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The idea behind " sperm wars" is this: men and women compete, even if they don't know it, to produce the maximum number of offspring. This means the two members of a couple will not always be co-operating in their sexual behavior.
Evidence which suggests sperm wars really do happen comes from research done in the USA. Some intrepid sex-boffins built an artificial vagina and a variety of plastic penises, then experimented with starch suspension to represent semen. And guess what?
They discovered that the shape of the penis is exactly right for removing any semen that already happens to be in the vagina when a man has sex.
They discovered that the coronal ridge of the penis could scoop out more than 90% of the starch mixture in one of their artificial vaginas with just one thrust.
A false penis with no coronal ridge only managed to remove 35%. And the vigor of thrusting mattered too: a deeper thrust would scoop out more of the offending semen.
You may say that just happens to be a matter of chance. After all, other scientists have suggested the shape of the penis is designed to keep a woman's lubrication inside her vagina during sex.
Though of course those two ideas are not necessarily mutually exclusive. But here's another interesting fact that may support the idea of semen clearance: a couple's sex tends to be more energetic if a women is suspected of cheating, or if the couple has been apart.
This might imply there's a sub-conscious desire on the part of the man to rid his partner of any trace of another man's semen. (Alternatively, it might just mean he's horny because he hasn't had sex for a while.)
The research team concluded that the human penis was designed to "enable males to substitute their semen for the semen of their competitors. As a consequence of competition for paternity, human males evolved penises that displace semen from the female vagina left by other males."
The research is published in the journal Evolution and Human Behaviour.
A very different kind of evidence comes from some amazing and hard-to-believe statistics about how many children born into married couples or long-term partnerships are not actually fathered by the woman's partner, but by another man.
Here are the dates, locations and results of a small number of these studies.
The percentages are the percentages of children who were not the genetic
offspring of the apparent father:
Source: UK Child support agency website.
This data seems hard to believe because we have always been told that men are more likely to be unfaithful to their partner. These figures don't seem to bear that out, although we don't know how they compare with unfaithfulness in childless couples, or even if the woman above were so desperate to get pregnant that they chose to be unfaithful for that reason alone.
Anyhow, it's clear that something strange is going on. It's easier to understand what this might be if you take a sociobiological view.
Simply put, this means that you shift from seeing everyone as driven by social conventions (like " don't commit adultery" ) and instead think about which biological imperatives might lie behind their behavior.
And of course, then it all becomes much simpler: women are subconsciously looking for the best man (the fittest, healthiest, richest, most socially successful male), while men are looking to inseminate as many women as possible - and get as many pregnant as possible.
In biological terms, the only winners in the race to reproduce are the males and females of any species who leave most offspring in the next generation.
And for males, one aspect of this strategy is to inseminate many women, and to con the woman's mate into bringing up the children.
For women, seeking out a successful male's sperm may be a riskier strategy if she is dependent on her cuckolded mate to support her and her offspring, for he may leave her if he discovers the deception.
Now, all of this happens all the time in the animal kingdom, but it's unusual to think of ourselves as so driven by our animal nature that the placement of a penis in a vagina becomes nothing more than the expression of our deepest animal instincts.
Sadly, though, that often seems to be the case. Robin Baker has written a book called " Sperm Wars" in which he entertainingly outlines about thirty different human sexual scenarios and then offers an interpretation of the biological motives behind them.
For example, he describes a case of partner swapping between two couples, one of which had no children, and the other of which had two children.
The nature of the events that happened, and the timing and exchange of sexual favors between the women and the men, turns out to have been an instinctive strategy which resulted in both women getting pregnant by the man who previously hadn't had any children.
Yet the other man raises the child which his own partner produces as though it were his own child (rather like the statistics in the table above).
It's hard to accept that both the man and the woman in a couple would behave in a way that maximizes the chances of each of them passing on their own genes, even if that means a woman getting pregnant by a man who is not her partner or a man fertilizing someone else's wife.
But the sad truth is that nature is only concerned with the passage of an individual's genes to the nest generation.
How that is achieved is an irrelevance. Robin Baker's book makes highly entertaining reading and it's very instructive in understanding human sexual behavior.
You may even gain some insight into episodes of your own sexual life if you come across a story in his book which describes events similar to something that's happened to you in your own sexual career.
Baker relates the story of a couple who have sex, sex which leaves the woman feeling grumpily dissatisfied.
What she wanted was to come during foreplay and then have the pleasurable sensations of her partner's penis inside her after she had come what she got was her partner stopping foreplay just as she was about to come and then penetrating her and coming too soon.
(Find out more about ejaculation control here.) As he entered her, both her interest and her arousal decreased rapidly.
One hour later, though, they are both in bed and have an incredibly strong urge to have sex - quite unexpectedly: accordingly, he thrusts into her and they reach orgasm rapidly and passionately. What does this mean?
Baker interprets the events in terms of the changes that the woman needed to happen inside her vagina, cervix and uterus to maximize her chances of getting pregnant.
The cervix dips into the pool of semen in the vagina when a woman orgasms: this increases dramatically the number of sperm which reach the uterus and therefore have a chance of fertilizing her egg.
In this case, the fact that she had not reached orgasm during the first bout of intercourse meant that her body instinctively told her to have sex again and then ensured she came.
Additionally, the man's fresh supply of sperm would increase his chances of fathering a child, and they would be more likely to get into her uterus since she came during sex.
Events like this may be more common with a new partner: you have, at some level, selected a partner because they seem likely to be reproductively successful (we're talking instinctual behavior here, not conscious motives, remember), and the best way to capitalize on that reproductive potential is to get pregnant as quickly as possible.
The events in the story above - which you may yourself have experienced in real life as a wave of passion sweeping across you unexpectedly during the night after you have already made love - are instinctive ways in which the body can ensure the maximum chance of a successful fertilization.
Of course, such strategies are deeply instinctual - they are not affected by the reality of contraception or being on the pill or the threat of disease.
It's the profoundly powerful nature of our instincts that makes sex so hard to resist when we are excited, aroused and passionate. Also read about the sperms' journey to the uterus.
There are other male and female sexual dysfunctions, problems which are in fact much more common than most people realize.
We describe a treatment for the condition in which men have trouble reaching orgasm during intercourse on another of our websites this is also known as retarded ejaculation or delayed ejaculation.
Men who have anorgasmia (inability to reach orgasm) will find information here on achieving orgasm during sex and becoming fully orgasmic.
And because it depicts sensuous and loving sex it is perfect to view with your partner if you wish to introduce a new level of sexual excitement into your lovemaking.
Other pages on this site
Check out this great information on
premature ejaculation and how to
control it so that you can become a longer lasting lover in bed.