Scientists have figured out one of the big reasons that dogs are — in scientific terms — impossibly cute.
Researchers compared the facial muscles in dogs to those in wolves. They found that domesticated dogs have far more "fast-twitch" muscles than their close canine cousins. These muscle fibers contract quickly, allowing dogs a greater degree of short-term control over their facial expression.
That comes in handy when a pooch wants to ask for a morsel of food but, sadly, doesn't have the words.
Co-author Anne Burrows says the findings "suggest that having faster muscle fibers contributes to a dog’s ability to communicate effectively with people." The researchers presented their findings Tuesday at the Experimental Biology 2022 meeting in Philladelphia.
Humans and dogs really do have a special connection
Humans and dogs go way, way back. Researchers aren't sure when humans first started breeding especially friendly wolves, but it was probably more than 30,000 years ago. Researchers in Siberia recently found the 18,000-year-old remains of a puppy that might be the earliest dog ever uncovered (though it could also be a wolf).
Whenever their shared history began, humans and dogs have grown a bond sets the pair apart.
"Dogs are unique from other mammals in their reciprocated bond with humans," Burrows says. That relationship "can be demonstrated though mutual gaze, something we do not observe between humans and other domesticated mammals such as horses or cats."
In other words, dogs and humans can easily capture the other's attention — and find deep meaning — just from their facial expressions. That kind of communication across species is incredibly rare.
Humans changed canine facial muscles to more closely match our own
For this new study, the researchers compared the ratio fast-twitch and slow-twitch muscle fibers in certain facial muscles. Fast-twitch muscle fibers can contract quickly, but it doesn't take long for them to grow tired. In humans, sprinters tend to have more fast-twitch muscles in their legs. Slow-twitch muscles aren't as responsive, but they're more efficient and don't tire nearly as quickly. These are more common in marathon runners.
They analyzed fibers from two muscles (the orbicularis oris muscle and the zygomaticus major muscle) in gray wolves and several breeds of dogs.
The results were stunning. Muscles from the dogs' faces contained between 66 percent and 95 percent fast-twitch fibers. For wolves, that number was just 25 percent. Conversely, dogs had just 10 percent slow-twitch fibers in their faces while wolves averaged 29 percent.
Burrows says the findings say a lot about what humans desired in dogs.
“Throughout the domestication process, humans may have bred dogs selectively based on facial expressions that were similar to their own," she says. "[O]ver time dog muscles could have evolved to become ‘faster,’ further benefiting communication between dogs and humans.”
These results are another piece in the puzzle
This research isn't the first evidence that explains how dogs became so darn cute.
A few years ago, Burrows and other researchers reported evidence that dogs have a special muscle that enables them to raise their eyebrows in that special puppy dog sort way. They also compared the behavior of dogs and wolves, finding that dogs raise their eyebrows more often and far more intensely than wolves do.
Those facial movements make adult dogs look more like puppies and resemble "an expression that humans produce when sad," the authors write. "We hypothesize that dogs with expressive eyebrows had a selection advantage and that 'puppy dog eyes' are the result of selection based on humans’ preferences," they said.
As for the dogs, they're still waiting on that morsel of food.