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Relatedness within colonies of social Hymenoptera is often significantly lower than the outbred population maximum of 0.75. Several hypotheses address the widespread occurrence of low relatedness, but none have measured the covariation of colony fitness and relatedness. In a polyandrous harvester ant, Pogonomyrmex occidentalis, average within-colony relatedness in the population is low but highly variable among colonies, and relatedness is negatively correlated with colony growth rate. Differences in growth rate strongly influence survival and the onset of reproduction, leading to a 35-fold increase in fitness of fast-growing colonies. Benefits of a genetically diverse worker population may favor polyandry in this species.

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Using four highly polymorphic microsatellite markers (12-28 alleles), we gentoyped workers from 63 colonies of Pogonomyrmex occidentalis. Colonies have a single, multiply mated queen, and an average number of 6.3 patrilines per colony. Colony growth was measured over an 8-year period in the study population. Intracolonial relatedness and colony growth are correlated negatively, indicating a substantial fitness benefit to multiple mating.