Between humans, a kiss on the mouth or cheek is a clear signal of warm feelings. But what’s going on in a dog’s brain when it plants a slobbery lick on a person’s face or hands? There’s no single definitive answer, though canine cognition experts have theories.
“If we want to distill it down to one thing, it’s communication,” says Ellen Furlong, an associate professor of psychology and neuroscience at Transylvania University in Kentucky, where she studies dog behavior. Dogs are highly social and well-attuned to humans. If a pup is interacting with you, it’s often with purpose. “But there are lots of different things that [dogs might be] communicating through a lick,” she adds.
To discern the intent and meaning behind a dog’s actions, “I always suggest that people look at the entire context,” Furlong says. For instance, if a familiar dog approaches you energetically, with repeated licks and loose “wiggly” posture, then a lick on the face or hand is likely an expression of greeting or enthusiasm.
Similarly, it may be “an attempt at creating affiliation or reaffirming affiliation,” suggests Zachary Silver, a professor of psychology at Occidental College where he leads the Canine Intelligence Lab. Arousal and over-stimulation probably play a role, he adds. “Dogs lick things when they’re excited,” says Silver. Though he points out that most dogs don’t lick indiscriminately, and that, anecdotally, “the behavior is disproportionately directed towards people they’re familiar with.” If a dog doesn’t greet everyone with the same licks, then the action likely has a social meaning– reinforcing “pack” relations.
Social hierarchy may also be involved as, among dogs, sometimes one animal will briefly lick another in a show of deference and pre-emptive placation. Puppies more often lick older dogs, says Furlong. “It’s like, ‘hey you’re the boss, and I know you’re the boss,’” she says. With human-dog interactions, too, the gesture could carry an aspect of appeasement and submission–depending on the situation.
Yet other kisses might not be quite so conciliatory. A lick, “might be communicating that they’re seeking closeness and affection, or they could be communicating the exact opposite: that they want some distance and space,” Furlong notes. The “lick to dismiss” she says, is a well-known behavior among canine researchers and trainers that signals a pet is trying to redirect someone or escape unwanted attention. A perfunctory lick or two on the face or hand, coupled with signs of stress like a rigid body, attempts to turn or move away, and visible eye whites are all signs of dismissal over greeting.
“People miss a lot in their dog’s behavior,” she says. “I think we tend to pick up really well on cues that a dog is happy and we tend to miss cues that dogs are stressed or looking for disengagement.”
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And going one level deeper, even the apparently happy licks may be more nuanced than just a dog’s keen hello. Behavior is complicated and often driven by multiple factors, says Furlong. In addition to eager greetings, there could be a component of curiosity or even flavor involved in licking–especially after a dog and its human come together after time apart. “They’re going to lick to taste things. They might lick to explore. They might lick because you’ve come home from a run and you’re very salty,” she says.
Among wolves, the wild ancestors of our pets, pups will often lick their mother’s face when she returns to the den, begging for regurgitated food. Some dog cognition researchers, like Alexandra Horowitz, a Barnard College psychology professor, interpret domestic dog licks through the same instinctual lens. “Your dog’s lick of you when you come home is absolutely a greeting, they’re happy to see you, but also it’s a little bit of a request for whatever you just ate,” Horowitz explained in an October CBS Morning appearance.
Even if we’re not throwing up lunch for our furry friends to eat, dogs do get something out of licking us. For one, there’s the unintentionally reinforcing feedback of squeals, pets, and writhing that a bout of fervent canine kisses generally induces in a human recipient. “We have such strong reactions to it,” says Furlong. Given that attention may be just what a tongue-trigger-happy dog is looking for, that outsized response probably encourages more and future licking, she says.
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Another bonus for dogs: licking relieves stress, says Furlong. Dogs will repeatedly lick surfaces (or their own paws) to self-soothe when they don’t feel well, she says, in a related behavior to animal wound licking. And at least one 2014 study found that licking, along with other types of “affiliative behaviors” raises dogs’ levels of oxytocin, a hormone associated with social bonding and positive emotions. Some online sources further claim that licking releases endorphins like dopamine in dogs’ brains–activating the classic neural reward system.
So there you have it. Why do dogs lick? Ultimately, “we don’t know,” says Silver, because the question hasn’t been well-studied. To get a firm answer or rule out any of the above hypotheses, researchers would have to conduct empirically controlled experiments “where we see exactly when dogs lick people and in what context,” he explains. In lieu of definitive explanations, we have “informed guesses,” he adds.
Yet the research Silver has conducted has revealed dogs to be deeply attached to their owners and aware of subtle social cues. An August 2023 study that he co-authored found that dogs were friendlier to strangers who helped their owners on a task than those who didn’t. “Dogs are really keyed into what the main humans in their lives are doing… They’re perhaps a bit more perceptive to social relationships than we previously thought,” he says. If dogs care enough to observe our human-to-human interactions and shift their behavior accordingly, then you can bet that their kisses aren’t without consideration.
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This story is part of Popular Science’s Ask Us Anything series, where we answer your most outlandish, mind-burning questions, from the ordinary to the off-the-wall. Have something you’ve always wanted to know? Ask us.
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