Sperm Competition

Sperm competition is a term used to refer to the competitive process between spermatozoa of two or more different males to fertilize an egg of a lone female. Competition can occur when females have multiple potential mating partners. Greater choice and variety of mates increases a female's chance to produce more viable offspring. However, multiple mates for a female means an individual male has decreased chances of producing offspring. Sperm competition is an evolutionary pressure on males, and has led to the development of adaptations to increase males' chance of reproductive success. Sperm competition results in a sexual conflict of interest between males and females. Males have evolved several defensive tactics including: mate-guarding, mating plugs, and releasing toxic seminal substances to reduce female re-mating tendencies to cope with sperm competition. Offensive tactics of sperm competition involve direct interference by one male on the reproductive success of another male, for instance by physically removing another male's sperm prior to mating with a female.

Sperm competition is often compared to having tickets in a raffle; a male has a better chance of winning (i.e. fathering offspring) the more tickets he has (i.e. the more sperm he inseminates a female with). However, sperm are not free to produce, and as such males are predicted to produce sperm of a size and number that will maximize their success in sperm competition. By making many spermatozoa, males can buy more "raffle tickets", and it is thought that selection for numerous sperm has contributed to the evolution of anisogamy with very small sperm (because of the energy trade-off between sperm size and number). Alternatively males may evolve faster sperm to enable their sperm to reach and fertilize the ovum first. Dozens of adaptations have been documented in males that help them succeed in sperm competition.