Cryptic Female Choice: This is a Woman’s World, but it Wouldn’t be Nothing Without a Man or a Boy

Nature has a reputation for being red in tooth and claw – a warzone with bloody battles ensuing between the biggest and bravest males of a population. The prize? The hand of a fair maiden and the guarantee of an heir to carry on the family line. If nature were to be hollywoodised the film might be called “Clash of the Titans: Survival of the Fittest” – a lesser known sequel to a well known franchise. It would be a macho, action-packed thriller starring the animal equivalents of Tom Cruise, Bruce Willis and Liam Neeson. The trailers would be full of large male mammals drawing blood and fighting to the death – there would be close-ups of bared teeth and wounded rhinos. The only light relief would come as the audience watches two giraffes whip their long necks and propel their heads into each other’s backsides (for those of you who haven’t been following David Attenborough’s “Africa” you need to watch the end of episode one to see this highly entertaining display!). Females would occasionally appear but they would not venture far from the sidelines as they looked upon the scene, passively awaiting their fate. The victor would claim his prize and walk off with her into the sunset, and the trailer would finish with the big booming Hollywood voice declaring; “Clash of the Titans – who will get the girl?! Only the fittest survive!”

The above view of sexual selection is rooted in Victorian sexism. The idea is that exaggerated male weaponry evolved in order to win battles against rival males. Darwin tried to convince people that, whilst this is almost certainly true, females do actually have a choice in the matter. Male elaborations that seemingly serve no adaptive purpose evolved purely as a way to attract the opposite sex. A female can therefore use these elaborations as a proxy for detecting the quality of her mate and make her choice according to the brightness of his plumage or the length of his tail feathers.

Further research into sexual selection has uncovered an even higher degree of female control. “Cryptic female choice” occurs after mating and it is a way in which females are able to bias the outcome of sperm competition. A situation might arise in which a male forces himself on a female, and the cost of resisting the forced mating is too high so the female lets it happen. However, the female may well have already decided not to mate with this particular male because she did not deem him to be good enough. Through mechanisms of cryptic choice the female is able to prevent her eggs from being fertilised by this male’s sperm and therefore prevent her offspring from having unworthy genes.

There are many mechanisms of cryptic female choice. Female insects use sperm storage organs (spermatophores) which prevents direct fertilisation of the eggs. If a female has mated multiple times then this delay between copulation and fertilisation allows her to sieve off the sperm which contain “less fit” genes that she does not want for her offspring. Female mice prevent inbreeding by producing enzymes in the vaginal passage that break down sperm containing the same MHC genes as their own. MHC genes are responsible for control of the immune system – the more diversity an individual has in these genes the greater the flexibility of the immune system. Individuals with flexible immune systems are more likely to fight off infection and survive to the age of sexual reproduction. A female mouse, therefore, increases her offspring’s survival chances by stopping her eggs from being fertilised by the sperm of a male that possesses an immune system that is too similar to her own.

Ducks provide us with one of the best examples of “girl power” that nature has to offer. Males have evolved to have extremely long and elaborate penises that rotate in an anticlockwise direction in order to allow “forced copulation” of the female – a polite, scientific way of saying rape. For a long time it was thought that there was nothing that the female could do to prevent this from happening and that her offspring would be fathered by the last male that raped her. However, recent research has shown that female ducks are not powerless and that they have evolved mechanisms to counter these forced copulations. In many duck species females have evolved vaginal passages that rotate in the clockwise direction – this reduces the efficacy of forced copulation as males are unable to fully penetrate. Only the sperm of the fittest males will be able to navigate the vaginal passage and fertilise the female’s eggs – the fittest male will have the fittest sperm containing the best genes. Whilst the female still has to put up with periodic, and unpleasant, forced copulation, she is able to bias the outcome of sperm competition so that ultimately she has the best offspring with the best genes.

If Hollywood were to make “nature” into a film it would be much more interesting to follow the female of the species. It would be a tale of intrigue and espionage in which the females play the damsels in distress in order to determine the fate of their male counterparts. The female would test her potential mate in ways unknown to him in order to assess his suitability to father her offspring. Ultimately, all females will produce offspring but only the very best males that can prove themselves worthy will be chosen to help females propagate the species. In reality nature is run through the choices that females make, not the aggression that males display – this is a woman’s world, but it wouldn’t be nothing without a man or a boy!