for small movements such as a raised eyebrow and the short, powerful muscle contractions involved in barking. Slow-twitch fibers, on the other hand, are important for prolonged muscle movements such as those wolves use when they howl.

In previous research, the team found that dogs have an additional mimetic muscle that’s absent in wolves that contributes to the expression “puppy eye.” The scientists note that further research is needed to confirm their new findings with antibody stains to differentiate other types of myosin fibers, which could shed new light on the anatomical differences between dogs and wolves.


The evolution of puppy eyes


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2022 Annual Meeting at Experimental Biology

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