Cryptic female choice

Cryptic female choice is a form of mate choice which occurs both in pre and post copulatory circumstances when females in certain species use physical or chemical mechanisms to control a male's success of inseminating them; namely by selecting whether sperm are successful in fertilizing their eggs or not. It occurs in internally – fertilizing species and involves differential use of sperm by females after the sperm are in the reproductive tract. Our present understanding of cryptic female choice is largely thanks to the extensive research and analysis done by William G. Eberhard. The term ‘cryptic’ according to Eberhard is meant to describe an internal and thereby hidden choice some female organisms are able to make following insemination with regards to sperm selection. In male species with intromittent organs, during sexual intercourse, a male inserts his reproductive organ into that of a female's so as to inseminate her with his genetic material. Through the development of mechanisms that either prematurely inhibit copulation or act following male insemination, females are able to prevent undesirable males from successfully fertilizing their eggs. Thus, not every copulatory event is successful – there are many factors that combine to determine whether or not an offspring is created. It is likely that cryptic female choice is a consequence of the conflict between the reproductive desires of males and females. While males increase their reproductive success by maximally fertilizing each female they mate with, females incur costs to their personal health as a result of such behavior. Cryptic female choice reduces these costs by allowing them to also benefit from and select for favorable matings.

Females not only exert sexual control but also benefit from exerting such control over male reproductive success. It has been observed that in some species males continue to court females following copulation, which illuminates the fact above. This female control compels males to continue to impress their female counterparts following copulation. The assumption above can be made because there is an energetic cost for a male to continue to court a female following insemination because he has to invest energy to do so. And, because there is an energetic investment he must benefit in some way. As such, sexual selection does not only act on traits that influence female mate choice of males but it also acts on male traits that determine his success following copulation.

There are circumstances in nature in which a male's interaction with a female is of detriment to her personal well-being and that of her offspring, one such situation occurs when a male harasses a female prior to copulation. In such circumstances where the female's lifespan and fertility are compromised, it benefits females to develop evasive mechanisms. In addition, in some species where multiple males inseminate a female, a female is able to select the most desirable sperm for her offspring by rejecting that which she desires less. In both of the aforementioned situations it is a female's reproductive actions that affect male reproductive success following copulation. As discussed below, female species that are able to use cryptic choice have developed various mechanisms to manipulate male reproductive success. Species that are able to use cryptic choice only use one of the below mechanisms to do so. Regardless of the mechanism, this ability allows females to respond differently to conspecific males depending on whether the male that inseminates her is favorable to her or not.

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