Lets talk sexual selection, Postcopulatory sexual selection.

Greetings everyone, 

We do hope that you are all enjoying the autumnal weather that has been engulfing the UK recently! In today’s post we bring to you a very interesting topic that is still open to more investigation and testing; Sexual Selection, this topic was explained beautifully by the very father of evolutionary biology Charles Darwin who wrote that sexual selection “... depends, not on a struggle for existence, but on a struggle between the males for possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring.” Don't confuse the theory of sexual selection with the theory of natural selection. Natural selection derives from an organisms  struggle to survive where as sexual selection derives from the organisms struggle to reproduce. 

Within sexual selection you have a split of two distinctive categories: intersexual selection which before sex is often termed female mate choice; and intrasexual selection which before sex is known as male-male competition. Any selection that takes place after copulation is then labeled as post-copulatory sexual selection, and this ladies and gentlemen is the main focus of our article today!! Nothing like a bit of sex talk to get you started on a Monday. 

Interestingly, the idea of post-copulatory sexual selection was lost on Charles Darwin, we suppose that he thought that once the male had copulated with the female that competition then ceases to exist. Basically Darwin either believed or just generally assumed that every female in every species practices the fine art of monogamy… yeah this is not the case take Chimpanzees for example that one female isn’t getting it once but a possible 40 times all with different males! Lets be realistic here monogamy has proven to be a challenge for some humans as well. 

Post-copulatory sexual selection is built on two things sperm competition and cryptic female choice. Species that exhibit a high amount of sperm competition are easily identifiable. They are usually the ones who’s male members have bigger testes or bigger testes in relation to their body size- which hold more sperm. It goes onto thinking that males with more sperm will have a higher chance of fathering their mates next offspring. We all know that nature is not this simple, if another male happens to chance across a previously copulated female (could have happened minutes ago) he will inevitably copulate with her. This is the moment where postcouplatory sexual selection comes into play. Inside the females ova competition has already started between the two rival males sperm to make it to the jackpot… the females egg. 

Sperm competition in more detail:

We know that you guys are curious about what type of competition can possibly occur between sperm... well here you go:

1. Sperm displacement: male flour beetles have modified genetalia in order to translocate other rivals sperm. 
2. Sperm incapacitation.
3. Sperm loading and allocation.
4. The use of chemicals to prevent further mating thus reducing sperm competition completely: Drosophilia melangasta also known  as those annoying little fruit flies! When the male is finished with the female he releases a toxic protein known as ACP (accessory gland proteins) they act as an anti aphrodisiac thereby reducing future sperm competition. 

Even after all of this sperm competition there is one major obstacle to overcome. Cryptic female choice we also like to think that this is pretty empowering for all ladies in nature, it allows the female to ultimately make the final decision on who gets to fertilize her egg. 

Q.What is cryptic female choice?:

A. Females choose which sperm they use, rather than male ejaculates competing. 

Picture above shows sperm storage in Drosophila Melanogaster females.

 

1. Some female species have ovarian fluid that is able to sort out the 'good' sperm from the 'bad' sperm as seen in Salmon. 
2. Females can even store sperm and choose which sperm load to use! Pretty awesome right? The females in the tortoise family (Testudinidae) happen to do sperm storage meaning that they have access to a range of genetically different male sperm so they are able to influence a clutch’s paternity during individual fertilizing events. 

 

All in all...: 

This is a pretty advancing field of reproductive biology, there is still a lot withing sexual selection that needs further evidence! In fact just the other day in preperation for this article we were reading through an amazing paper that was talking about the 'Evolution of Sex'. From this we found that this whole postcopulatory sexual selection deal is built on the males in a species having more than one female mate (male lions and their pride of females) which causes sperm competition and females not committing to one male mate either (like the female chimps) thus leading to cryptic female choice. Almost as if this entire process is built on a species incapability of being monogamous. 

Well that bring us to the end of this blog post but tell us what do you guys think?? And keep a look out for more articles and of course our amazing Friday Facts!! 

BioBunch.

Over and out.