Dogs have a small muscle, which allows them to intensely raise their inner eyebrow, which wolves do not.

New research comparing the anatomy and behavior of dogs and wolves suggests that the facial anatomy of dogs has changed over thousands of years specifically to allow them to better communicate with humans.

In a detailed analysis comparing the anatomy and behavior of dogs and wolves, researchers found that the facial musculature of both species was similar, except above the eyes. Dogs have a small muscle, which allows them to intensely raise their inner eyebrow, which wolves do not.

The authors suggest that the inner eyebrow raising movement triggers a nurturing response in humans because it makes the dogs’ eyes appear larger, more infant like and also resembles a movement humans produce when they are sad.

The research team, led by comparative psychologist Dr. Juliane Kaminski, at the University of Portsmouth, included a team of behavioural and anatomical experts in the UK and U.S.

It is published in the journal Proceedings of the National Academy of Sciences (PNAS).

The researchers also studied dog and wolf behavior, and found that, when exposed to a human for two minutes, dogs raised their inner eyebrows more and at higher intensities than wolves.

“The findings suggest that expressive eyebrows in dogs may be a result of humans’ unconscious preferences that influenced selection during domestication,” Kaminski explained. “When dogs make the movement, it seems to elicit a strong desire in humans to look after them. This would give dogs that move their eyebrows more a selection advantage over others and reinforce the ‘puppy dog eyes’ trait for future generations.”

Kaminski and co-author, evolutionary psychologist Professor Bridget Waller, also at the University of Portsmouth, previously mapped the facial muscular structure of dogs, naming the movement responsible for a raised inner eyebrow the Action Unit (AU) 101.

“The AU101 movement is significant in the human-dog bond because it might elicit a caring response from humans but also might create the illusion of human-like communication,” Kaminski said.

Co-author Rui Diogo, an anatomist at Howard University, Washington D.C., said: “I must admit that I was surprised to see the results myself because the gross anatomy of muscles is normally very slow to change in evolution, and this happened very fast indeed, in just some dozens of thousands of years.”

The only dog species in the study that did not have the muscle was the Siberian husky, which is among more ancient dog breeds.