Sexual selection drives the evolution of diverse traits that provide an advantage in reproduction. In polyandrous species the traditional premating processes of male-male competition and female choice have as counterparts sperm competition and cryptic female choice (CFC) occurring during and after mating; these processes determine paternity shares of multiple males mated to the same female. I conducted experiments to demonstrate CFC in the red flour beetle Tribolium castaneum Herbst (Coleoptera: Tenebrionidae).

In two studies, I manipulated male phenotypic quality by starvation and female choice by killing a subset of females to investigate possible female control over insemination during copulation, and control over sperm quantity transferred. Live, but not dead, females accepted spermatophores more often, and also accepted more sperm when mating with fed males than with starved males. Male paternity share was also lower for starved compared to fed males, likely due to differences in sperm quantities transferred and stored from these males rather than to differences in male copulatory courtship.

I also describe the male spermatophore and female sperm storage organ (spermatheca) in T. castaneum. The spermatophore has a complex tubular structure that undergoes preprogrammed transformations within the female bursa, where it is injected with sperm. Spermathecal structure is variable among females, and larger spermathecal volumes correlated with higher second-male paternity share. The latter was also higher in females that were anaesthetized between two matings, suggesting that female muscular control over sperm storage could represent another CFC mechanism.

I also demonstrated that female choice of mates based on olfactory cues correlates with a male's subsequent insemination success: copulations by males that were more attractive to females than their first mates were 2.5 times more likely to result in insemination than copulations by less attractive second males. Thus, females may improve on their previous mate's quality by using CFC during remating.

In all, these studies demonstrate that T. castaneum females possess several sequential mechanisms of CFC allowing paternity adjustments from multiple males. More generally, this work shows that CFC is an important selective force generating differential paternities among mating males.