Primary and secondary sex ratios in monogyne colonies of the fire ant

Abstract

Patterns of sex ratios and sex investment ratios play an instrumental role in theoretical and empirical considerations of the evolution and maintenance of insect sociality. An assumption of sex-ratio studies in the eusocial Hymenoptera is that workers, who reproduce indirectly by rearing the queen's offspring, have the ability to distinguish the sex of larvae and direct preferential treatment towards developing females. To determine if workers of the monogyne (single queen per colony) form of the fire ant, Solenopsis invicta, have this ability and the extent to which they might use it to influence the sex ratio of the colony's reproductives, the primary sex ratio (ratio of male/female-determined eggs produced by the queen) and the secondary sex ratio (ratio of male/female reproductive adults or pupae) were compared, in mature colonies producing sexuals of almost exclusively one sex or the other, a common condition in monogyne colonies of this and other ant species. Queens of male-producing colonies laid about 19% haploid (male) eggs, whereas queens of female-producing colonies laid about 11% haploid eggs. In both cases, the proportion of haploid eggs laid was far higher than the proportion of adult males reared by the workers. These results suggest that workers strongly influence the colony's secondary sex ratio by selective elimination of male larvae, but the magnitude of this influence depends upon the primary sex ratio produced by the queen. Highly male-biased secondary sex ratios produced by workers in relation to more male-biased primary sex ratios may be caused by ergonomic constraints that could limit the ability of workers to skew the sex-investment ratio.