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 fertilizing their ova or ovum; i.e. 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 when sperm are available in the reproductive tract.
The 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.[1]: 7–9 In male species with intromittent organs, during copulation, 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.[1]: 5 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.[1]: 22 While males commonly increase their reproductive success by maximally fertilizing each female they mate with, females can 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.