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Human selection has resulted in dogs evolving more expressive faces. They have a facial muscle for making the “puppy dog eyes” that melt many peoples’ hearts that does not exist in wolves – the ancestors of dogs.

This muscle allows dogs to lift up their inner “eyebrow”, which makes their eye look larger. This makes them look more like childlike and also rather sad – the puppy dog eyes look.

It really does make dogs more appealing to us. In 2013, Juliane Kaminski at the University of Portsmouth and colleagues videotaped dogs interacting with strangers at a shelter to see what made them more likely to be adopted.

“The only thing that seemed to have an effect is this eyebrow movement,” she says. Dogs that made this movement more often .

“It was a surprising result,” says Kaminski, who studies dog-human communication. “That got us really interested.”

In 2017, her team showed that .

Now Kaminski and some anatomists have dissected 6 dogs and 4 grey wolves to compare their facial muscles (they used existing specimens – no animals were killed for this study).

In dogs, the eyebrow motion is made by a muscle above their eyes, on the inner side nearer the nose, called the levator anguli oculi medialis. Five of the 6 dogs had this muscle. The one exception was a Siberian husky – an ancient breed more closely related to wolves than most dogs.

In the wolves – which cannot raise their eyebrows as much – this muscle did not exist. In its place there was a small tendon partially connected to another muscle.

So Kaminski thinks this muscle evolved because people favoured dogs that make this expression.

“It is really different in structure,” she says. “It’s not just something that is used more.”

The movement appears to be under voluntary control, but there’s no evidence that dogs are intentionally using it to manipulate us.