Sperm competition

The existence and evolutionary significance of sexual selection through sperm competition was first realized by Geoff Parker in a prescient and influential review published in 1970. Parker recognized that competition between males for fertilizations will continue after mating, if the sperm of two or more males overlap near the site of fertilization in females. The process of sperm competition was originally defined as ‘the competition within a single female between the sperm of two or more males for the fertilization of the ova’. This definition was later broadened to ‘the competition between the sperm of two or more males for the fertilization of a given set of ova’. This was to reflect the fact that sperm competition is also possible in externally fertilising species that broadcast their gametes into water, such as sea urchins. The prerequisites for sperm competition are simple: females must mate at least two males, whose sperm must exhibit spatial and temporal overlap. However, multiple mating by females, and even the presence of sperm from different ejaculates within females, do not necessarily guarantee that sperm competition will occur. For example, females may remate only when sperm stores are exhausted, thus minimising sperm overlap. There is also a variety of ways by which sperm from different ejaculates can be prevented from mixing inside females. For example, sperm from different males may be stored in different locations, or sperm from earlier matings may be displaced from storage before incoming sperm are stored. In its strictest sense, sperm competition occurs when sperm from different males are in direct contact with one another. However, sperm competition as a research field generally also encompasses adaptations that serve to minimize sperm competition.