The Evolutionary Ecology of Sexual Conflict and Condition-dependence in an Insect Mating System

Sexual conflict and condition-dependent trait expression have emerged as major themes in sexual selection. There is now considerable evidence suggesting that both conflict and condition-dependence can drive the evolution and expression of sexual traits; still, important questions remain concerning the extent to which conflict shapes sexual traits and the role of condition in mediating conflict. Here, I address these two themes in studies of a ladybird mating system. One set of studies investigates the function and economics of potentially antagonistic traits – nuptial gifts and female mating resistance – while another examines condition-dependence in mating resistance and male ejaculate composition. Nuptial gifts are often considered beneficial to females, but recent thinking suggests they may also allow males to manipulate females, raising the possibility of conflict. I demonstrate that male ladybirds benefit from nuptial feeding by their mates through reduced re-mating frequency. Benefits to female reproduction and lifespan, however, are weak or non-existent. These results show that although males gain from transferring gifts that influence female behaviour, females experience neither harm nor benefit. I next tested the hypothesis that nuptial feeding is maintained – despite an absence of benefits – because female foraging is generally elevated after mating. However, although females indeed display strongly increased foraging after mating, this response did not increase nuptial feeding. Recent studies suggest that individual condition may affect the economics of mating and extent of conflict. Female ladybirds vigorously resist mating, and I show that (1) resistance is condition-dependent, with low-condition females displaying more resistance, and (2) resistance functions to minimize superfluous matings (sexual conflict), rather than to select among males (indirectly benefiting females). Resistance generates selection favouring large males; thus, this work demonstrates that ecological circumstances, through influencing condition, affect the strength of sexual selection. Finally, male condition may influence investment in ejaculate components, but condition-dependence in ejaculate composition is currently poorly understood. I show that, in agreement with theory, males in poor condition transfer smaller ejaculates that nonetheless contain more sperm, but less seminal fluid. Taken together, this work highlights both the value of economic studies in evaluating sexual conflict, and the significance of condition-dependence for sexual selection.Ph