Dogs use fast-acting face muscles to help them make puppy-dog eyes

Dogs have evolved face muscles that move much quicker than those of their wolf relatives – which means their faces move in a way reminiscent of human ones. These faster facial muscles allow for better communication between dogs and humans and may help explain why people find dogs’ faces so appealing.

“Dogs are really unique from any other domesticated animal in that they reciprocate a bond with their humans. They truly are our companions,” says Madisen Omstead at Duquesne University in Pennsylvania. “They demonstrate this through their mutual gaze – that ‘puppy-dog eye’ look that they give us.”

A previous study found that dogs evolved a muscle in their eyebrows that wolves don’t have through human selection, which also helps them produce facial expressions humans find appealing.

Now, in research presented at the Experimental Biology 2022 meeting in Pennsylvania today, Omstead and her colleague Anne Burrows, also at Duquesne University, have probed deeper into the evolution of canine facial expressions by looking at how the facial movements of dogs and wolves differ.

In humans, most of our facial muscles are dominated by fibres made from the protein myosin that contract rapidly, but these tire quickly. This is why we can make quick but short-lived facial expressions. Muscles that are made up of more “slow-twitch” fibres are better for longer, sustained facial movements.

For dogs and wolves, the pair counted the number of fast-twitch and slow-twitch fibres in the orbicularis oris muscle (which is around the eye) and the zygomaticus major muscle, which is around the mouth. They did this with muscle samples from seven dog species, including chihuahuas, huskies and Labradors, as well as grey wolves.

The researchers found that between 66 and 95 per cent of muscle fibres across the dogs could be identified as comprised of fast-twitch fibres, but only 25 per cent of muscle fibres in wolves were like this. Slow-twitch muscle fibres made up around 10 per cent of identifiable muscle fibres in dogs, while around 29 per cent of identifiable fibres in wolves were slow-twitch.

By having more fast-twitch fibres in their muscles, dogs can speedily form a range of facial expressions, including their signature “puppy-dog eyes”, and make short, sharp barks. This is key for human-dog communication, says Omstead.

In wolves, however, slow-twitch fibres are necessary for the extended movements associated with activities such as howling.

“These results suggest that humans may have, either consciously or unconsciously, selectively bred dogs that make these faster facial expressions that are more similar to how humans express themselves,” says Omstead.

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