Sex wars and the great egg race: Masturbation may help couples to conceive, reports Chris Barnard

ACCORDING to folklore, it inflicts blindness, saps strength and leads to sterility and impotence. The evils of masturbation are as legion as the pejorative names deriving from it. Despite its alarming mythology, however, masturbation turns out to be a useful mechanism for sharpening reproductive performance - in both sexes.

While copulating with the partner of one's choice might seem like the pinnacle of sexual success, negotiating the minefield of social skills is only the half of it as far as reproductive success is concerned. Once sperm are shed into the vagina, a new, more subtle battle of the sexes ensues. There are two aspects to this: first, there is a struggle between the sperm of rival males to fertilise the egg (sperm competition); second, the female may be able to manipulate the outcome of that competition.

Biologists at the University of Manchester have discovered that patterns of masturbatory and copulatory orgasm are important weapons in the conflict.

In common with many other superficially monogamous species, the apparent stability of human pair bonds is regularly undermined by sex with other partners. With the help of volunteers who were supplied with condoms, Robin Baker and Mark Bellis have studied changes in the number of sperm ejaculated in relation to the risk of sperm competition from rival males. The possibility of his partner mating with a rival poses a problem for the 'regular' male. If he has been away from his partner for any length of time, there is a chance that when he next copulates with her she may already be carrying the sperm of a rival. One way to deal with this is to up the number of sperm in his ejaculate and swamp the opposition.

Condom samples from copulations with regular partners confirmed this; men ejaculated more sperm the longer they had been away from their partners. The number of sperm in control masturbatory samples showed no such variation, so the effect was not due simply to replenishment.

However, ejaculating a great quantity of sperm has its drawbacks. The more sperm that are inseminated, the more are retained in the female reproductive tract. If this number becomes too great, the chances of successful fertilisation can drop because development of the egg is impaired by multiple fertilisation. So if the risk of sperm competition is low, increasing output may not be such a good idea. Variation in ejaculate content can thus amount to a flexible 'topping-up' strategy, with a male attempting to maintain an optimum number of sperm in his partner's tract - where the optimum depends on the likelihood of competition.

This situation is further complicated by the female. According to Baker and Bellis's evidence, far from being passive receptacles for incoming sperm, females actively manipulate the sperm content of their reproductive tract by using different patterns of copulatory and masturbatory orgasm. Baker and Bellis found that during sex with their regular partner, the timing of female orgasm relative to the male resulted in low levels of sperm retention.

Adultery is different, however. During copulations with a male other than her regular partner, the timing of orgasm shifted so that the suction effect of the contractions drew sperm deeper into the tract. However, almost by definition, the regular partner is still likely to to have left residual sperm in the tract. In addition to the timing of orgasm, females pursuing extra-pair relationships stepped up their frequency of masturbatory and involuntary nocturnal orgasms.

The likely effect of this was to increase the acidity of the cervical environment and thus kill or reduce the mobility of the regular partner's sperm retained in the tract. Such additional orgasms could be an effective means of loading the dice in favour of an extra-pair male's sperm at the next copulation. This fits with the finding that women indulging in extra-pair matings tend to do so at a point in their cycle when they are most likely to conceive.

While there's not much a male can do about this surreptitious tweaking of the odds, he can go some way to redressing the balance during in-pair copulation. Baker and Bellis discovered that masturbation adjusted the content of ejaculates in favour of sperm that were more likely to be retained by the female. These were, on average, younger sperm made available by the tendency for males to masturbate less than 48 hours before copulation, so getting rid of older, less competitive stock.

Far from being an unproductive avenue of self-relief, therefore, masturbation may be a powerful customising agent, allowing both sexes to manipulate opportunities for fertilisation in a complex struggle for reproductive advantage. The conflicts of interest between the sexes so obvious at the social level thus extend to the internal female environment, fine-tuning the race for the egg among competing ejaculates.

Suggested Topics
News
Susan Sarandon described David Bowie as
peopleSusan Sarandon reveals more on her David Bowie romance
Sport
Lewis Hamilton walks back to the pit lane with his Mercedes burning in the background
Formula 1
Arts and Entertainment
The new characters were announced yesterday at San Diego Comic Con
comic-con 2014
Sport
Arsenal supporters gather for a recent ‘fan party’ in New Jersey
football
PROMOTED VIDEO
News
i100
News
Bryan had a bracelet given to him by his late father stolen during the raid
people
News
A rub on the tummy sprang Casey back to life
video
Sport
sportDidier Drogba returns to Chelsea on one-year deal
Arts and Entertainment
The Secret Cinema performance of Back to the Future has been cancelled again
film
Life and Style
Balmain's autumn/winter 2014 campaign, shot by Mario Sorrenti and featuring Binx Walton, Cara Delevingne, Jourdan Dunn, Ysaunny Brito, Issa Lish and Kayla Scott
fashionHow Olivier Rousteing is revitalising the house of Balmain
News
i100
Arts and Entertainment
Christian Grey cradles Ana in the Fifty Shades of Grey film
filmFifty Shades of Grey trailer provokes moral outrage in US
News
people
News
BBC broadcaster and presenter Evan Davis, who will be taking over from Jeremy Paxman on Newsnight
peopleForget Paxman - what will Evan Davis be like on Newsnight?
Life and Style
fashionCustomer complained about the visibly protruding ribs
News
ebookA unique anthology of reporting and analysis of a crucial period of history
News
newsComedy club forced to apologise as maggots eating a dead pigeon fall out of air-conditioning
Arts and Entertainment
Jo Brand says she's mellowed a lot
tvJo Brand says shows encourage people to laugh at the vulnerable
Life and Style
People may feel that they're procrastinating by watching TV in the evening
life
News
Tovey says of homeless charity the Pillion Trust : 'If it weren't for them and the park attendant I wouldn't be here today.'
people
Sport
Rhys Williams
commonwealth games
Independent
Travel Shop
the manor
Up to 70% off luxury travel
on city breaks Find out more
santorini
Up to 70% off luxury travel
on chic beach resorts Find out more
sardina foodie
Up to 70% off luxury travel
on country retreats Find out more
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

Employment Solicitor

Highly Competitive Salary: Austen Lloyd: MANCHESTER - Senior Employment Solici...

Senior Risk Manager - Banking - London - £650

£600 - £650 per day: Orgtel: Conduct Risk Liaison Manager - Banking - London -...

Commercial Litigation Associate

Highly Attractive Package: Austen Lloyd: CITY - COMMERCIAL LITIGATION - GLOBAL...

Systems Manager - Dynamics AX

£65000 - £75000 per annum + Benefits: Progressive Recruitment: The client is a...

Day In a Page

Evan Davis: The BBC’s wolf in sheep’s clothing to take over at Newsnight

The BBC’s wolf in sheep’s clothing

What will Evan Davis be like on Newsnight?
Finding the names for America’s shame: What happens to the immigrants crossing the US-Mexico border without documents who never make it past the Arizona desert?

Finding the names for America’s shame

The immigrants crossing the US-Mexico border without documents who never make it past the Arizona desert
Inside a church for Born Again Christians: Speaking to God in a Manchester multiplex

Inside a church for Born Again Christians

As Britain's Anglican church struggles to establish its modern identity, one branch of Christianity is booming
Rihanna, Kim Kardashian and me: How Olivier Rousteing is revitalising the house of Balmain

Olivier Rousteing is revitalising the house of Balmain

Parisian couturier Pierre Balmain made his name dressing the mid-century jet set. Today, Olivier Rousteing – heir to the house Pierre built – is celebrating their 21st-century equivalents. The result? Nothing short of Balmania
Cancer, cardiac arrest, HIV and homelessness - and he's only 39

Incredible survival story of David Tovey

Tovey went from cooking for the Queen to rifling through bins for his supper. His is a startling story of endurance against the odds – and of a social safety net failing at every turn
Backhanders, bribery and abuses of power have soared in China as economy surges

Bribery and abuses of power soar in China

The bribery is fuelled by the surge in China's economy but the rules of corruption are subtle and unspoken, finds Evan Osnos, as he learns the dark arts from a master
Commonwealth Games 2014: Highland terriers stole the show at the opening ceremony

Highland terriers steal the show at opening ceremony

Gillian Orr explores why a dog loved by film stars and presidents is finally having its day
German art world rocked as artists use renowned fat sculpture to distil schnapps

Brewing the fat from artwork angers widow of sculptor

Part of Joseph Beuys' 1982 sculpture 'Fettecke' used to distil schnapps
BBC's The Secret History of Our Streets reveals a fascinating window into Britain's past

BBC takes viewers back down memory lane

The Secret History of Our Streets, which returns with three films looking at Scottish streets, is the inverse of Benefits Street - delivering warmth instead of cynicism
Joe, film review: Nicolas Cage delivers an astonishing performance in low budget drama

Nicolas Cage shines in low-budget drama Joe

Cage plays an ex-con in David Gordon Green's independent drama, which has been adapted from a novel by Larry Brown
How to make your own gourmet ice lollies, granitas, slushy cocktails and frozen yoghurt

Make your own ice lollies and frozen yoghurt

Think outside the cool box for this summer's tempting frozen treats
Ford Fiesta is UK's most popular car of all-time, with sales topping 4.1 million since 1976

Fiesta is UK's most popular car of all-time

Sales have topped 4.1 million since 1976. To celebrate this milestone, four Independent writers recall their Fiestas with pride
10 best reed diffusers

Heaven scent: 10 best reed diffusers

Keep your rooms smelling summery and fresh with one of these subtle but distinctive home fragrances that’ll last you months
Commonwealth Games 2014: Female boxers set to compete for first time

Female boxers set to compete at Commonwealth Games for first time

There’s no favourites and with no headguards anything could happen
Five things we’ve learned so far about Manchester United under Louis van Gaal

Five things we’ve learned so far about United under Van Gaal

It’s impossible to avoid the impression that the Dutch manager is playing to the gallery a little