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. 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).