Cats groom each other to express affection and mark each other with their scents, indicating that they belong to the same family group. They also groom each other to bathe difficult-to-reach spots, such as the insides of the ears.
Cats groom themselves as a pastime, but they only groom cats and humans that they feel affection towards. Outdoor cats are more likely to engage in this behavior to create a unified group scent that helps them identify each other, but grooming is common among friendly indoor cats as well. Some cats take advantage of other cats to help them groom themselves, presenting their heads for cleaning as a sign of trust. Grooming also helps cats relax, with some cats grooming each other as a source of comfort or reassurance. Female cats also groom their kittens to remove debris and liquids and to encourage them to breathe.
While grooming is safe and normal behavior, it doubles the likelihood of hairballs among one or more cats.
Cats show affection in a variety of ways besides grooming. More vocal cats meow at their owners, while others rub their heads and cheeks against each other. Known as bunting, the cheek rubbing allows them to spread their scent on each other, accomplishing a goal similar to that of grooming.