Sperm competition

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 (Olsson et al, 1997; Wedell et al, 2002) 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).

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 adaptation of sperm traits, such as length, viability and velocity might be constrained by the influence of cytoplasmic DNA (i.e. mitochondrial DNA) (Dowling et al 2007).

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.


 * A million million spermatozoa,
 * All of them alive:
 * Out of their cataclysm but one poor Noah
 * Dare hope to survive.
 * And of that billion minus one
 * Might have chanced to be
 * Shakespeare, another Newton, a new Donne--
 * But the One was Me.
 * Shame to have ousted your betters thus.
 * Taking ark while the others remained outside!
 * Better for all of us, froward Homunculus,
 * If you'd quietly died! --Aldous Huxley
 * Better for all of us, froward Homunculus,
 * If you'd quietly died! --Aldous Huxley