Male reproductive investment and queen mating-frequency in fungus-growing ants

Sperm number and male accessory gland compounds are often important determinants of male mating success but have been little studied in social insects. This is because mating in social insects is often difficult to manipulate experimentally, and first evidence for an explicit influence of accessory gland secretions on male mating success in social insects was obtained only recently. Here we perform a comparative analysis of male sexual organs across 11 species of attine fungus-growing ants, representing both genera with single- and multiple-queen mating. We found that the general morphology of the male sexual organs was very similar across all species, but the relative sizes of the accessory glands and the sperm-containing accessory testes vary significantly across species. Small testes and large accessory glands characterize species with singly mated queens, whereas the opposite is found in species with multiply mated queens. However, in the social parasite Acromyrmex insinuator, in which queens have secondarily reverted to single mating, males have accessory gland characteristics reminiscent of the lower attine ants, but without having significantly reduced their investment in sperm production. We hypothesize that the main function of accessory gland compounds in attine ants is to monopolize male paternity in similar ways as known from other social insects. This would imply that the evolution of polyandry in the terminal clade of the fungus-growing ants (the leafcutter ants) has resulted in selection for decreased investment by males in accessory gland secretions and increased investment in sperm number, in response to sperm competition for sperm storage.