Mutual grooming between two individuals, in which each grooms parts of the other’s body the other cannot easily reach.

Common in horses (the classic nose-to-tail mutual withers-grooming), in primates (where allogrooming is a central feature of primate social life and can occupy significant portions of the daily activity budget), in some bird species (allopreening), and in many other social mammals. The behaviour involves cooperation: both individuals must be willing participants for allogrooming to occur, and the choice of partners reveals important information about social relationships.

Allogrooming serves both hygiene and social-bond functions. The hygiene function is real (parts of the body that are difficult to self-groom benefit from another individual’s attention) but the social function appears to be primary in most species studied. Allogrooming bouts are typically longer than required for hygiene purposes, partners are chosen non-randomly (preferred partners receive more grooming), and the behaviour is associated with physiological markers of stress reduction in both groomer and groomee.

The frequency and partner choice patterns of allogrooming are among the most useful indicators of affiliative behaviour in any social species. A horse herd, dog pack, or primate group with rich allogrooming patterns is typically a group with strong affiliative bonds and good welfare. A group with limited allogrooming may have weaker social bonds or welfare concerns affecting social engagement.

In horses specifically, allogrooming partners tend to be stable over time. Two horses who allogroom together regularly typically do so for years, and the bond is reflected in other affiliative behaviours (synchronised resting, preferred-partner proximity, allomimicry of movement and posture). The persistence of these partnerships is one of the features of equine social life that does not fit a simple dominance-hierarchy framework: the partnerships are not about rank but about individual preference and bond formation.

In welfare assessment, the presence and richness of allogrooming opportunities is one of the social-behaviour indicators considered. Animals housed in conditions that prevent normal allogrooming (single stabling for horses, single housing for primates) often show welfare consequences.