Split sex ratios and queen-male conflict over sperm allocation

Abstract

In populations of eusocial Hymenoptera with single-queen colonies, where sterile workers realize split sex ratios due to variable queen mating frequency, there is an extreme conflict over the colony-level sex ratio between the parents of these workers. Because of the later sex ratio consequences, queens are under selection to minimize the relatedness asymmetry of daughter workers to sexuals, whereas males are under selection to maximize worker relatedness asymmetry. After an earlier analysis of selection forces on queen mating behaviour, this paper explores two specific adaptations for males to influence the allocation of sperm to maximize relatedness asymmetry among female offspring: sperm clumping and reduced sperm transfer. These male mating traits are against the interests of queens, so that evolutionary arms races between the sexes are expected. A series of testable predictions on sperm clumping, male pheromones and variation in paternity distributions is given. Evenness in paternity in colonies with a double-mated queen is predicted to increase as queen mating frequency increases, which is opposite to the observed trend in non-social insects with sperm displacement. It is concluded that the best options for paternal manipulation in highly eusocial insects may occur during the mating process, before the first worker offspring are around.