Cats are the most popular companion animals worldwide, and many people have multiple cats at home. In these multi-cat households, not all the cats get along equally well. Until recently, it was thought that if a cat licks another cat (a behaviour known as ‘allogrooming’), the two cats were friends. But is this really true? Researchers of Ghent University and the University of Lincoln investigated this behaviour.
The researchers performed a citizen science research project for which fifty-three households with two cats were recruited, and the caregivers filmed their cats when allogrooming. Using these videos, we analysed the behaviour of the cats. The study examined the characteristics of the allogrooming behaviour itself and all other behaviours that occurred in connection with it, as well as the characteristics of the cats.
The findings revealed that allogrooming is certainly not always a sign of affection. On the contrary, it is sometimes a sign of conflict between cats. How, then, can we understand what cats mean when they lick each other? By understanding everything that takes place around allogrooming – the context – we can understand the function of the behaviour. Below, we discuss the two main contexts in which cats lick each other: social bonding and social tension.

Allogrooming … to establish or maintain friendships
Allogrooming can be used to strengthen social bonds, serves a hygienic purpose, or helps the recipient to relax. We have observed that seeking physical contact – such as snuggling up together in a cat basket – is followed by allogrooming in 41% of cases. The body posture adopted by the cats also plays a role here: if the cats synchronise their posture (for example, they both lie down or sit), this indicates a positive setting. A cat will often focus the grooming specifically on the head or ears. Cats’ ears are very sensitive and have scent glands in the area, making this pleasant for the recipient (cats, for example, also often enjoy being stroked in this area).
Furthermore, we also observed that allogrooming is linked to play. When cats play, they will wrestle (i.e. roll around whilst grasping each other with their front paws and kicking with their hind legs). We saw that many cats wrestled, and allogrooming was sometimes used to initiate play. In such instances, licking typically occurs in the neck region: the area where cats also frequently bite each other during play.

Allogrooming … when there is tension between the cats
Allogrooming can also be used as a signal to avoid conflict or to convey a passive-aggressive message in a subtle way. Cats prefer to use a subtle warning rather than their claws or teeth, as they themselves run the risk of being injured in a fight. Allogrooming can therefore also be used as a subtle aggressive signal, for example when both cats want the favourite spot in the sun to rest. We observed that when one cat licks the other, often on the neck, the other will flatten its ears because it does not particularly enjoy the interaction. A further pattern was identified involving swiping with the paw, scratching and biting. We also observed many subtle signs of tension, such as a cat shaking its head, scratching behind its ears, yawning or licking its lips. These are subtle stress signals, and they occurred particularly when the cats did not have the same body posture. For example, one cat leaning over the other.

Summary
The meaning of cats grooming each other (allogrooming) depends on the context, with two important contexts for recognition:

Social bonding	Social tension
Behaviours: The cat engages in physical contact for long periods while allogrooming (huddling, rubbing each other …). The cats wrestle during playful interactions initiated with allogrooming. The grooming focusses on the head and ears. The cats mimic each other’s body posture (both cats lie down or sit).	Behaviours: The cats rotate their ears backwards, bite or claw at each other, and show aggressive body postures. The cat grooms the neck area, which is often followed by biting. The cats show subtle signs of tension by displaying random behaviours (displacement): they briefly shake their head, lick their lips, yawn, or lick their shoulder or front leg a couple of times. The cats have a different body posture or move during the interaction (one cat leaning over the other or a cat trying to get away). The interaction ends when one of the cats leaves.
Functions of allogrooming: Initiate or maintain a play session. Establish or maintain friendships. Social service to promote hygiene or relaxation.	Functions of allogrooming: Conflict mediating or passive aggressive sign (for example to claim a desired sleeping spot without fight)