From the article:

To check whether this behavior was more widespread, Ms. Van Belle and her colleagues looked at 53 households across Europe with two or more cats. After telling the pet owners what to look for, the researchers had them submit videos of their cats’ interactions. The scientists then randomly selected a submission from each participant and used statistical analyses to tease apart the hidden nuance in cat-licking behavior.

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The results revealed two things. The first was consistent with typical grooming behavior: The cats licked each other on the head, neck or ears. In these videos, the cats were much more likely to mimic each other’s body postures, either cuddling together or sitting next to one another before and after the grooming. The licks were clearly friendly gestures.

The other side of the cat-licking coin revealed something more in line with bullying. A subset of the videos showed that licking often preceded conflict. These interactions were defined by differing body postures, where one cat might stand and lick the other sitting cat. The aggressive licks were followed by signs of stress in the licked cat, including staring, yowling, rotating the ears, licking the lips or swiping at the other cat. The results were inconsistent with the prior conception of cat allogrooming.

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The researchers suggest that unwanted licks might be an easy way to jab at another cat without getting into a fight. While a full scuffle could result in injury, a precisely placed irritating lick might be a safer way to tell your furry “friend” to get lost.