Polygyny and competition in the acacia-ant, Crematogaster mimosae

Polygyny is common in social insects even though it inevitably decreases nestmate relatedness and often reduces the inclusive fitness returns for cooperating non-reproductive individuals. We studied the prevalence and mode of polygyny in the African acacia-ant Crematogaster mimosae. These ants compete intensively with neighboring colonies of conspecifics and with three sympatric ant species for resources associated with the whistling thorn acacias in which they all obligately nest. We used the genotypes of alate males at 10 microsatellite loci to reconstruct queen genotypes and found that C. mimosae colonies are frequently secondarily polygynous, in that they include multiple closely related (and sometimes full-sib) queens, and (more rarely) unrelated queens. We also found that individual queens in both monogynous and polygynous colonies had mated with multiple males, making C. mimosae one of the few ants known to engage in both polygyny and polyandry. The number of queens in a colony is negatively associated with nestmate relatedness but positively related to colony size, suggesting that colonies with greater numbers of queens produce more workers. These increased worker populations may give a competitive advantage to polygynous colonies, as inter-colony conflicts are typically won by the colony with the larger number of workers.