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Saturday, 18 March, 2000, 02:44 GMT
Absence 'boosts sperm levels'
Sperm count
Sperm counts can vary dramatically
Men's fertility levels almost double when they spend the majority of their time away from their partners, scientists have discovered.

A report in the scientific journal Personality and Individual Differences says a man's sperm count varies significantly depending on how much he sees his partner.

The researchers found that when couples spend all their time together men inseminate 389 million sperm per ejaculate.

This increase in sperm insemination increases the odds of crowding out and displacing a competing man's sperm

Paper in Personality and Individual Differences
But when only 5% of their time is in each other's company, the figure almost doubles to 712 million.

Some scientists believe this is evidence of a phenomenon they have named Sperm Competition.

The researchers say it could provide a biological explanation for why people can find it difficult to hold down long-term, stable relationships.

The researchers wrote: "This increase in sperm insemination increases the odds of crowding out and displacing a competing man's sperm, which is precisely what would be expected if humans had an ancestral history of some casual sex and marital infidelity."

They suggest that men may have developed a powerful desire for sexual variety in response to infidelity - technically known as short-term mating - among women. Females were often keen to maximise their chances of becoming pregnant, the researchers said.

"An ancestral woman married to a man uninterested in sex may have had a more difficult time getting pregnant," the report states.

Chimpanzees are well endowed....
"If this is correct, then fertility backup is a possible function of a woman's short-term mating."

The report continues: "Alternatively, a partner's lack of sexual interest may signal to the woman that he is channelling his sexual interest and perhaps commitment elsewhere, in which case the woman might benefit by doing likewise."

The report also suggests male sexual jealousy may have evolved as a way to combat women's natural desire to seek sex elsewhere.

It goes on to explain how the size of the testicles is a reliable measure of the level of promiscuity across primate species.

The greater the promiscuity, the larger the testicles relative to body size.

.....Gorillas less so
For example, male gorillas have relatively small testicles and the female of the species is highly monogamous.

In contrast, chimpanzees are highly promiscuous and the males tend to have testicles that, in relative terms, are ten times the size of those of gorillas.

Human males are somewhere between chimps and gorillas.

This suggests that ancestral humans were not as promiscuous as chimpanzees - but neither were they entirely monogamous.

'Not convinced'

Dr Amin Gorgy, clinical director of the London Fertility Centre, said he was yet to be convinced by the concept of sperm competition.

However, he said other scientists believed that men produced two types of sperm - one to fertilise the egg, and another to fight off sperm from other males.

He had also been told that 4% of women have sperm from different males in their reproductive system at the point of conception.

However he told BBC News Online: "The bottom line is that it is an interesting theory, but it has not been proven, and I would think it is unlikely to change the cultural behaviour of human beings."

"However, some men find it difficult to stay monogamous, and maybe there is a biological reason for that."

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See also:

02 Jan 00 | Health
Hope for infertile men
17 Mar 00 | Health
Modern man still virile
05 Oct 99 | Health
UK may import sperm
02 Mar 00 | Health
Sperm count fall blamed on salt
13 May 99 | Health
Sperm analysis 'varies wildly'
13 Sep 99 | Health
Ignorance over male infertility
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