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 egggetters
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 egggetters
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 me 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 In 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 sperm 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.
Lower down in the woman's tract, in her cervix, many of her
partner's sperm are trying to enter her cervical crypts. But the crypt
entrances too are patrolled by killers and, in any case, inside they
are virtually full of the lover's sperm. Just occasionally, some of the
partner's sperm stumble across a channel leading to an empty crypt, but the majority get stranded in
the mucus where they fall prey to the combined forces of the
lover's sperm and the woman's white blood cells Clearly, the Wednesday encounter in this war has gone very
much in favour of the lover, and nothing much happens over be
next two days to redress the balance. As the woman goes about as
normal on Thursday and Friday, the population of blocking sperm
from both men in her cervical mucus slowly declines. Some drip
into her vagina, carried by mucus. Others are mopped up by
rearguard white blood cells. Even when some of these blockers are
replaced higher up in her cervix with sperm from the cervical
crypts, the new recruits fail to make good the loss and the blockers
dwindle. The killer sperm in her womb, after an initial decline
from battle losses, actually build up in numbers on Thursday with
a fresh recruitment from the cervical crypts. Then the killers also
begin to decline in number. A steady flow of egg-getters from both
men (but mainly the lover) leave the crypts, heading for the rest
areas in the oviducts. En route, they have to run the gauntlet of
rival killer sperm in the womb, and most of the partner's egggetters
fail because these killers are almost all from the lover. By
Friday night, with only hours now to go to ovulation, the partner's
egg-getters in the oviducts are outnumbered by about one hundred
to one.
When the woman and her partner have sex on the Friday night,
there is only one more hour to ovulation. Now, the partner's sperm
have a much easier passage through the cervical mucus because
the number of blockers is much reduced. Although most of his
sperm head for the now half-empty cervical crypts, a vanguard of
egg-getters and killers head straight for the oviducts. Many are
killed or slowed down in the womb by the lover's patrolling killers,
but enough get through to reduce the odds in the oviducts from a
hundred to one to about ten to one. It is at this point that the
woman produces an egg from one of her ovaries and a chemical
signal passes down the adjacent oviduct. This signal activates
hundreds of the sperm in the rest area, and a wave of sperm begin to make their way up the oviduct towards the
fertilisation zone. It is now a race, or rather an obstacle race because
there are still killers in the oviduct, mainly from the lover. The
partner's sperm, particularly those few which have just arrived straight
from the insemination, are actually fastball than the lover's - if all else
were equal, the partner could still win the prize of fertilisation.
But all else is not equal. One after another, the partner's egg-getters
run into the lover's killer sperm. As the egg reaches the zone of
fertilisation in the oviduct and the first sperm arrive, the odds against the
partner have reduced to five to one, but that is not enough. The first
three sperm to arrive are all from the lover, and one of these claims the
prize. An hour later, with the partner's fresh sperm now overwhelming
the lover's at all points in the woman's tract, the odds in the oviducts
swing heavily in the partner's favour. But it is too late. The lover has
made it and the woman's daughter, to be born in nine months' time, will
not have been sired by the man she will call her father. But nobody will
ever know.
