Sperm competition is "competition between sperm of two or more males for the fertilization of an ovum" (Parker 1970). 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 costly to produce and the energy may be spent elsewhere such as defending a territory to the exclusion of other males; the distribution of resources are called strategies. The optimum amount is the evolutionarily stable strategy (ESS). Male fruit flies have been shown to release 250% more sperm when another male is present, compared to being alone with the female .
Sperm competition may lead to evolutionary adaptations for producing more sperm, such as larger testes. Such adaptations cost and so species with low sperm competition invest in mate competition instead. Other means of sperm competition could include improving the sperm itself or its packaging materials (spermatophore). These sorts of competition can occur within a single male, if they involve genes that are expressed in the haploid sperm itself. The male black-winged damselfly provides a prime example of sperm competition. Female black-winged damselflies are known to mate with several males over the span of a only a few hours and therefore possess a receptacle known as a spermatheca which stores the sperm. During the process of mating the male damselfly will pump his abdomen up and down using his specially adapted penis which acts as a scrub brush to remove the sperm of another male. This method proves quite successful and the male damselfly has been known to remove 90-100 percent of the competing sperm .
Sperm competition has led to other adaptations such as larger ejaculates, prolonged copulation, deposition of a copulatory plug, or the application of pheromones that reduce the female's attractiveness.
The adaptation of sperm traits, such as length, viability and velocity might be constrained by the influence of cytoplasmic DNA (i.e. mitochondrial DNA) .
In primates Harcourt et al (1981) studied the relative size of testes compared to body mass against the mating system. They found that promiscuous chimpanzees have larger testes compared to polygynous gorillas.
The British biologist Geoffrey Parker proposed the concept of sperm competition in a 1970 paper.
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