At this point, the sperm, fully developed but still sluggish, set out on the first and last journey of their lives. They pass into the epididymis, a small convoluted tube encased in tough protective tissue. Stretched out to its full length, the epididymis is about twenty feet long. Yet, in its casing within the scrotum, doubling back on itself continually, it measures only about an inch in length. Slowly, sluggishly, for ten to fifteen days, the sperm move through the twenty feet of tube, maturing as they go. Then they enter another tube, the vas deferens, which continues for an inch and a half before swelling to form a waiting station for the sperm, pending their journey into the outside world.
When sexual stimulation occurs, the sperm finally get excited. They begin dashing around in something of frenzy. Their behaviour may be influenced by secretions from another organ, the seminal vesicles, according to some authorities. They secrete a sugar-rich fluid into the waiting areas, and in some way the sugar may energize the sperm. As lovemaking proceeds, their departure approaches. The sperm slip into the urethra, the passage through which they will leave the male body. Surrounding the urethra on all sides at that point is a gland approximately the size and shape of a walnut: the prostate. The urethra runs through its Center. As sexual stimulation increases, the prostate grows firm. At the moment of orgasm it begins to spasm, and the sperm are instantly deluged with a milky, sticky fluid produced in the prostate and squirted out of it by the contractions. What we would now call semen, the fluid containing the sperm, is then ejected by strong muscular contractions from the penis into the dark, moist and potentially hostile environment of the vagina.
From a man's point of view, as a man and woman make love, he thrusts his penis in her vagina, and, after a few moments or a few minutes of intercourse, depending on his staying power, he feels that glorious moment of impending ejaculation, and then falls headlong into his orgasm. As he ejaculates, his penis ejects up to 500 million spermatozoa into his partner.
But as we have seen elsewhere, a large proportion of the semen evaluated into a woman's vagina is expelled again quite soon after intercourse as flow back. How, then, does a woman get pregnant?
There isn't an open channel between the vagina and the uterus. The neck of the cervix, which is effectively the entrance to the uterus, is not empty. It is filled with mucus, which serves both to prevent bacteria entering her uterus and to give her some control over how many - and, if she has mated with more than one man, whose - sperm can enter her and potentially fertilize her egg. This mucus contains both fibres and extremely narrow channels, the latter no wider than the head of a sperm.
A woman's ability to block or clear the channels in her mucus, by changing its consistency or flow rate, or by blocking it with menstrual debris, white blood cells or even invading sperm, is the primary weapon she has in fending off unwanted sperm and encouraging the sperm from men she has decided would be suitable fathers for her children. According to Baker, for example, if she is motivated to have a child by a man who is not her long-term partner, she will unconsciously manipulate the time of intercourse and the composition of her cervical mucus so that she maximizes the chance of being fertilized by the "right" man.
As you may have guessed, evolution has not left men behind in the battle to fertilize a woman's eggs. But a man's sperm have two potential obstacles to overcome: the first, a woman's reluctance to be fertilized by him; the second, the competition of any sperm left inside her by another male. We'll see how men overcome this in a moment.
In the moments after intercourse, the cervix dips down into the seminal pool left by the man's ejaculation, gapes open, and a complex interaction between the sperm and the seminal pool develops during which sperm leave the pool and enter the cervical mucus. How many of the sperm make it depends on the woman's receptiveness: as described above, she can control this and often does so - during pregnancy, for example, no sperm can enter the cervical mucus and they are all ejected in the flow back. But the other factor which controls how many make it is whether the woman has an orgasm or not: the passage of sperm into her cervical mucus is greatly enhanced if she orgasms, for the contractions of her muscles repeatedly dips her cervix into her mate's semen and also opens up the channels in her cervical mucus to allow many more sperm to pass upwards out of her vagina.
So, every healthy sperm struggles for survival against overwhelming odds when it arrives in the vagina. Almost none will reach the breeding ground and fulfil themselves by reproducing. But what happens to the ones which are allowed into the cervix? Therein lies another, most incredible story.
Each time a man ejaculates, he ejaculates no more sperm into his partner than are needed to top up (to the ideal number for fertilization to have most chance of occurring) those already there from the last time they had intercourse. The reason for this is easier to understand if you follow the activities of the sperm after they are introduced into the vagina. A small number of sperm swim straight through the cervix and into the uterus, then on into the oviducts, where they temporarily rest. Eventually they may meet the egg on its way down the oviduct to the uterus. Other sperm come to rest in the walls of the cervix. After
A man's reproductive strategy is based several things. First, on ejaculating as many sperm as are needed to keep the "reservoirs" in the walls of his partner's cervix topped up, so that in effect his partner has fresh sperm in her oviducts all the time. If more than a week has passed since the last session of intercourse, he will ejaculate a full load of sperm - up to 500 million. If it's been only three days since the last session of intercourse, he ejaculates only 200 million. Remarkably, the number of sperm he ejaculates also seems to vary depending on how much time the couple have spent together since their last intercourse - the less time they spend together, the more chance the woman has to be unfaithful, and the more sperm he ejaculates.
So the number of sperm he ejaculates is an unconscious process determined by these two factors. But how does his body do this? Simple - how many sperm he ejaculates depends on how much sperm his body moves from his epididymis into his urethra in the moments before he ejaculates. (You feel your sperm moving out of the epididymis as a subtle swelling sensation, or a feeling of fullness, at the base of your penis in the seconds or minutes before you reach the point of no return, the point where you know you are going to ejaculate. If you watch out for it, you'll feel it.)
Second, sperm are not all created equal. As we saw above, some sperm stay in the cervical mucus - their function is to block the channels through the mucus so that other men's sperm cannot physically penetrate it. These blockers are large sperm designed for the job. But there are other types of sperm, too: egg getters and killers. We'll come back to this in a minutes
Third, a man can physically guard his partner from other men by spending time with her. As we saw above, his body adjusts how much he ejaculates according to how much time they have spent apart, and therefore how possible it is that another man may have inseminated her.
But let us now suppose that a woman has - at some level, conscious or not - chosen a lover because he represents a better bet for fathering her child than her partner. She is going to be unfaithful to her partner, and the first battle in the forthcoming sperm war is that when she commits her act of "adultery", her lover will inject many more sperm into the woman than her partner already has in her - assuming, of course that the lover is inseminating her much less often than the partner. So he begins the war to fertilize her egg with a bigger sperm army. And because she has been aroused and wet all day, excited in anticipation of her infidelity, much of her partner's blocking sperm located in her now freely flowing cervical mucus have been swept away - another disadvantage faced by her partner's sperm in their battle to fertilize her egg.
There's another type of warrior sperm, a killer, which seeks out any sperm that don't match it, and kills them with enzymes located on its head. The woman's partner has some of his killer sperm in her uterus from his last ejaculation, but so of course does her lover - and he has many more, since he has ejaculated a huge army of all kinds of sperm into her very recently. These sperm will seek out and destroy her partner's sperm, just as her partner's sperm are doing to her lover's - but the greater numbers injected by her lover will win this battle, leaving the lover's egg-getters much more likely to fertilize her egg. In such a way can a man be "tricked" into raising a child who is not his own. That would be a reproductive success story for the biological father, and the mother, but potentially a disaster for the supposed father, especially if he does not have anymore children of his own.