Influences of sex, size, and symmetry on ejaculate expenditure in a moth

Although sperm fundamentally function to fertilize eggs, forces arising from both sexes select for optimal ejaculate composition. Sperm competition is one recognized agent in the evolution of sperm and ejaculate structure. Few studies, however, have examined how female factors influence ejaculate structure, despite some behavioral evidence for male mate choice. Male Plodia interpunctella (Lepidoptera, Pyralidae) accrue all resources for reproduction as larvae. Adults emerge with a limited sperm complement and are therefore under intense selection to optimize gamete allocation. I detected no effect of male body weight on ejaculate size. However, female reproductive potential (ovary masses) was dictated by body weight. In addition, heavier females had greater spermathecal volumes, but there was no such relationship with bursal size. Finally, heavier females showed a higher mating frequency. I found that mating males were sensitive to female size and produced larger ejaculates when mating with heavier females. Males may ejaculate more sperm into larger females either because it pays them to "spend" more reproductive resources on matings that provide greater reproductive potential, or because heavier (longer lived and more attractive) females mate more frequently and have larger spermathecal volumes. Alternatively, females may control spermatophore formation and "accept" an appropriate ejaculate to maximize fertility. Males may therefore be also selected to ejaculate more sperm into larger females to counteract greater risks of sperm competition associated with heavier females. There was no association between male or female femur asymmetry and ejaculate size. P. interpunctella may be selected to exercise modulation of ejaculate size because males invest paternally, sperm for the single reproductive episode are limited, and female fecundity and mating pattern vary between individuals and are associated with body weight. More obvious variability in male reproductive behavior and choice may therefore be paralleled at the cryptic gametic level by plasticity in ejaculate allocation.