Dogs have evolved their "sad eyes" facial expression in order to help them get on better with humans, according to new research.
Scientists at the University of Portsmouth carried out a study comparing the anatomy and behaviour of dogs and wolves.
Their results suggest the facial structure of dogs has changed during thousands of years specifically to enable them to communicate with humans.
"Researchers found that the facial musculature of both species was similar, except above the eyes," a university spokesman said.
"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."
Psychologist Dr Juliane Kaminski, who led the research which is published in the American journal PNAS, said: "The evidence is compelling that dogs developed a muscle to raise the inner eyebrow after they were domesticated from wolves.
"The findings suggest that expressive eyebrows in dogs may be a result of humans' unconscious preferences that influenced selection during domestication."
Dr Kaminski's previous research showed that dogs moved their eyebrows significantly more when humans were looking at them compared to when they were not looking at them.
Australian Associated Press