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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.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Animal Ecology. Summary 1. The western harvester ant Pogonomyrmex occidentalis is frequently the dominant insect species in the deserts and arid grasslands of western North America. 2. The lek mating system of this species provides an opportunity to relate local reproduction to dispersal. Because reproductives from individual colonies aggregate at a limited number of sites for mating, there is a small number of propagule sources. 3. We studied the spatial distribution of a large population of this species. The population as a whole was highly overdispersed, although the patterns of recruitment were highly clumped and correlated in 2 years. 4. In addition to recruitment limitation, we also found evidence of space limitation in the population. Mortality rates were related to the nearest neighbour distance, even when taking into account the fact that small colonies have higher mortality. 5. The spatial structure of the population developed along with the age structure of the population, and was a complex product of the production of new queens, the availability of recruits and the availability of space.

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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.