Sperm competition is the physical competition between the sperm of two separate males to fertilize the eggs of a lone female. A male's fitness is usually measured as a function of the number of females inseminated, however in many animal species fertile females mate with many male partners. When this happens, whose sperm will fertilize her eggs? Males in many species have evolved mechanisms to give their own sperm a special advantage after deposition in the female reproductive regions.
The male black-winged damselfly is a prime example of sperm competition. A female will mate with several males in the span of several hours, storing their sperm in a receptacle known as the spermatheca. The male's penis is specially adapted to act as a scrub brush. During mating, the male will pump his abdomen up and down, effectively cleaning between 90 and 100 percent of competing sperm from the female's spermatheca (Alcock 452). He then will deposit his own sperm.
Some males, instead of or in addition to their own mechanisms of sperm competition, will guard their female partners from more copulation even after sperm deposition. These postcopulatory interactions do produce fertilization benefits for the guarding male (Alcock 452). Mate guarding exists in a variety of forms including prolonged copulations, mating plugs, mate grasping, and mate mounting. In the parasitic wasp Cotesia rubecula competition for mates is intense and there is a short window of time immediately following copulation in which a second male may induce a mated female to copulate again (Field and Keller 1183-1189). In order to distract rival males, a recently mated male will mimic a female long enough for the mated female to become unreceptive. Female mimicry in this species acts as a post-copulatory mate guarding tactic employed by males to increase paternal reproductive success. This is one mechanism that would allow subordinate males to increase the likelihood that they would father offspring.
Cotesia glomerata, a close relative of Cotesia rubecula. Both are known for their scrub-brush like penises that can remove almost 100 percent of competing sperm from a female's reproductive tract. Photo from Cornell University Biological Control: A Field Guide to Natural Enemies in North America.