Relatedness does not explain geographic variation in queen cooperation in the seed-harvester ant Messor pergandei

The desert seed-harvester ant Messor pergandei shows sharp regional differences in social structure: in central Arizona, queens initially form group nests but become aggressive following worker emergence and reduce to a single queen (secondary monogyny). In much of the rest of the species range, however, co-founding queens do not display aggression and retain multiple queens throughout the colony lifecycle (primary polygyny). One hypothesis to explain why queen behavior differs between regions is that relatedness among co-foundresses and, therefore, the potential for kin-selected cooperation, varies geographically. To test whether primary polygyny is associated with greater kin association, we used highly polymorphic microsatellites to estimate within-group relatedness for co-foundress associations in the field at two secondarily monogynous sites and five primarily polygynous sites. To determine whether queens can potentially use nestmate identity as a proxy for genetic relatedness, we compared these values to similarly sized samples of worker nestmates from adult colonies at the same sites. We found that foundresses do not preferentially form groups with relatives regardless of the ultimate fate of foundress groups. Mean relatedness values for co-foundresses did not differ significantly from zero irrespective of social structure. In contrast, adult colony worker nestmates were significantly positively related at all sites. These results indicate that kin-selected benefits are not likely to be responsible for the absence of fatal competition in the polygynous region; instead, the cause of geographic variation in queen cooperation must lie in ecological factors that alter the costs and benefits of retaining additional queens into colony maturity.