Sex ratio bias in the dung beetle Onthophagus taurus: adaptive allocation or sex-specific offspring mortality?

Sex allocation theory predicts that females should adjust the sex of their offspring when the fitness returns of one sex are higher than the other. However, biased sex ratios may also arise if mortality differs between the sexes. Here, we examine whether offspring sex ratio bias in the dung beetle, Onthophagus taurus, represents adaptive sex allocation by females or is due to sex-specific mortality. First, we re-analyze an existing data set to show that females produce an excess of daughters when mating to smaller, less attractive males and near equal sex ratio with large, more attractive males. We show, that this results from females adjusting larval provisions after mating to males of variable attractiveness which in turn influences the likelihood that sons die during development. Second, we conduct a manipulative experiment varying the quantity and quality of larval provisions and show that the mortality of sons increased when larval provisions were reduced. Collectively, our work demonstrates that offspring mortality is contingent on the amount of resources provisioned by females and that sons have greater nutritional demands than daughters during development, leading to higher mortality. Our results therefore demonstrate the importance of considering sex-specific offspring mortality in studies of sex ratio evolution.