In cartoons, a dog licking his lips is probably anticipating something tasty. Just think of the Big Bad Wolf in the Three Little Pigs. But according to new research, published in the journal Behavioural Processes, lip-licking could also be a form of dog-to-human communication.
Scientists from the UK and Brazil observed the behavior in pups exposed to a series of emotionally charged images and sounds. When shown a picture of an angry human face, the dogs licked their lips. Audio cues (negative, positive, or neutral) did not produce the same response.
"Humans are known to be very visual in both intra and inter-specific interactions, and because the vision of dogs is much poorer than humans, we often tend to think of them using their other senses to make sense of the world," explained study co-author, Daniel Mills from the School of Life Sciences at the University of Lincoln, in a statement. "But these results indicate that dogs may be using the visual display of mouth-licking to facilitate dog-human communication in particular."
During the study, the mutts were shown images of dogs and humans (both male and female) displaying either a positive or a negative facial expression. At the same time, the dogs were exposed to an audio clip containing emotionally charged vocalizations, which, again, could be positive or negative. The researchers also added in a neutral control sound.
Each pup was tested on two separate occasions two weeks apart and was shown every possible image-audio combination. Their response was captured on a digital video camera.
Interestingly, the lip-licking occurred more frequently when the dogs were presented with an image of an angry human and not an angry dog. The researchers put this down to domestication. Years of cohabiting with Homo sapiens has given canines a functional understanding of human emotions.
Lead author Natalia Albuquerque, from the University of Sao Paulo, adds that “the findings indicate that this behaviour is linked to the animals' perception of negative emotions."
For anyone who has ever owned a dog, this might not come as a surprise. After all, pets seem to have a canny way of picking up on their owner's feelings and science seems to confirm this.
Recent studies have shown that dogs can visually discriminate between blank and smiling human faces and that they exhibit different gazes depending on the facial expression of humans or other dogs, specifically whether they're happy or angry. Just last month, a paper came out announcing that puppy dog eyes are purely for human benefit. Yes, your dog is (knowingly or not) manipulating you.
The researchers hope this new study will add to this growing body of research and shine a light on dogs' inner emotional world.