Last week we followed the early adventures of Mr. Sperm as he grew from a run-of-the-mill round cell into a lean, mean fertilizing machine. The last we saw of him, Mr. Sperm had just been ejaculated into a vagina; about to begin the spermatological equivalent of running the Boston Marathon. Sure, only one can win but they all get to participate, right? Wrong. For a closer analogy, think of the first fifteen minutes of Saving Private Ryan.
Firstly, the sperm have to get off the beach. The vagina is an acidic environment. It has to be to prevent incipient infection. I know it doesn't feel that way. I know it seems like the warmest, yummiest place imaginable, but your skin has a much broader range of PH tolerance than does your ejaculated sperm. The sperm are protected briefly by the seminal plasma which provides a very short-lived alkaline environment. The seminal plasma also contains a gelatin-like substance that prevents it from draining from the vagina too quickly, and fructose from which the sperm can draw energy. Your ejaculate really is like an army in hostile territory. But unless they can breach the initial defenses and get into the safety of the uterus within hours, if not minutes, it will be more Dieppe than D-Day for your wiggling troops.
Trouble is, to get into the womb your sperm must first get past the cervix. The cervix is the woman's main line of defense against microbial infection. Most of the time, it's a veritable fortress, plugged with mucous. This is primarily an evolved mechanism to keep bacteria out. Unfortunately, it also acts as a formidable obstacle for sperm. For 90 percent of the time the mucous is highly viscous. A few strong swimmers may make it through but not enough to seriously threaten impregnation. But for just a few days a month, shortly before ovulation, the mucous becomes watery and tiny channels form that the sperm can swim through.
Had they landed a day or two earlier, Mr. Sperm and his brethren would have been just another lost episode in reproductive history. As it is, they caught the right phase of the moon. The woman's body they now inhabit has just begun ovulation. The mucus has thinned and Mr. Sperm and his comrades begin to make their way through the tiny channels towards the womb. Most of them don't get very far though because unbeknownst to the sperm and the man that produced them, somebody's already been here.
It's been estimated that up to 10 percent of any given population are the products of an extra-marital affair. In the vast majority of these cases, the children and the fathers who raise them have little or no idea that they are not biologically related. Cuckoldry, the fine art of tricking a male mate into raising offspring that aren't his progeny is a well established reproductive strategy for almost every species that pair-bonds. This includes most species of birds and a number of mammals. The idea behind this is to not just ensure the survival of the offspring but to enhance the offspring's chances of being reproductively successful.
Mr. Sperm's originator (let's call him Lloyd) may be a good provider but he's geeky, gawky and nobody's idea of a Friday night shagfest. Mr. Sperm himself carries a subset of these same geeky, gawky genes and if he makes it, the boy that is produced will likely also have a hard time getting laid. Not so for the sperm that were deposited the previous night. They belonged to Raoul, a charming tango instructor and strapping hunk of a man with a package that's the talk of the dance class. The boys that his sperm produce will break many hearts and likely leave as many offspring. Had the woman (let's call her Bernice) gone to the dance class any other time of the month it's likely that nothing would have happened. But the hormone surge that directs the release of the egg from its follicle had the collateral effect of making Bernice horny. Not horny for hubby mind you, but horny for a strapping hunk of a man with a package that's the talk of the dance class. It's been estimated that over half of infidelities occur when a women is ovulating. There's nothing conscious behind this of course, just a couple of hundred million years of evolution, but it's had the effect of making life very difficult for Mr. Sperm and his comrades.
This is because in any given ejaculate, about 40 percent of the sperm are flawed. They have two tails, three tails. They swim in circles. They swim the wrong way. They don't stand a hope in hell of impregnating anybody. Until recently it's been assumed that these sperm were the result of natural flaws in the spermatogenetic process. But recent advances in the understanding of sperm competition have led to speculation that these sperm serve an evolutionary function by blocking the low-viscosity channels that the sperm, subsequently released, must traverse. This is still a controversial idea and because the mechanisms of spermatogenesis are as yet poorly understood, it's far from universally accepted, but there is logic to it. Robin Baker, the leading proponent of this idea, has also conjectured that a significant proportion of the remaining sperm (up to 98 percent) exist only to act as "killer" sperm, designed to attack any other sperm which sports surface molecules that differ from it's own. The "Kamikaze sperm hypothesis" has been somewhat discredited lately but the debate isn't over and I'll be discussing it more at length in a future column.
Wherever the truth lies in the Sperm Wars debate there is no denying that Raoul's defective sperm, which are now blocking the mucus channels, have put Mr. Sperm and his genetic siblings at a disadvantage. Luckily, the channel that Mr. Sperm swims up is largely clear and he and several of his comrades gain entrance to the uterus. Mr. Sperm is one of the front runners of his pack but he's a full 12 hours behind the front runners from Raoul's ejaculate which are by now most of the way up the fallopian tube. Ordinarily this would give Raoul's sperm an almost insurmountable advantage except for one small fact. The egg has yet to be released. It can take up to 36 hours after the lutenizing hormone surge for the affected follicle to release its egg. Luckily for Mr. Sperm (and Lloyd), Raoul's best sprinters showed up too early. This levels the playing field somewhat. A series of uterine contractions propels Mr. Sperm through the length of the uterus in under 30 minutes. Upon arriving at the top of the uterus he has the option of swimming up one of two fallopian tubes that each lead to an ovary. For Raoul's sperm this was pretty much a 50/50 gamble. But by the time Mr. Sperm arrives, Bernice's ovary has begun releasing chemoattractants into the fallopian tube. The mechanisms of chemotaxis (the ability of sperm to follow the trail to the egg) are still poorly understood, but it's likely that sperm are programmed to follow a chemical gradient, much as dogs follow a scent.
On the trail of this scent, Mr. Sperm enters the fallopian tube. Already, the chance of any given sperm entering the fallopian tube - even during the ovulation period - is less than one-in-a-million. Thanks to Raoul's deformed laggards, the odds against Mr. Sperm are even higher. He is one of only a couple of dozen (out of the four hundred million originally ejaculated) that have a realistic chance of getting to the egg first. Raoul's sperm already in the fallopian tube outnumber Lloyd's by a factor of ten.
Once inside the tube, Mr. Sperm and his comrades become capacitated (armed). It's believed that chemicals in the fallopian tube trigger chemical changes on the surface of the acrosome, a membrane that covers the head of the sperm. Without these changes, Mr. Sperm is incapable of delivering the enzymes that allow him to burrow through the protective layers of the egg's surface.
By now, the egg has been released and already Raoul's sperm have started worming their way through the cumulus cells that protect the egg. As Mr. Sperm approaches, the chemical gradient surrounding the egg causes Mr. Sperm to become hyperactivated. He begins swinging his head wildly back and forth. This motion clears a path through the cumulus cells. Some of Raoul's sperm have already reached the egg's thick protein shell called the zona pellucida. But admission through the zona is determined less by first-come-first-served, than by a precise docking of surface proteins. By chance, when Mr. Sperm reaches the zona, his proteins match-up perfectly. He becomes locked in and undergoes what is known as the acrosomal reaction. The top of Mr. Sperm's head disintegrates releasing enzymes that dissolve a hole through the zona. By now, a couple of Raoul's sperm are burrowing their own hole. But it's Mr. Sperm that reaches the egg first. The membranes of the two cells fuse, and Mr. Sperm disgorges his genetic material into the cytoplasm. Job done! Almost instantaneously a cellular reaction occurs that hardens the egg surface, effectively locking out Raoul's sperm and, ironically, the tail that served Mr. Sperm so well.
So Mr. Sperm wins the most important battle that any of us will ever undertake. Was it a case of the best sperm wins? Probably not. It's a popular misconception that sperm competition is a Darwinian struggle to ensure the best genes are delivered. As far as we know, the fitness of any one sperm is a function of cytoplasmic determinants, not the cell's genes. Who you are, it seems, is almost entirely a matter of luck.
Nine months later a geeky, gawky baby boy (Lloyd Jr.) was born to beaming parents. Six months after that Raoul knocked up another dance class participant and is now being sued for paternal support.
