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

Sperm precedence is a useful descriptor of the outcome and possible process es of sperm competition between (usually) two males. Although sperm precede nce values are known for a variety of taxa, a characteristic feature has be en the profound, and largely unexplained, variance in precedence. In this s tudy we partition some of the precedence variance in Tribolium castaneum (H erbst) (Coleoptera, Tenebrionidae) by examining how: (1) sperm precedence c hanges with time over a one-month oviposition period, (2) precedence is inf luenced by the relative timing of the two competitive mating periods, and ( 3) sperm precedence is affected when three, rather than the normal two, mal es are placed in sperm competition. In general, last male sperm precedence is high (approximate to 90%) for the first few days after mating, but this changes significantly with time as previous males' sperm become used in fer tilization. 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 the re are three males in the competition are not significantly different to pr ecedence results when only two males are in competition. Our results are co nsistent with the hypothesis that, soon after inseminations, male ejaculate s '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 m ay 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.