Behaviour that establishes or maintains positive social bonds between individuals.

Mutual grooming (in horses, the classic nose-to-tail flank grooming; in primates, allogrooming of inaccessible body parts; in dogs, gentle mutual licking), synchronised resting (animals choosing to rest near specific preferred partners), preferred-partner association (consistent proximity and interaction with specific individuals), gentle physical contact (touching, leaning, nuzzling), and play (across age groups, not only juveniles) are all affiliative behaviours.

Across social species, affiliative behaviour is central to group stability and individual welfare. The 2023 systematic review of equine social behaviour found that affiliative interactions are far more common in stable horse groups than aggressive interactions, despite the research literature having historically focused more on the aggressive category. The same imbalance in the literature exists for several other social species, where the everyday reality of affiliative behaviour has been less studied than the more dramatic agonistic interactions.

The functions of affiliative behaviour include social bond formation and maintenance, stress reduction (animals with affiliative bonds typically show lower stress responses to challenges), reproductive success in some contexts, and the development of social skills particularly in young animals. The behaviours have parallels across mammals and many bird species, suggesting deep evolutionary roots.

In welfare contexts, the presence and quality of affiliative behaviour is one of the most useful indicators of positive welfare. A group of animals with rich affiliative behaviour is typically a group in good welfare. A group where affiliative behaviour is absent or limited, even if no obvious aggressive behaviour is present, may have welfare concerns.

The disruption of affiliative bonds is increasingly recognised as a welfare concern. Moving a horse away from preferred paddock partners, separating a dog from a long-term social companion, breaking up established cat groups, all involve disrupting affiliative relationships that have welfare significance for the individuals involved.