A million million spermatozoa
All of them alive
Out of their cataclysm but one poor Noah
Dare hope to survive.
And among that billion minus one
Might have chanced to be
Shakespeare, another Newton, a new Donne...
...But that One was Me.
— Aldous Huxley – 'Fifth Philosopher's Song'
Having yet to learn that clicking links in the catbox can often lead to badness (I can't remember who it was that dropped this in there one time in the distant ago, but I still haven't forgiven you) I was directed today to go read the node cryptic female mate choice; it didn't seem such a dangerous link to click, and in fact was an interesting read... right up until the offhand comment at the end about the existence of 'poisonous sperm'. Then interest turned to mild panic, as I clicked the hardlink to find out more of this previously unknown danger. Denied! 'Here's the stuff we found when you searched for "poisonous sperm".' Nooooo! E2, how could you do this to me? I'm in danger of death by sperm, and you're not providing me with the information that I need to face this danger pro-actively.
Once I'd overcome this uncharacteristic act of hyperbole, I decided that more must be learned about this newly discovered interesting quirk of evolution.
Get in there, my son.
Ah, the dance of life. You're born, you die, and in the middle bit you get to have sex and sire some progeny, thus condemning them to have to go through the same boring, predictable existence as yourself. That is... assuming you've got what it takes to actually get with a member of the opposite species in the first place. There's lots of competition out there for mates, and evolution favours those individuals with traits that advantage them to being the pick of the crop, judged by looks, strength and/or the ability to provide shelter, food, and so on.
These fertilisation advantages manifest in a variety of ways across the animal kingdom, for example, you could just put all your resources into producing the hardiest sperm possible – the most bang for your buck, as it were. However, producing Grade A sperm isn't cheap, resources-wise, and so it is usually more efficient to establish a mating advantage behaviourally rather than biologically; this is called 'mate competition'. One version of mate competition is pair bonding, where a strong affinity develops between two individuals of the same species, who then go on to mate, with each of them eventually caring for the welfare of their offspring. These pair bonds are of varying lengths and strengths depending on the species, from the lifetime pair-bonds of swans to the dynamic pair-bonds seen in gibbons. However, who wants just the one mate when you can have the pick of all the women around you? Other species, e.g., peacocks and antelope, square up and have a 'tournament' to fight for the prize of who gets to have a lot of sex this week: sometimes the tournament is just a show of size, strength and virility, allowing the females to pick their suitor; other times it's a fight to the death and the winner takes it all. Both lion prides and gorilla troops are led by a single male whose leadership must be challenged and won in order to take over the herd and be the main gene-donor to all the females contained within.
Now, I can already hear the cries from the XY camp of 'why do we have to do all the work?' Well, pretty much because you want your genes to be passed on. Females are guaranteed by biology that the children they produce will be half them; males get to ensure this by stamping out the competition. If you don't want your genes to be passed on, then you don't have to go to all of this hullabaloo. Those that do want their genes passed on, do; sometimes they take it to the next level. Lions, on taking over a pride, will kill all the young so that the nursing females will be free to mate; wild stallions will harass any pregnant mares in his newly acquired herd till they miscarry; some male rodents are able to "clear the way" by secreting a pheromone that causes failure of egg implantation in already impregnated females. The act of achieving procreation is a messy, cutthroat affair. Do not enter into it lightly.
Sperm competition
Females of all species are a sneaky gender – those of you who've already read cryptic female mate choice will have had a glimpse of just how sneaky when it's necessary to ensure the long-term survival of their own young. I'd like to carefully skirt the discussion of just how evil female-kind is for doing this, and concentrate on what the males do to ensure that their genetics live on once they themselves have passed on to the next life. Aside from aforementioned advantageous behavioural traits, he can also ensure his genetic domination by making sure that, in the event of a female mating with more than one male, his boys are the ones that win the race. The fantastic term used to describe these evolutionary and biological adaptations is 'sperm competition'.
Two obvious examples of cornering the market with your brand of sperm is the development of larger testicles in order to produce more sperm (playing the numbers game); or increasing the actual quality of the sperm such that it is hardier and more able to negotiate the inhospitable female genital tract whilst simultaneously fending off all comers who seek to thwart them in their goal. You know, when I phrase it like that, I get a mental image of fertilisation being a 1950's pirate movie where they board the baddies' boat during a tempestuous storm to go rescue the princess (or similar).
Yes, yes, I know. I need to get out more.
Anyway, getting around to the point (as I eventually do), 'poisonous sperm' is a consequence of one particular type of sperm competition, where the seminal fluid of the males contains proteins that reduce the ability of other male's spermatozoons to successfully fertilise the eggs. In Drosophila melanogaster, the common fruit fly, these proteins are actually toxic to the female fruit fly; this is an example of the conflict that arises when traits that increase reproductive success for one gender actually reduces the chance of reproductive success for the other. The fruit fly is one of many animal species in the world that practices multiple mating – in the wild, females may mate with up to six different males; what, then, is the species-survival advantage provided by the female flies mating such a number of times that she is actually cutting short her own life by doing so? Well, there isn't really an answer to this. Drosophilia melanogaster has had the pleasure of being under constant scientific scrutiny now for over a century because they are the perfect specimens in the field of genetic research: they only have four chromosomes; they are small and easy to breed; they have a lifespan of 30days and a generation time of ~10days from egg to adult; and they lay a large number of eggs, ~400 in total by each female.
One way of looking at it is that there probably isn't an advantage to a reduced lifespan in the females, but there also isn't a disadvantage; from the female point of view there's no genetic push towards males who secrete less of this seminal protein, but from the male point of view there is a genetic push towards secreting the protein. I suspect that another way of looking at it is that what the female loses in lifespan, the species gains in genetic diversity.
Like I said: the act of achieving procreation is a messy, cutthroat affair. Do not enter into it lightly.
References
- Wiki article on animal sexuality: http://en.wikipedia.org/wiki/Animal_sexuality
- Imhof M, Harr B, Brem G, Schlötterer C, 1998, "Multiple mating in wild Drosophila melanogaster revisited by microsatellite analysis", Molecular Ecology; 7:915-917
For the record, in the human species, male semen is not that welcome an addition to the female vagina, even if she has been screaming your name for the last twenty minutes. The human vagina is, admittedly, a fairly hostile place; it's naturally acidic because of the Lactobacillus microfloras that reside there. Also, the tissues that make up the vagina are brimming with immune cells; sperm does not have an easy time of it once it's left home base. But there's a reason for this; an upset to the native microflora equilibrium allows other not-so-nice microscopic inhabitants to come to prominence; thrush and bacterial vaginosis (BV) are usually the result, and neither condition is a particularly pleasant experience. With the normal flora in place, the vagina is self-cleansing, and I'd happily go out on a limb and say that most vagina owners prefer it that way. However, male semen doesn't think like that, the selfish bastard. Because sperm can't survive the acidity of the vagina, the seminal fluid that contains the sperm is made alkaline thanks to the inclusion of various basic amines. This can sometimes mess with the native environment, and it's not uncommon for a woman to develop BV soon after starting sexual relations with a new partner.
Another hazard contained within semen is its tendency to harbour sexually transmitted infections, including and not limited to: Chlamydia, gonorrhoea, syphilis, HIV, hep B, HPV, and Trichomoniasis.
Oh, and let's not forget human seminal plasma hypersensitivity; previously thought to be a rare occurrence, it is now recognised as being a lot more common than previously thought, though the jury's still out on providing an accurate population prevalence percentage*. It is an allergic reaction on contact with seminal fluids that can vary from local itching and hives, to whole-body itching, blisters, facial oedema, and breathing difficulties. A fatality has yet to be recorded, but that shouldn't stop any of you crazy atopic kids out there from trying.
And last but not least, it's chock-full of spermatozoons, and invariably one of the little fuckers will manage to break on through to the other side; nine months later you're knee-deep in nappies and threatening to rip the nutsac off any man who even thinks of touching you ever again.
Okay, that might just be me.
*I would just like to also say how proud I am that the second returned entry for human seminal plasma hypersensitivity on google is Us. Well done, Us. Unfortunately the 5% quote is a 'made up on the spot, sounds about right' figure – I looked through several journal papers without success for an official figure but none seems to be available. It's only in the last three years that people have started to include human seminal plasma hypersensitivity as a differential, so hopefully there'll be more research published soon.
Last but not least, can I please direct you to Lord Brawl's fantastic writeup, hostile mucus, which nicely complements my writeup with a discussion of women's gential secretions being not so nice either. Another thing that's not so nice, now that I think about it, are the words 'genital' and 'secretions', when placed side-by-side. Euw.