The authors of the study, published in the journal of Proceedings of the National Academy of Sciences, suggested 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 evidence is compelling that dogs developed a muscle to raise the inner eyebrow after they were domesticated from wolves. We also studied dogs' and wolves' behaviour, and when exposed to a human for two minutes, dogs raised their inner eyebrows more and at higher intensities than wolves,' explained lead author of the study, Dr Juliane Kaminski.
"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. This would give dogs, that move their eyebrows more, a selection advantage over others and reinforce the 'puppy dog eyes' trait for future generations,' she added.
Dr Kaminski's previous research showed dogs moved their eyebrows significantly more when humans were looking at them compared to when they were not looking at them.
It is not known why or precisely when humans first brought wolves in from the cold and the evolution from wolf to dog began, but this research helps us understand some of the likely mechanisms underlying dog domestication.