Female Sperm Choice

A female's control over reproduction does not end after the sexual act. In many species the female will mate with several males, allowing her to decide which male's sperm she will utilize to fertilize her eggs. The female moth Utetheisa ornatrix will mate many times, selecting only the sperm of males with large spermatophores to fertilize her eggs. Females somehow sense the size of spermatophores deposited in them and selectively store only the sperm from large vesicles. This is known as cryptic female choice.

Utetheisa ornatrix. This moth demonstrates a unique system of cryptic female choice. The female detects the size of spermatophores deposited in her reproductive tract and only allows sperm from the large spermatophores to fertilize her eggs. Photo courtesy Institut National de la Recherche Agronomique.

Males have developed a way to respond to such female choice. A male will try to mate as much as possible with the same female to obtain a numerical advantage over competitors in the amount of sperm stored in the female's reproductive tract. Some females actually take the initiative, initiating copulation repeatedly with the same male. This may allow the female to "swamp" the sperm of unwanted males with the sperm of other, more desirable males. This behavior could also be a form of mate guarding, in which the female prevents her male from copulating with other females and risking the loss of male parental care.