A social relationship described between two specific individuals rather than within an overall group structure.

In horses, dominance relationships are best understood as bilateral. This particular mare displaces this particular gelding at the feed bin, regardless of where either of them sits in any overall ranking. This particular gelding consistently moves out of the way for this particular other gelding, but holds his ground with everyone else. Each pairwise relationship has its own pattern, and the overall picture of “the herd hierarchy” is in fact a network of bilateral relationships rather than a single ordered list.

The shift from thinking about group-wide hierarchies to thinking about bilateral relationships is one of the most useful conceptual moves in modern equine social science, and applies to many other social species. Dogs in established multi-dog households often have bilateral patterns that do not map cleanly onto group-wide rankings. Cats in multi-cat households typically have a complex network of pairwise relationships. Primate social life has long been understood as substantially bilateral in its structure.

The bilateral framework matters for management and handling. A horse who has been bullied by one specific paddock-mate may show no problems with other paddock-mates, and the welfare-positive intervention is to address that specific pairing rather than to relocate the horse from group living entirely. A dog who has tension with one specific household-mate may be perfectly compatible with other household members, and the intervention is to address that specific relationship.

Bilateral relationships are typically more stable than group-wide hierarchies in the species where both have been studied. Two horses who have a particular pattern of displacement at the feed bin tend to maintain that pattern over years, even as other members of the herd change. The relationship is between the two specific individuals, not a function of their abstract rank.

The bilateral framework also helps explain why social interventions often need to be targeted. Generic “improving herd dynamics” is less effective than specific interventions on the bilateral relationships that are causing concern.