When the Nobel Prize-winning ethologist Konrad Lorenz forced a dog to lie flat on the floor in a submissive position, the dog stopped struggling, went slack, and then flicked its tongue as if it was licking its lips or the air. While most dog owners tend to look at all forms of licking by their pets as being signs of affection, Lorenz recognized that in this situation the dog's behavior had a totally different meaning. Some new research now shows that when dogs are stressed, perhaps by recognizing negative emotional states in others, it may trigger this form of non-social contact licking.
Over the past few decades, researchers have begun to recognize that a dog's licking behavior can communicate information about dominance, intentions, and state of mind. The current consensus is that lip or air licking is mainly a pacifying behavior. One thing in common among all pacifying signals is that they contain elements of puppy-like behavior. Juvenile behavior is the canine equivalent of a "white flag." Most adults tend to nurture the young of their own species and there appears to be a strong inhibition against actually attacking them.
Thus, non-dominant, frightened or weak adults will adopt puppy-like postures and perform juvenile actions to avoid aggression. These behaviors usually soften the mood of the threatening animal and will normally avert any sort of physical attack. Many aspects of pacifying behavior contain forms of licking and it makes sense to look at the actions of young puppies to interpret what these signals were meant to communicate in their earliest stages.
As puppies mature in their litter, they begin to lick and clean themselves and their littermates. This mutual licking and grooming serve several social functions. Obviously it helps keep the puppies clean, but in the process, it helps to strengthen the bonds between the puppies. The actual mechanism that builds this affection is mutual satisfaction. A puppy can thus have companions to get at those hard to reach places, like ears and backs and faces and they can pay them back by licking their littermates in their inaccessible regions.
Because friends and familiars groom friends and familiars as a considerate gesture, the very act of licking another dog develops significance as a means of communication. Licking thus shifts from being a utilitarian and useful act to becoming a ritualized gesture. The meaning of this gesture at this time in a puppy's life involves goodwill and acceptance. In effect, each puppy is saying something like, "Look how friendly I am." As the puppy matures, the message sent by licking continues to be friendly but is widened to also mean, "I am no threat," and perhaps also the submissive plea, "Please accept me and be kind."
Licking takes on another meaning a little bit later in the puppy's life, usually around the time that they become less dependent on their mother's milk. In the wild, when a mother wolf returns from hunting she will have already fed herself on her quarry. When she enters the den, the puppies gather around her and begin to lick her face. To a romantic, this may look like a loving greeting with all of the puppies overjoyed at the mother's return after her absence of several hours. They are seen as simply kissing her in happiness and relief.
The actual purpose of all of this face-licking, however, is really much more functional. Wild canines have a well-developed regurgitation reflex and the puppies lick their mother's face and lips to cause her to vomit up some food. It is most convenient for the mother to carry food in her stomach rather than trying to drag things back to the den in her mouth. Furthermore, this partially digested material makes ideal dining for young puppies. This puppy induced regurgitation is not as often seen in domestic dogs unless the pups are not being well-fed.
Understanding the development of licking behavior helps to interpret another place where it occurs. Face-licking in adult canines can be a sign of respect or deference to a more dominant dog. The dog doing the licking usually lowers his body to make himself smaller and looks up, adding to the effect of juvenile behavior. The dog receiving the face-licks shows his dominance by standing tall to accept the gesture but does not lick the other dog in return.
It is often a stressed and fearful dog who is exhibiting licking behavior and these behaviors have become so ritualized that an anxious dog may lick even when there is no dog or person close enough to be licked. It may lick its own lips, much like stressed humans may bite their lip. Sometimes the dog will simply extend its tongue quickly and appear to be licking the air. At other times the dog may drop down to the floor and nervously lick at its own paws or body.
I frequently see this kind of lip-licking or air-licking on the first day of a beginner's dog obedience class. The dogs are often stressed because their handlers are a bit nervous, the surroundings are strange and there are unfamiliar dogs in the room. As the classes proceed, however, the room, the situation, and the other dogs all become familiar, and the licking behavior quickly disappears. I have been told by veterinarians that they often observe the same behaviors in their consulting rooms. The dog licks the air and its lips while it seems to fret about this new environment, filled with strangers and unknown future events.
Some new research indicates that simply recognizing negative emotions in other dogs or people can trigger this kind of licking behavior. The research team was headed by Natalia Albuquerque from the School of Life Sciences at the University of Lincoln in the UK. The dogs used in this study were pet dogs of various breeds. The investigators presented the dogs with photographs of unfamiliar human or dog faces displaying different types of emotions (in this case happy or playful faces versus angry or aggressive faces). The pictures were presented side-by-side and the scientists videotaped which pictures the dogs looked at and what reactions the dogs displayed.
The results show that roughly one out of every five times when the dogs were presented with a canine or a human face showing an angry or aggressive expression, it would trigger a lip-lick or an air-lick from the dog. The investigators interpret this as an indication that seeing such a negative emotional image produced a momentary, low level, stress response in the dog. This mild stress or anxiety was enough to trigger an involuntary quick lick from the dog. They conclude that this confirms that such licking behavior represents an expression of worry or concern on the part of the dog.
There was one interesting quirk in these data. When confronted with a negative facial expression in a human being the dogs were twice as likely to lip-lick or air-lick than they were when confronted with a negative expression from another dog. This is a puzzling finding, for which the researchers really don't have a clear explanation.
However, one speculative possibility is that humans are much more responsive to facial expressions than are dogs. A subtle gesture like a tongue flicking out of the mouth is more likely to be recognized and responded to by humans rather than other canines. If that is the case then as dogs and humans co-evolved that gesture would come to be more likely to be given when the dog's anxiety was triggered by something associated with people. But of course, that is pure speculation.
What is not speculative is that it now appears clear that air-licking and lip-licking in social situations are signs that a dog might be feeling worried or anxious.
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Natalia Albuquerquea, Kun Guo, Anna Wilkinson, Briseida Resende, Daniel S. Mills (2018). Mouth-licking by dogs as a response to emotional stimuli. Behavioural Processes, 146, 42-45. doi.org/10.1016/j.beproc.2017.11.006