Why Dogs Like to Lick People

When a dog licks its owner or a family member, most of us interpret this as a sign of affection. In fact, we usually tell children that when a dog is licking them he is giving them "kisses." The truth of the matter is that science has not established the specific reasons why dogs engage in this behavior, although there are a few popular hypotheses.

Is It Affection?

It is possible that dogs are indeed expressing affection through licking. Licking appears to be a natural activity that dogs may have learned from the grooming and affection given to them by their mothers when they are pups. Dogs do seem to lick each other in a friendly way as part of social interactions. When it comes to people, dogs might try to lick someone's face if they can get to it, but if not, they may just go for any available patch of skin. Thus licking, or being licked, may provide comfort to the dog and may be regarded by the dog as a means of showing warm feelings toward a person.

How About a Snack?

One fairly sensible supposition is that this behavior evolved from the behavior of wild canines, such as wolves. When an adult wolf goes on a hunt, they don't have any shopping bags to bring food back to the pups in the den. So they gorge themselves and when they return the wolf pups lick the adult around the mouth area which causes them to regurgitate and thus provides warm pre-digested food for their young. So it could be that licking is a holdover from that behavior and the dog is simply signaling that it would appreciate something to eat.

A Taste of Salt?

Perhaps the most common explanation (beyond the notion that licks are kisses) is the idea that dogs lick us because our skin tastes salty due to perspiration. This hypothesis never seemed fully sensible to me since dogs actually have many fewer taste receptors for salt than do humans. Furthermore, while salt receptors in humans extend all the way around the edge of the tongue, including the tip, there are no salt receptors at the tip of dogs' tongues; rather, there are two small strips of salt-responding taste buds along the side of their tongues. From an evolutionary standpoint, extra salt detectors should not be needed because dogs are carnivores, and normally should get all of the salt they need from the sodium in meat. To me, this suggests that dogs would not seek out salt preferentially the way people seek out salty snacks.

I did a literature search to see if there had been any experimentation done on why dogs lick people. I thought that at least there should be some work on the salt hypothesis, since that seemed quite easy to experimentally test. Unfortunately, I found no published literature on this question. Given that gap in the available data, I decided to conduct a simple experimental test.

Testing Procedure

I set up the test so that it could be conducted by individuals in their homes. I managed to recruit 20 participants who lived in single-dog households who were willing to test their own pets. The measure was how long a dog would lick each of their owner's knees, when one would have a saltier taste than the other.

Each person wiped their knees with warm water and blotted them dry. A salt solution [10 mL (2 teaspoons) table salt dissolved in 125 mL (1/2 cup) warm water] was applied to one knee with a paper towel and allowed to dry for a few minutes.

The dog's owner sat with both knees exposed and called their dog over. When the dog approached and began to lick one knee the timing started. The time spent licking each knee was recorded over a one-minute test interval. The test was repeated three times with at least one hour between each test. The average time that the dog licked the salty knee versus the non-salted knee (rounded to the nearest second) was recorded as the data points.

The Results

There was a lot of variability in the results since some dogs licked a lot and some licked very little over the one-minute intervals. It appeared to some of the dog owners that some of the dogs would arbitrarily choose one knee on a single trial and lick that one, without ever tasting the other knee. However, over the three trials, it seemed as though all dogs eventually tasted both salty and non-salty knees.

When averaged over all trials and all dogs there was virtually no difference between the amount of time spent licking the salty versus the non-salty knee (average of 22.6 seconds for salty versus 22.9 seconds for the non-salty). There was no significant statistical difference between the two [t=0.09 (df=19) p=0.92 ns].

What We Can Conclude

Given how easy it was to conduct this particular experiment, I am surprised that no one has done it before. I certainly hope that some canine behavior laboratory will retest this under more controlled conditions.

At the very least, these results suggest that one of the popular explanations for why dogs tend to lick people—specifically, that they seek a salty taste—seems to have no empirical support. So that leaves us with the first two hypotheses: (1) Dogs lick us because in their evolutionary past this was a way for wild canine pups to solicit food from adults, and licking is simply a holdover indicating that the dogs would like a snack; or (2) It is a gesture of affection that is a residual from the affection felt by puppies when their mother groomed them by licking. As yet we have no reason to favor one or the other of these explanations. In any event, if these new data are to be believed, the salt hypothesis is off the table of possible explanations.

[Anyone desiring a copy of the data set should contact me directly.]

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