When your best friend catches you in a bad mood, does she try to console you, give you space to cool off or lick her own face in an uncontrollable slobber? If your best friend is a dog, this third reaction may be familiar to you. Certain cuteness-obsessed Internet communities call it a "mlem"; some animal behavior researchers prefer to call it mouth-licking, and offer many possible explanations for the quirky canine behavior. Mouth-licking has been described as a stress-coping mechanism, a spontaneous display of arousal or a way to communicate desire to play with a certain toy or munch a certain treat. But according to a new study by animal behavior researchers from the University of Sao Paulo, Brazil, mouth-licking may actually be one of a dog's best tools for reading and responding to human faces -- in particular, angry faces.

They may not have the ability to speak our languages directly, but over thousands of years of domestication, dogs have likely developed certain'signals' to help them communicate with humans. When they're confronted with an angry face, a new study has discovered that dogs tend to their lips as an immediate response – and, this was more often the case when a human was involved, instead of another dog. Animal behaviour researchers say this may be linked to the dogs' perception of human emotions, acting as a way for them to communicate in response to visual cues of anger. In the study, dogs were shown two facial expressions (one positive and one negative, from the same individual). When they're confronted with an angry face, the researchers discovered that dogs tend to their lips as an immediate response Historically, animal's facial expressions have been considered to be inflexible and involuntary displays, which reflect an emotional state rather than active attempts to communicate with others.