Effects of risks of sperm competition on the numbers of eupyrene and apyrene sperm ejaculated by the moth Plodia interpunctella (Lepidoptera: Pyralidae)

Abstract

This study examines the effects of different risks of sperm competition upon ejaculate characteristics in the moth Plodia interpunctella. In this short-lived species, females will remate and thus generate sperm competition, while males have a limited sperm supply. We therefore predict males to have evolved prudence in ejaculate allocation and investigate the effects of (1) rival male presence, (2) female mating history and (3) female age, upon the ejaculation of eupyrene (fertilizing) and apyrene (non-fertile) sperm numbers. We found no effect of the presence of rival males upon ejaculate characteristics, and conclude, due to the mating system of P. interpunctella, that rival males do not represent a proximate risk of sperm competition. We controlled female mating history by allowing females to receive different and predictable numbers of sperm which they then store for at least 7 days. In subsequent matings (7 days later) we found that new males ejaculated significantly more eupyrene sperm to females that had previously received larger numbers of sperm. We conclude that males increase numbers of eupyrene sperm to maintain success in sperm competition with rival sperm already in storage in the female. We found no effect of female mating history upon the ejaculation of apyrene sperm. Female age, however, had a significant negative effect upon both sperm types. We discuss these results in relation to sperm competition theory and apyrene sperm function.