When a dog looks at you with puppy eyes and a crinkled brow, chances are they will get exactly what they want, whether that be food, a walk or simple attention. And who can resist that look?
But dogs didn’t come up with this cunning little trick to manipulate us on their own, or was it by pure chance that they look adorable. Domesticated dogs possess a higher ratio of faster facial muscles than their wild ancestor, the wolf, according to an 18-month study, whose preliminary results were presented at the annual Experimental Biology 2022.
Dogs instead have a muscle composition that is closer to humans. Through domestication, these distinct features have evolved.
The Science Behind Puppy Eyes
Humans contributed to the development of a more human-like anatomy and features in dogs through domestication and the breeding of new dog species.
“We think humans selected dogs to have a lot of fast-twitch fibers in their mimetic muscles so they could communicate with us using facial expression,” says Dr. Anne Burrows, senior author of the study and professor in the department of physical therapy at Rangos School of Health Sciences at Duquesne University in Pittsburgh, Pa.
“Humans have about 70 to 75 percent of their mimetic muscles dominated by fast-twitch fibers. In our current study, we found that dogs also have more fast-twitch muscle fibers in their mimetic muscles than wolves,” says Burrows.
Mimetic muscles in humans control facial expression and fast-twitch fibers dominate those muscles. These result in readable facial expressions. We also have around a quarter of slow-twitch fibers that hold a smile for any length of time.
“Fast twitch muscle fibers contract quickly, but fatigue quickly. Slow-twitch fibers contract slowly but are fatigue-resistant. If you're a marathon runner, most of your lower limb muscles are probably dominated by slow-twitch fibers, whereas if you are a sprinter, your lower limb muscles are probably dominated by fast-twitch fibers,” explains Burrows.
Those puppy eyes indeed came from humans rather than natural selection, according to past research published in the Proceedings of the National Academy of Sciences.
“Our initial study located a muscle called the levator anguli oculi medialis muscle in dogs, but not in wolves. The current study is aimed at understanding how mimetic muscles contract in dogs versus in wolves,” says Burrows.
The study in progress will further research microanatomy to assess the range of motion in dog faces versus wolf faces.
That Special Bond
Did we then breed dogs to communicate more as humans than as wolves?
“We generally think that these behaviors were unconsciously selected for by humans during the dog domestication process. We humans have selected dog behaviors that simply pleased us,” Burrows says.
Breeding alone can’t explain the bond between human and dog. Brain activity from the mutual gaze between humans and their personal pets is similar to human mothers and their children, according to research published in 2015 in Science.
Several studies also show that both dogs and their owners release oxytocin into the bloodstream right after engaging in a mutual eye gaze. This is the same hormone that is released by a mother when she gazes into the eyes of her infant.
Yes, humans may have selected – intentionally or unintentionally – puppy eyes and crinkled brows as an adorable feature when breeding domesticated dogs. And dogs now communicate with their owners through facial expressions, sometimes to manipulate us. But there surely is no denying the special bond between us.