When cats groom each other it is called ‘allogrooming’ by scientists. It is essentially a bonding behavior which is what it looks like. Humans also engage in social grooming.
It can happen after two cats greet each other with the tail up signal which means that they are not a threat to each other. In other words they want to be and are friendly. Once exchange of tail ups has been established another social exchange can take place which is mutual licking. Mutual licking also takes place between relatives in family groups.
We know that domestic cats spend a lot of time grooming themselves by licking. It is therefore unsurprising that if one cat lies down close to another that they sometimes lick each other. In fact cat ‘owners’ participate in this. If you put your hand near your cat while she is grooming herself she’ll lick you provided the bond is good. You feel the relationship with your cat strengthened by this behaviour.
Further, cats tend to groom the top of the other cat’s head and between the shoulders. This is for the obvious reason that they are difficult to access places by the other cat. Although they are not impossible places to access because cats can lick their foreleg and then deposit saliva on the desired target area with their leg. However, this cannot happen with respect to the area between the shoulders.
Mutual licking has a profound social significance in many other animal species and that argument probably applies to domestic cats as well. It performs the same function as mutual rubbing which is to cement an amicable relationship.
In situations where there are large groups of cats consisting of more than one family, the majority of mutual licking takes place between relatives.
There is evidence that allogrooming reduces conflict. Artificial cat colonies are created inside rescue centres. Evidence suggests that aggression between cats is less prevalent than might be expected when strange cats are forced to live together when they engage in mutual grooming.
It has also been suggested that if an aggressive cat licks another cat it may be an “apology” for a recent loss of temper, says Dr Bradshaw in his book Cat Sense (a source of the information for this article).
An alternative theory is that when a cat allows herself to be groomed by a cat who may have been aggressive towards him, he accepts it because it is more pleasant than being bitten. Accordingly, mutual licking is an alternative to aggression. Dr Bradshaw says that this places it in a dominance framework in which one animal controls another’s activities.
In general, when one cat licks another what happens is what one expects to happen namely that there is an improvement in friendship between the two parties.
The definition of ‘allogroom’ is to clean and maintain the appearance of an individual of the same species (comment from Michael: it also occurs between different species) . It is a ritualised social behaviour and “it has been repeatedly and consistently transformed into conciliatory and bonding signals” – Edward O Wilson sociobiology 1980 (Merriam-Webster dictionary)