The dynamics of second- and third-male fertilization precedence in Tribolium castaneum

Sperm precedence is a useful descriptor of the outcome and possible processes of sperm competition between (usually) two males. Although sperm precedence values are known for a variety of taxa, a characteristic feature has been the profound, and largely unexplained, variance in precedence. In this study we partition some of the precedence variance in Tribolium castaneum (Herbst) (Coleoptera, Tenebrionidae) by examining how: (1) sperm precedence changes with time over a one-month oviposition period, (2) precedence is influenced by the relative timing of the two competitive mating periods, and (3) sperm precedence is affected when three, rather than the normal two, males are placed in sperm competition. In general, last male sperm precedence is high (≈ 90%) for the first few days after mating, but this changes significantly with time as previous males? sperm become used in fertilization. We find that the duration between two competitive mating bouts has a significant effect on precedence (even when relatively brief): longer intervals between mating bouts result in more complete and persistent last-male sperm precedence. The dynamics of last-male sperm precedence when there are three males in the competition are not significantly different to precedence results when only two males are in competition. Our results are consistent with the hypothesis that, soon after inseminations, male ejaculates 'stratify' in the narrow and elongate storage tubules of the spermatheca. With time we see increased variance in individual male precedence as sperm from previous matings achieve more fertilizations. This change over time may result from depletion (either via fertilizations or passive/active sperm loss) of the last males? sperm and/or increased mixing of different males? sperm within the spermatheca.