Female sperm storage is a biological process and often a type of sexual selection in which sperm cells transferred to a female during mating are temporarily retained within a specific part of the reproductive tract before the oocyte, or egg, is fertilized. This process takes place in some species of animals, but not in humans. The site of storage is variable among different animal taxa and ranges from structures that appear to function solely for sperm retention, such as insect spermatheca and bird sperm storage tubules (bird anatomy), to more general regions of the reproductive tract enriched with receptors to which sperm associate before fertilization, such as the caudal portion of the cow oviduct containing sperm-associating annexins. Female sperm storage is an integral stage in the reproductive process for many animals with internal fertilization. It has several documented biological functions including:
Supporting the sperm by: a.) enabling sperm to undergo biochemical transitions, called capacitation and motility hyperactivation, in which they become physiologically capable of fertilizing an oocyte (e.g. mammals) and b.) maintaining sperm viability until an oocyte is ovulated (e.g. insects and mammals). Decreasing the incidence of polyspermy (e.g. some mammals such as pigs). Enabling mating, ovulation and/or fertilization to occur at different times or in different environments (e.g. many insects and some amphibians, reptiles, birds and mammals). Supporting prolonged and sustained female fertility (e.g. some insects). Having a role influencing offspring sex ratios among some insects possessing a haplodiploid sex-determination system (e.g. ants, bees, wasps and thrips as well as some true bugs and some beetles). Serving as an arena in which sperm from different mating males compete for access to oocytes, a process called sperm competition, and in which females may preferentially utilize sperm from some males over those of others, called female sperm preference or cryptic female choice (e.g. many invertebrate animals, birds and reptiles).
Increased diversity of offspring
One important advantage female insects that store sperm gain is increased genetic diversity in their offspring. There are many ways that females can alter offspring genetics to increase their success and diversity. An example of how this can be accomplished is in female Scathophaga that preferentially fertilize eggs to survive in different environments. Since many environments require different traits for success, females are somehow able to match sperm (acquired from multiple mates) that have the best genes for whichever environment in which they will develop. Many of the different properties of the environment, including temperature and thermal properties affect the female's sperm choice. Studies have also shown that ovipositing is nonrandom and females lay eggs with varying PGM(phosphoglucomutase) genotypes in different environments in order to optimize offspring success. Females are acutely aware to their environment and manipulate the genetic diversity of their offspring in appropriate ways to ensure their success.
Another way sperm-storing females can alter the diversity of their offspring is controlling the relatedness to the males that provide them with sperm. Inbreeding depression can have a deleterious impact on populations of organisms and is due to mating of closely related individuals. To combat this effect, female insects appear to be able to sort out the sperm of relatives from the sperm of non-relatives to choose the best option to fertilize their eggs. Female crickets are able to preferentially store sperm of multiple unrelated males to that of their siblings; this results in more of the offspring having unrelated parentage. Being able to choose between sperm after coupling might be advantageous to females because choosing between mates precopulation may be more costly, or it may just be too difficult to tell males apart before mating. Females possess remarkable abilities to select sperm to store and to fertilize their eggs that will make their offspring diverse. Though it has been shown that a majority of female insect species can store sperm, specific examples that have been studied could include field crickets, dung flies and Mediterranean fruit flies. Females largely benefit from this mechanism, but males often can experience low fitness because their paternity is not secure.
Sperm stored often determines which male ends up fertilizing the most eggs. An example of this is seen in Red Flour Beetles, where most of the sperm is stored in the first mating. Another male can remove previously stored sperm to better chances of successful mating.