Variation in sperm size within and between ejaculates in a cockroach

1. Interspecific diversity in the morphology of animal sperm suggests adaptive evolutionary changes in size and structure. Little attention has been directed at patterns of intraspecific phenotypic variation in sperm morphology, although such variation is a prerequisite for the extreme diversity seen among male gametes. Despite well-developed sperm competition theory providing a basis for exploring how variation in sperm morphology may affect male fitness, the patterns of variation in sperm morphology remain unclear for most species. 2. We report systematic size variation in sperm length among different males and between successive ejaculates within the same male in the cockroach Nauphoeta cinerea. Both flagellum length and mid-piece length decreased between the first and second ejaculate. However, within the same ejaculate the sperm head (acrosome and nucleus), midpiece (centriole adjunct region) and tail length exhibited less variation than that present between males for both first and second ejaculates. 3. Our findings, combined with previous studies, suggest a focus on episodes of selection and lifetime selection on ejaculates may be instructive in understanding the selective forces influencing sperm evolution in a predictive framework.