A new study suggests that dogs’ facial expressions have evolved over tens of thousands to appeal to humans.
Compared to a dog’s ancestor—the wolf—the dog has a higher proportion of “fast-twitch” muscle fibers in its face. In the same study, it has been discovered that dogs have developed a muscle above their eye that helps them create the “puppy dog eyes” look. The muscle allows the dog to make its eyes look larger.
This muscle was never found in wolves, which implies that “puppy dog eyes” is one expression used to manipulate people. According to Anne Burrows, a professor of physical therapy at Duquesne University, all muscle changes suggest that dogs’ faces have evolved to create a connection with people.
“It’s quite a remarkable difference between dogs and wolves,” she said. “They just don’t move their faces in the same way.”
The preliminary findings from the study were presented at the Experimental Biology 2022 meeting in Philadelphia.
Burrows and her colleague and animal psychologist Kailey Omstead discovered that the muscles in dogs’ faces are 66-95% fast-twitch fibers. In wolves, however, they average 25% fast-twitch muscle fibers.
In general, every muscle contains millions of protein fibers called myosin. Each muscle is composed of fast-twitch fibers and slow-twitch fibers. Fast-twitch fibers contract quickly but fatigue slowly, while slow-twitch fibers are the exact opposite.
For instance, most muscle fibers in human faces are fast-twitch to express emotions quickly, while back muscles are slow-twitch to tolerate heavy loads.
The study done by Burrows and Omstead suggests that the fibers in a dog’s face are closer to a human’s rather than a wolf’s. It is also recommended that the reason for these recent changes in a dog’s features is domestication. The puppies who were most responsive to humans were the ones who eventually became domesticated, thus causing dogs’ faces to become “faster” over time.
The changes in muscles may have also been due to prehistoric people’s preference for a dog barking over howling. Barking requires fast-twitch fibers, while howling requires slow-twitch threads.
With all this being said, however, evolutionary biologist and animal behaviorist Marc Bekoff warns that the study results are still preliminary, and the facial muscles may not change their personality.
Bekoff has worked with wolves and coyotes and found that they can still communicate well with humans, but it is uncertain whether they can communicate as well as dogs.
Burrows and Omstead also note that the facial muscles of domesticated cats and horses are the same as their undomesticated counterparts.
It’s been proposed that dogs display a form of “neoteny.”
In this instance, dogs have the retainable characteristics of young wolves, such as no aggression, while wolves eventually do become aggressive. This difference made dogs a favorable choice for prehistoric people.