Genetic diversity benefits for social insect colonies headed by polyandrous queens have received intense attention, whereas sexual selection remains little explored. Yet mates of the same queen may engage in sperm competition over the siring of offspring, and this could confer benefits on queens if the most successful sire in each colony (the majority sire) produces gynes (daughter queens) of higher quality. These benefits could be increased if high-quality sires make queens increase the percentage of eggs that they fertilize (unfertilized eggs develop into sons in social hymenopterans), or if daughters of better genetic quality are over-represented in the gyne versus worker class. Such effects would lead to female-biased sex ratios in colonies with high-quality majority gynes. I tested these ideas in field colonies of Lasius niger black garden ants, using body mass of gynes as a fitness trait as it is known to correlate with future fecundity. Also, I established the paternity of gynes through microsatellite DNA offspring analyses. Majority sires did not always produce heavier gynes in L. niger, but whenever they did do so colonies produced more females, numerically and in terms of the energetic investment in female versus male production. Better quality sires may be able to induce queens to fertilize more eggs or so-called caste shunting may occur wherever the daughters of better males are preferentially shunted to into the gyne caste. My study supports that integrating sexual selection and social evolutionary studies may bring a deeper understanding of mating system evolution in social insects.