Dogs lick their mouths to communicate with angry looking humans and not simply in response to food or uncertainty, according to a study.
Scientists, including those from the University of Lincoln in the UK, examined the behaviour of dogs in response to emotionally significant images and sounds.
They found that mouth licking in domestic dogs is not simply a response to food or uncertainty, but appears to be used as a signal to try to communicate with humans in response to visual cues of anger.
Significantly, audio cues of angry human voices did not elicit the same response, researchers said.
Dogs were exposed simultaneously to two facial expressions (one positive and one negative from the same individual), which could be either human or canine of either sex, along with a sound, which could be positive or negative from the same species and gender.
The findings, published in the journal Behavioural Processes, throws new light on our understanding of the emotional world of dogs.
"Mouth-licking was triggered by visual cues only (facial expressions). There was also a species effect, with dogs mouth-licking more often when looking at humans than at other dogs," said lead author Natalia Albuquerque from the University of Sao Paulo in Brazil.
"Most importantly, the findings indicate that this behaviour is linked to the animals' perception of negative emotions," said Albuquerque.
The researchers believe that this behavioural trait may have been selected during domestication.
The findings suggest that dogs may have a functional understanding of emotional information and greatly increase our understanding of their emotional world.