A study of facial muscles in dogs and wolves has revealed key anatomical differences that are likely a result of human selective breeding.
Scientists say that the 33,000-year relationship between humans and canines has relied on a reciprocal bond between the two species, which developed through mutual gazing and dogs' ability to 'communicate' with facial expressions similar to ones made by humans.
It's likely that humans selectively bred dogs based on their effectiveness in communicating this way, which may have led to our furry friends' facial muscles evolving to become faster and more responsive.
The new research compared dogs with wolves and focused on specific muscles called mimetic muscles. These are muscles found in mammals that connect to nerves in the face, and they help us communicate our many emotions – pulling our eyebrows into a frown, or our lips into a smile. The facial nerve is often affected by stroke, leading to paralysis of the mimetic muscles.
In humans, the mimetic muscles are made of fibres that enable us to form facial expressions almost instantaneously, said biological anthropologist Dr Anne Burrows, the study's senior author. But while these 'fast-twitch' muscle cells help us to flash a smile the moment someone holds up a camera, they also fatigue quickly, making school picture day a cheek-aching experience.
Mimetic muscle cells with slow-twitch fibres are not as quick to react, but they are better at controlling and maintaining a position.
As facial expressions help us regulate our social interaction and bonds with dogs, Burrows wanted to find out if the mimetic muscles in our canine companions had evolved to enable faster facial movement. Did we selectively breed the puppy-dog eyes?
Compared to samples taken from wolves, dogs' facial muscles had a higher percentage of fast-twitch fibres. This difference likely contributed to dogs' ability to communicate with us, and throughout the domestication process "dog muscles could have evolved to become ‘faster’, further benefiting communication between dogs and humans," said Burrows.
"The classic 'puppy-dog eyes' facial expression is one that dogs make in front of their humans. While we can't know exactly what dogs are thinking when they make this expression, it seems to trigger a care-giving response in humans," said Burrows.
Next, the team want to expand their research to cover a variety of dog breeds. "From the very small breeds to the very large – we want to understand any breed differences and whether some breeds use their facial muscles in different ways," said Burrows.
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