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

Affiliative interactions have important fitness consequences for individuals. Yet, precisely how social roles emerge across ontogeny outside of the context of agonistic interactions is largely unknown, particularly in free-living species of nonprimates. Here we used longitudinal data on yellow-bellied marmots, Marmota flaviventris, residing around the Rocky Mountain Biology Laboratory in Colorado, U.S.A., to investigate the development of social partner choice in free-living rodents. We first characterized the symmetry of affiliative interactions between mother-offspring pairs and within cohorts of individuals as they developed across discrete life history stages. Affiliative behaviours generally increased in symmetry such that exchanges became more reciprocal as individuals matured. We then evaluated the extent to which individuals biased their affiliative behaviour towards, and agonistic behaviour away from, genetic relatives. As predicted by kin selection theory, rates of affiliation increased with the degree of relatedness among individuals at all three life history stages. Thus, nepotism emerged within cohorts of newly weaned pups and persisted into adulthood despite prolonged annual separations due to hibernation. Surprisingly, close relatives also exchanged the highest rates of agonistic behaviour. After accounting for the degree of relatedness, rates of affiliative behaviours given and agonistic behaviours received remained positively correlated, suggesting that close affiliates are each other's closest competitors. Thus, although reciprocity of affiliative exchanges within cohorts increased as individuals matured, competition among genetic relatives emerged early in ontogeny and persisted into adulthood. Taken together, these results highlight changes in social roles shown by individuals across ontogeny and shed light on theories of life history and social evolution. (C) 2013 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.