Ontogeny and symmetry of social partner choice among free-living yellow-bellied marmots

► Precisely how social preferences develop across ontogeny is largely unknown for free-living animals. ► Affiliative exchanges of yellow-bellied marmots increased in symmetry across each successive life history stage. ► Nepotism emerged within cohorts of pups and persisted into adulthood. ► Close kin also exchanged the highest rates of agonistic interactions. ► Conflict and cooperation among kin emerges early and endures throughout ontogeny.