Opposing effects of sperm viability and velocity on the outcome of sperm competition

Sperm competition is often envisaged as a raffle where differences in sperm quality load the raffle in favor of the male with the higher quality sperm. Sperm quality, however, is a complex trait, and the effects of its various components on the outcome of sperm competition are likely to depend upon their role in sperm transport, storage, and fertilization. Here, I use artificial insemination to assess the relationships between sperm viability, velocity, and storage on siring success in the swordtail Xiphophorus nigrensis (Teleostei: Poeciliidae), an internally fertilized fish with alternative reproductive tactics. Males with greater sperm viability sired more offspring than their rival, as predicted if the number of fertilization-capable sperm influences sperm competition in a numerical raffle. In contrast, males with faster swimming sperm sired fewer offspring, but only when sperm were stored prior to fertilization. Faster swimming sperm were thus a liability following sperm storage, suggesting a trade-off between sperm velocity and longevity in vivo. These results provide experimental evidence that the sign and magnitude can differ among components of sperm quality depending upon the mechanisms that mediate their effects.