The differences in facial musculature between wolves and dogs suggest that facial expressions were taken into account in their domestication.
A puppy’s adorable expression as he begs for a toy or treat can be irresistible. Can you say no to that little face? A new study reveals key anatomical features that could explain what makes dog faces so attractive. And how can we not like them, if we encourage them? The findings, released today at the Experimental Biology annual scientific meeting, also suggest that humans contributed to canines’ ability to form facial expressions over thousands of years of selective breeding.
“Dogs are unique in their reciprocal bond with humans by mutual gaze, something we don’t see between humans and other domesticated mammals, such as horses or cats,” says lead author Anne Burrows of Duquesne University in Pittsburgh.
“Our preliminary findings provide a deeper understanding of the role facial expressions play in dog-human interactions and communication.”
Dogs and wolves are closely related. Although the exact timing is unclear, scientists estimate that the two species diverged genetically about 33,000 years ago when humans began selectively breeding wolves, the first species to be domesticated.
The new study focuses on the anatomy of the small 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 explains why we can form facial expressions in an instant but not maintain them for very long. Muscle cells with more ‘slow twitch’ fibers are more efficient for long, controlled movements and don’t tire as quickly.
For the study, the researchers compared myosin fibers in facial muscle samples from wolves and domesticated dogs. The results revealed that, like humans, both dogs and wolves have facial muscles dominated by fast-twitch fibers, but wolves have a higher percentage of slow-twitch fibers relative to dogs.
“These differences suggest that having faster muscle fibers contributes to a dog’s ability to communicate effectively with people,” says Burrows. “Throughout the domestication process, humans may have selectively bred dogs based on facial expressions that were similar to their own, and over time, dogs’ muscles may have evolved to become ‘faster.’, further benefiting dog-human communication.”
Having more fast-twitch fibers allows for greater facial mobility and faster muscle movement, allowing for small movements like raising an eyebrow and the short, powerful muscle contractions involved in barking. Slow-twitch fibers, on the other hand, are important for extended muscle movements, such as those used by wolves when howling.
In previous research, the team found that dogs have an additional mimetic muscle that is absent in wolves and contributes to the ‘puppy eye’ expression. The scientists note that further research is needed to confirm their new findings with suitable antibody stains to differentiate additional types of myosin fibers, which could shed new light on anatomical differences between dogs and wolves. And understand a little better why our pets have conquered us.
