Sexual selection in harvester ants (Hymenoptera: Formicidae: Pogonomyrmex)

Abstract

1. In mating aggregations of some Pogonomyrmex harvester ants (P. desertorum and P. barbatus), communally displaying males compete for females, and large males are disproportionately successful in gaining access to mates (Figs. 1 and 2). 2. Males indiscriminately seize females and attempt to mate, but females actively resist copulation. As a result, large females mate disproportionately with large males in both P. desertorum and P. barbatus (Figs. 3 and 4). 3. In P. desertorum, significant between-colony differences in male body size are approximately correlated with the total reproductive biomass of the parental colony (Fig. 5). Female coyness may have evolved in response to sexual selection for females to choose mates from large and, presumably, long-lived colonies. 4. Pogonomyrmex males are much larger in relation to conspecific females than are males of many species in which males spend long time periods in hovering flight. As Pogonomyrmex males display in vegetation or on the ground, they are not constrained to small body size by the energetics of flight, and have been free to evolve large body size under the influence of sexual selection. 5. Two factors may explain why selection for large males does not escalate indefinitely. Since a male's probability of mating depends on the sizes of his competitors as well as on his own size, selection for large male body size should be frequency-dependent and inversely related to the magnitude of the trade-off in the production of large and small males. Secondly, in P. barbatus the mating advantage shows a significant decline at very large body sizes. 6. Over all colony offspring, the ratio of investment in the sexes is significantly less than 1:1. The female bias also slightly but consistently exceeds that predicted by worker control of sexual investment, and this excessive bias is not well explained by local mate competition.