Social grooming

In social animals, including humans, social grooming is an activity in which individuals in a group clean or maintain one another's body or appearance. A related term, allogrooming, indicates social grooming between members of the same species. Grooming is a major social activity, and a means by which animals who live in proximity may bond and reinforce social structures, family links, and build relationships. Social grooming also is used as a form of and a means of conflict resolution in some species. Mutual grooming typically describes the act of grooming between two individuals, often as a part of social grooming, pair bonding, or a precoital activity.

It is a reuse of ordinary grooming behavior, a means of achieving hygiene and good health, in that an animal helping another animal to clean itself also is helping to form a social bond and trust between them.

Mutual grooming in ponies

Three macaques grooming one another near Lonavla, India

Social grooming in hyacinth macaws

Female budgerigar preening the male (video)

One lion grooming another at the Rietvlei Nature Reserve

Allopreening in Yellow-billed Babbler

Bonnet macaque allogrooming while infant suckles

Animals regularly clean themselves to keep their fur, feathers, scales, or other skin coverings in good condition. This activity - known as personal grooming, preening, or auto-grooming - promotes hygiene. Dead skin and foreign objects such as insects, ectoparasites, and leaves, dirt and twigs, are some of the items typically removed.

...
Wikipedia