Dog owners have long been aware of the phenomenon of puppy dog eyes -- just a simple glance from a canine companion can soften even the hardest of human hearts, CNN reports.

Researchers think there is a reason for this -- with dogs evolving new muscles around the eyes to help bond with their two-legged friends.

Experts in the UK and USA who studied the anatomy and behavior of dogs and wolves found that the muscular composition of canine faces has changed, allowing them to "better communicate with humans."

Researchers dissected dog and wolf heads and found that the facial muscle anatomy of both animals was almost identical -- except for an eyebrow muscle, found only in dogs, that allows them to "intensely" raise their eyebrows.

The study, published in the Proceedings of the National Academy of Sciences of the United States of America (PNAS) journal, found that the inner eyebrow raising movement makes the dogs' eyes "appear larger, more infant like and also resembles a movement humans produce when they are sad."

This prompts a "nurturing" response in humans, Juliane Kaminski, a comparative psychologist at the University of Portsmouth in the UK and the study's first author, said.

"The findings suggest that expressive eyebrows in dogs may be a result of humans' unconscious preferences that influenced selection during domestication. When dogs make the movement, it seems to elicit a strong desire in humans to look after them," she said in a statement.