(wikipedia)
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.
Many social animals groom each other, an activity known as social grooming, mutual grooming, or allo-grooming. Items removed during social grooming are identical to those removed by personal grooming. Social grooming also takes the form of stroking, scratching, and massaging.
Primates provide perhaps the best example of this activity. Primatologists have called grooming the social cement of the primate world. The trust and bonding it builds is critical to group cooperation. Among primates, social grooming plays an important role in establishing and maintaining alliances and dominance hierarchies, for building coalitions, and for reconciliation after conflicts; it is also a resource that is exchanged for other resources, such as food and sex.Primates groom socially in moments of boredom as well, and the act has been shown to reduce tension and stress. It is often associated with observed periods of relaxed behaviour, and primates have been known to fall asleep while receiving grooming.
Results of research shows that male crab-eating macaques will groom females in order to get sex. One study found that a female has a greater likelihood to engage in sexual activity with a male if he had recently groomed her, compared to males who had not groomed her.
Other animals groom socially as well. These include insects, fish, birds, ungulates, and bats. Whereas social grooming among primates has been very well-studied, less is known about social grooming in these other animals.
Mammals often perform social grooming. Domesticated animals, especially cats and dogs, will groom trusted humans as a sign of affection.
