The Science Behind Those Big Ol’ Puppy-Dog Eyes

Our canine friends evolved extra muscle fibers around their eyes and mouths that allow them to make facial expressions humans find adorable

Image of a dog laying down looking up with red bandana
The muscles that allow for the "puppy-dog eyes" in domestic dogs is undeveloped in wolves, suggesting that the adorable look evolved to captivate humans. (Pictured: The author's dog, Smoky.) Courtesy of Elizabeth Gamillo

When a canine companion wants an extra treat, one glance of those endearing puppy-dog eyes is all it takes. Now, scientists suggest domesticated dogs may have evolved extra facial muscles to win people over with their adorable expressions.

Humans may have contributed to the heart-warming look through thousands of years of selective breeding for the animated faces, a statement explains. Presented at the Experimental Biology 2022 Meeting in Philadelphia, the preliminary research offers a deeper look at how dogs communicate with us.

"Dogs are really unique from any other domesticated animals in that they reciprocate a bond with their humans. They truly are our companions," says study author Madisen Omstead, a biologist at Duquesne University in Pittsburgh, to Chen Ly at New Scientist. "They demonstrate this through their mutual gaze – that 'puppy-dog eye' look that they give us."

In humans, tiny muscles around our eyes and mouths are responsible for small, quick facial expressions like raising an eyebrow, reports Robyn White for Newsweek. Our so-called mimetic muscles are powered by fast-twitch myosin fibers that tire quickly, which is why we can can't hold them these expressions for very long, a statement explains. Other muscles contain slow-twitch myosin fibers used for long, controlled movements.

For the experiment, the research team quantified how many fast-twitch and slow-twitch fibers wolves and dogs in their mimetic muscles, New Scientist reports. Fast-twitch fibers made up 66 to 95 percent fast-twitch fibers, whereas wolves average 25 percent, reports Tom Metcalfe for NBC News. The wolves' facials muscles were dominated by slow-twitch muscles, which they might use for extended movements like howling.

Like humans, dogs have a higher percentage of fast-twitch fibers in their facial muscles, Newsweek reports. As dogs branched off from their wolf ancestors 33,000 years ago, the need for slow-twitch muscles may have decreased, and their facial expressions became more captivating and familiar to people.

The fast-twitching muscles around dogs' mouths may have evolved to produce the sharp and snappy barks pets use to communicate with their humans today, reports Anna Salleh for Australia's ABC News. Dogs may bark at their humans to be playful, get our attention, protect their territory, or warn us.

"It was part of the domestication process somehow — whether humans chose dogs consciously that were barking, or whether it was a by-product of domestication," says study author Anne Burrows, a biological anthropologist, to ABC News.

Scientists plan on further looking into how barking developed in dogs and why humans may have selected this trait during the domestication process. The team also plans to investigate if domestication shaped the mimetic muscles of other mammals, Burrows tells Newsweek.