Dogs have evolved their ‘sad eyes’ expression in order to help endear them to humans, a study has found.
Scientists compared the anatomy and behaviour of dogs and wolves.
Their findings suggest the facial structure of dogs changed over thousands of years specifically to enable them to communicate with humans.
A University of Portsmouth spokesman said: ‘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 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.
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.
‘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.
‘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.’