by Robin Baker
There is one further stage in her body’s strategy which her conscious mind is most unlikely to fathom. When she gets home, she works very hard to have sex with her partner. Consciously, she will have seen this as helping to avoid detection. If she can get her partner to inseminate her, any tell-tale damp patch on the sheets or any smell of semen will not arouse his suspicions. What her conscious mind will not realise, though, is that having collected sperm from her ex-boyfriend, her body is now very keen also to collect sperm from her partner. Her body has already decided that, on balance, her ex-boyfriend would make a better genetic father than her partner. The one thing it doesn’t know is how their ejaculates compare. She wants to have her egg fertilised by her ex-boyfriend only if his ejaculate is also the more fertile and competitive. The way for her to discover this is to pit one ejaculate against the other. In other words, her body wants to promote sperm warfare between the two men, and this is probably her only chance ever to do so.
Once a woman’s body contains sperm from two or more different men, those sperm compete for the prize of fertilising her egg. But the contest that takes place is not a simple game of chance, nor is it just a race. It is indeed a war – a war between two (or more) armies. And it is this warfare between ejaculates, or the threat of it, that has shaped the sexuality of every man and woman alive today, as well as the sexuality of just about every other animal that has ever existed.
Sperm warfare is more common and more important than most people suppose. A recent study in Britain concluded that 4 per cent of people are conceived via sperm warfare. In other words, one in every twenty-five owe their existence to the fact that their genetic father’s sperm out-competed the sperm from one or more other men within the reproductive tract of their mother. If this does not seem very many, it nevertheless means that since about 1900 every single one of us will have had an ancestor who was conceived via sperm warfare. Every one of us, therefore, is the person we are today because one of our recent ancestors produced an ejaculate competitive enough to win a sperm war.
Most often, because a woman only produces one egg at a time, sperm warfare can produce only one winner, as in Scene 6. Occasionally, however, a woman produces two eggs at the same time and gives birth to fraternal twins. Under such circumstances, a different outcome of warfare is possible – a draw. There are several remarkable cases on record – most obvious when the two rivals in sperm warfare are of different races – in which fraternal twins have different fathers.
Let us go back to the moment of conception that we witnessed earlier. Three sperm arrived at the egg simultaneously. They were all from the woman’s lover. If we move back down her oviduct to the collection of sperm waiting quietly in the rest area for their turn to swim up the oviduct, we find that nine out of every ten are also from the lover. The woman’s partner had lost this game of sperm wars in a big way, even though he had done all he could in preparation. We are back to routine sex.
So far, we have interpreted a man’s interest in routine sex simply in terms of his attempt to maintain a steady flow of fertile sperm through a woman’s oviduct. Routine sex, however, does more than just top up the partner with fertile sperm: it also prepares for sperm warfare. Moreover, the level of preparation depends on the risk of war. In deciding how many sperm to load and ejaculate during routine sex, the man’s body weighs up the chances that his partner might contain sperm from another man. His body does this very simply by registering how much time he has spent with the woman he is about to inseminate since they last had sex. If they have not had sex for over a week, then he registers how much time he has spent with her over the past eight days.
Crude though this strategy might seem, it does work. The less time a man spends with his partner, the greater the chance of her infidelity. If he spends more than 80 per cent of his time with her, there is virtually no chance of her being unfaithful. But if he is with her for as little as 10 per cent of his time, there is over a 10 per cent chance. In other words, as far as the man’s body is concerned, if he has spent less time with his partner since they last had sex, there is a greater chance that when he inseminates her her body will already contain sperm from another man. To increase his chances of winning the sperm wars that might follow, he needs to introduce more sperm. And this is just what he does.
The difference in the number of sperm he will ejaculate, depending on the circumstances just outlined, is quite large. In the scene of infidelity we have just witnessed, the woman returned home on the Wednesday night and had sex with her partner. In deciding how many sperm to introduce, his sleepy body will first have registered that it was three days since they last had sex (Sunday). The average top-up for such a gap is about three hundred million sperm. His body next registered that they had been together about 50 per cent of the time since then, giving an average chance of infidelity. So the average top-up of about three hundred million is about right, and that’s the number of sperm his body should have loaded and then ejaculated. If the couple had been in each other’s company continuously during those three days, the risk of her infidelity would have been near zero and he would have introduced only about a hundred million sperm. On the other hand, if one or other of them had been away from early Monday morning to late Wednesday night, the risk of her infidelity would have been greater and he would have injected about five hundred million.
For the woman’s lover, the situation was quite different. It was the first time he had inseminated this woman for over six years. Moreover, even over the last eight days he had spent only a few hours with her. His body judged, correctly of course, that her body had a very high chance of containing sperm from another man, and will have responded by loading and ejaculating six hundred million. Half an hour later, he injected a further hundred million or so. As far as ‘Infidelity Wednesday’ is concerned, sperm wars began with the lover putting a sperm army into the woman’s tract that was twice the size of her partner’s.
So the lover had a twofold advantage over the partner from the very beginning. By the time the war was coming to its climax and the prize of fertilisation was nearly ready to be claimed, however, the lover had a ninefold advantage. (Remember that in the rest area of the woman’s oviduct, nine out of every ten sperm waiting to set off for the fertilisation zone were the lover’s.) What had happened during sperm warfare to shift the odds even further in the lover’s favour? The first step in finding out is to meet and appreciate the soldiers themselves – the sperm.
The most common sperm in the human ejaculate is, of course, the magnificent, sleek athletic cell that most people know, with a head, a mid-piece and a long, slender tail. The head is paddle-shaped, oval in outline but flattened and wearing a cap. This cap is filled with important fluids. Inside the head, densely folded, is the package of DNA, the genes that a fertile sperm will deliver to the heart of the egg. The head perches like a lollipop on the short, stiff mid-piece which is the sperm’s powerhouse, the place where stored energy is mobilised to activate the tail for swimming. These sleek individuals travel effortlessly through the female’s body fluids, pushed forward by elegant waves whipping in slow motion down the length of their tail.
Familiar though this image will be to most people, such sperm comprise only just over half of the total in a normal ejaculate. A sperm army is a much more motley collection of characters than most people expect. For example, some sperm have a big head, others a small one. Yet others, the pin-heads, have a head that is so small that there is no room to carry the genetic package of DNA. Some sperm have a round head, some a cigar-shaped one, some a pear-shaped one, some a dumb-bell shaped one, and some a head that is so irregular in shape that it defies description. Some sperm, the real monster troops, have two, three or, very occasionally, four heads.
The shape of the head is not the only feature to differ. Some sperm have short tails, some have tails coiled like a spring, and some have two, three or occasionally four tails. Some sperm, the hunchbacks of the force, have mid-pieces that are bent into a right-an
gle. Yet others, like hikers carrying a rucksack, have bags of cell material on their mid-pieces. On average, only about 60 per cent of the army are the familiar, sleek athletes; the remainder are this collection of deviants. All, however, have an important part to play in sperm warfare.
On ‘Infidelity Wednesday’, it was the performance of the lover’s army as a whole that helped to increase the odds in his favour, as the sperm war progressed, from 2:1 to 9:1. To find out how, we need to climb into the woman’s body with a microscope and follow the war in detail through all its campaigns. We will begin at the moment the woman and her lover walk into his hotel bedroom, undress and begin to have sex on the floor.
SCENE 7
A Sperm War
As the woman and her lover sink to the floor, only moments away from intercourse, her body already contains sperm. Her partner inseminated a total of six hundred million during their routine intercourses the previous weekend. Most were ejected in her various flowbacks, but even so some are still inside her. Their ability to influence the outcome of sperm warfare, however, depends on where they are.
A few ineffectual sperm are at the top of her vagina, carried there by cervical mucus, which has been dripping out of her cervix and oozing down her vagina all day in anticipation of this moment of infidelity. Each drip of mucus carried with it a few of her partner’s sperm. As these sperm were lost from the future battleground in her cervix, they were partly replaced higher up by the last handfuls of sperm from her cervical crypts. These emerged and entered her mucus channels in a vain attempt to make good the numbers being lost into her vagina. However, the numbers being lost were greater than the number of their replacements, and all day her partner’s cervical defences had been slowly declining.
The sperm lodged in her cervical mucus are not the sleek types already referred to. Instead, they are sluggardly blockers – sperm whose role is to prevent any later sperm from passing through to her cervical crypts and womb. Sperm with coiled tails, a bent mid-piece, a large ‘rucksack’, a large head, or with two, three or four heads can block very effectively any of the very narrow mucus channels in which they lodge. So, too, can two sperm side by side. As her lover thrusts inside her, however, relatively few of her mucus channels are still blocked by this rapidly dwindling collection of her partner’s sperm.
These blocking sperm are not her partner’s only defence inside her body. Roaming around in the void of her womb are a few more of his sperm, though these too are dwindling in numbers. These sperm look familiar. They are svelte and athletic, but they are not there to fertilise. These are killer sperm, roaming around in search of sperm from other men to destroy. Each time a killer encounters another sperm, it tests the chemicals on the surface of the other’s head. If those chemicals are the same as on its own head, the killer recognises an ally and moves on to continue its search. So far, all encounters inside this woman have been with allies and the killers’ deadly services have not been needed. Many are now beginning to move slowly, and large numbers are dying of old age. The weakest have been in her womb for three days. The more active are more recent arrivals from the reservoirs in her cervical crypts.
The woman’s womb is not the only territory stalked by killers. A few more are scattered along her oviducts. There is even one swimming solitarily in her body cavity near to her left ovary. These killer sperm in her oviducts accompany the last handful of the partner’s fertile sperm, the egg-getters. Killer sperm and egg-getters look very similar. They are both sleek and athletic in form but, whereas the killers have average-sized heads, the heads of the egg-getters are slightly larger. If the woman ovulated now, her partner would still have a good chance of fertilisation. But ovulation is still two days away, and war is about to begin.
After very few thrusts, the lover deposits his seminal pool in the woman’s vagina. Her cervix dips into and stays in the pool, and the vanguard of his army begins to stream into the channels of her cervical mucus. This army contains about five hundred million killer sperm, about one million egg-getters, and about a hundred million blockers. Some are denied passage through mucus channels by the partner’s blocking sperm. So few of these blockers remain, though, that almost all of the cervical channels are now clear. The invaders pour through in waves. A few hundred egg-getters with support from killers travel straight through the cervix into the womb, heading immediately for the rest area in the oviduct. The remainder of the egg-getters, some of the killers and the youngest of the blockers – a few million in all – head for the cervical crypts. They pour in, settle down, and await developments. The remainder of the killers travel more slowly through the cervix into the womb, leaving behind the much slower blockers. These latter distribute themselves throughout the mucus channels, then settle down, many immediately coiling their tails as if in anticipation of a long wait.
Some of the vanguard of the lover’s egg-getters do not make it to the oviduct. As we have seen, the partner has relatively few killer sperm still active in the womb, but those that are there do their best to stem the lover’s tide. As soon as a killer from either man first encounters a sperm from the rival, it is alerted that war has begun. For an hour or so, the killers from both men swim much faster than normally, seeking out as many rival sperm as possible. Their aim is to poison the rival’s egg-getters and killers using the deadly cocktail of fluids in the cap which they each carry on their heads. They do this via head-to-head combat. First, as we have seen, they probe with the tips of their heads at every sperm they encounter, comparing its surface chemicals with their own and checking for similarities and differences. If a killer finds a sperm from the rival’s army, it tries to jab the deadly tip of its head against the vulnerable side of its opponent’s head, applying a small amount of corrosive poison with each jab. Having jabbed several times it moves on, leaving the other sperm to die.
A single killer sperm carries enough poison to kill many sperm from the rival army, but gradually its cap runs out of chemicals, for it has no reserves of energy to make any more. In a last-ditch attempt to kill just one more sperm, it tries to stick its head to a rival’s and apply the last drops of its lethal fluids. As the war progresses, there is a gradual increase in these pairs of dead and dying sperm, joined at the head in a deadly embrace.
In this initial skirmish, one or two of the partner’s killers do their job and some of the lover’s egg-getters and killers die from head-to-head combat, their heads coated in the poison. Any initial success of the partner’s sperm, though, is short-lived. In their turn, they are found by the invading bands of killers from the lover which accompany his egg-getters. In a frenzy of kamikaze mayhem, killer sperm from both sides attempt to annihilate each other’s troops. Outnumbered by at least a thousand to one, however, the last remaining sperm from the partner are soon killed.
The battlefront now moves into the oviducts, where the killing continues. With only a few losses of their own, the lover’s sperm systematically wipe out the partner’s last remaining egg-getters and killers. By the time the woman and her lover have sex again, an hour later, the first battle is over and none of the partner’s sperm remain alive inside her. Actually, the role of the second insemination in this particular sperm war is more complicated than it seems – but here that discussion would be a distraction.
So far, our war has been so one-sided that it has been little more than a brushing aside. The main battle is still to come. It begins when the woman gets home and her body urges her to sit astride her partner, push his penis into her vagina, and stimulate him to ejaculate. By doing so, she has staged a real war. Nevertheless, even though her partner now enters a fresh army of three hundred million sperm into the arena, the contest is still going to be one-sided.
As soon as the newly introduced sperm from the partner attempt to leave the seminal pool, they encounter problems. The channels of the woman’s cervical mucus are nearly all blocked, not only with sperm from the lover, but also with white blood cells from the woman herself. The huge number of the lover’s sp
erm and the matching number of white blood cells are doing their job almost perfectly, and the partner’s sperm are much more hindered in leaving the seminal pool than were the lover’s, a couple of hours earlier. Queues of the partner’s sperm develop in the blocked channels, producing tailbacks all the way to the seminal pool. As a result, only a small proportion of the partner’s army manages to escape the pool before the woman ejects her flowback.
Even those sperm which do escape the pool and find a clear channel are not yet out of trouble. The small vanguard of egg-getters and killers which head straight into the womb find themselves having to run the gauntlet of hordes of the lover’s killers. Inevitably, one or two get through without being poisoned, but only to run into further problems as they then try to leave the womb. The entrance into each oviduct is narrow, only just large enough to allow easy passage to a descending egg. Moreover, both oviduct entrances are blocked by lover’s sperm and patrolled by killers, and many of the partner’s sperm are killed as they try to push through. Even those few that do escape and eventually arrive safely in the oviduct rest area are still at risk to the lover’s killers, which patrol the whole area.
