From Wikipedia:

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 . . . 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.

Sperm competition between males has been brought up on TRP before, but I haven't seen discussion about the role females would play in deciding which sperm gets to fertilize the egg. There is already a vast variety of species that are demonstrated to exhibit some type of Cryptic Female Choice - insects, rodents, birds, mammals (including primates), etc. Here are some examples from the Wikipedia page:

Specifically in mammals [e.g. humans], by hiding the timing of their estrus cycle, females can avoid male control and pursue their own reproductive strategies. This way a female in estrus can sometimes avoid being swamped by competing males or she can prevent male harassment, coercion, or forced mating.

Human females are already known to mask estrus, but there is one issue: why do they still try to look sexy most of the time? Women are constantly flaunting and obsessing over their sexual characteristics in subtle and overt ways, which makes no sense if the goal is to hide fertility. This leads me to believe there is more going on.

In Drosophila flies and in field crickets, females mate-multiply and select for sperm or spermatophores of certain males.

Females can also exercise cryptic female choice by preventing the transfer of sperm to a storage organ or fertilization sites. This can occur through various means such as muscular contractions, hormones, nervous control, or bodily fluids or chemicals, etc. In some lab rats, females demonstrated their ability to halt sperm transport by re-mating more quickly after certain mating events.

Accidentally nut inside your girl? No problem! Just let Chad's sperm clean it out for you!

Females can sometimes store sperm in adjacent organs, especially when they mate multiply and thus give unequal preference to certain males. In many polygamous species, males often have the ability to displace a previous male’s sperm. In response to this, females retain control of sperm choice by using multiple sperm storage sites to exercise cryptic female choice.

Wasn't there a study about women retaining their previous sexual partners' DNA and having it end up in future offspring years later?

In rats and golden hamsters, females denied a male's success by failing to prepare their uterus based on male stimulation.

After insemination, male sperm are relatively slow and inert in the reproductive tracts or storage areas of the female and are only activated by calcium ionophores in vitro. Thus by releasing or withholding the necessary ions to activate the sperm, females can promote faster motility of sperm towards the egg for fertilization or hinder it by slowing their motility and allowing them to die before they reach the egg respectively.

In lions some females will reduce fertility after the male commits infanticide, resulting in low offspring counts after a new male takes over a pride.

Seems harem-building isn't as straightforward as it looks.

Some females can deny male reproductive success by controlling the development and maturation of eggs in her ovary. In the red garter snake, some females failed to produce eggs after a mating event.

This one comes from Jezebel of all places (I'm not gonna link, you can look it up if you really want to):

In a recent study of bison at Fort Niobrara National Wildlife Refuge, researchers collected behavioral observations of breeding behavior among the herd and compared their observations with genetic paternity data from tissue samples. They found that, while some behavioral characteristics—things like dominance, overall copulation frequency, and age—were significant factors contributing to paternity . . . observed matings had "no predictive power regarding likelihood of paternity."

In general, Cryptic Female Choice seems to arise whenever males are able to force copulation, or find a way to work around existing obstacles. Kind of like an evolutionary arms race. Human males can very easily overpower females - and it's been like that for a long time - so I think it's very likely that women have evolved ways to negate male reproductive advantages when it suits them.

If it is indeed discovered that women can exert Cryptic Female Choice - especially if it's to the same degree of precision that other animals are capable of - it could help explain some reproductive mysteries:

  • Why it takes longer for some couples to get pregnant, or not at all (the woman's reproductive tract could be actively resisting fertilization)

  • Irregularities in sexual behavior, ovulation, fertility, and pregnancy (adaptive responses to the environment, their current partner(s), and their partners' own sexual behavior. It is already theorized that lack of ovulation in anorexic women is an evolved response to famine conditions)

  • Why women tend to be slutty, even when they're with top-tier Alphas (post-sex mechanisms will judge whether Chad/Brad/Vlad is the best choice. It's just her job to arrange the meeting(s).)

  • Why women don't have well-defined standards for who they fuck (for every man with looks, and/or money, and/or status, there is a man who does just as well without it. As before, CFC would sort things out after she has sex).

  • Etc.

Also, thought I'd include this:

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.

Evolution of oneitis, anyone?

I know this is all fairly speculative, but let me know your thoughts, and whether you know anything more about this topic.