Are you confident your dog is smiling at you during an especially good scritch-scratch? You could be right. And it all has to do with how dogs evolved from wild wolves to faithful human companion.
A recent study done at Duquesne University in Pittsburgh looked into how the faces of dogs have evolved over thousands of years to become what we perceive as much cuter than their ancestors. The lure of puppy dog eyes is a real thing.
The research was presented at the Experimental Biology 2022 meeting in Philadelphia on Tuesday, as biological anthropologist Anne Burrows and animal physiologist Kailey Omstead explained what makes dogs' faces more attractive to humans.
Dogs' facial muscles have more fast-twitch muscle fibers than wolves, allowing them to have many more facial expressions when they try to communicate to owners and other animals.
The fast-twitch fibers in dogs' faces account for 65% to 95% of the muscles in their faces, compared to just about 25% in wolves.
"Humans domesticated dogs with attention to the facial expressions that dogs produce, selecting for a suite of facial movement," the study said. "Domestic dogs and humans are adept at accurately understanding one another’s facial expressions and movement around the eyes in dogs is highly valued by humans. This unique, mutual ability to accurately process facial expressions is part of the dog-human bond."
Burrows was also involved in a 2019 study that indicated that "puppy dog eyes" is a feature that dogs have over developed over time to use their facial expressions to convey feelings to humans.
"It’s quite a remarkable difference between dogs and wolves," Burrows told NBC News. "They just don’t move their faces in the same way."
Human faces have tons of fast-twitch muscle fibers, allowing people to make facial expressions at any given moment -- like when you stub your toe and shriek in pain. Burrows went on to explain how holding a facial expression for a long period of time isn't very possible, while humans' back muscles have slow-twitch fibers that allow for people to hold or lift items for a longer period of time.
"If you pick up a 10-pound weight, you can hold it for a full minute," Burrows said. "But if you try to hold a smile in the mirror for a full minute, you can’t do it. Your face muscles get tired, because your face is dominated by fast-twitch fibers."
Wolves in comparison to dogs are much stronger, and would be able to carry a heavier object for longer. The study backed that up by saying wolves averaged 29% slow-twitch fibers, compared to just 10% in dogs.
"The dog range of type II fibers mirrors the range found in human facial muscles, indicating that the domestication process in dogs also involved selection for rapid facial movement, similar to the movement seen in human faces," the study said.