Men who have lopsided ears might not make the earth move

Sex may be for procreation, but orgasms are really why we do it. Without this recreational incentive we probably would not go to all that bother. As Lord Chesterfield famously advised his son: "The pleasure is momentary, the position ridiculous and the expense damnable."

Biologically, however, that moment of pleasure, the orgasmic pay-off, is not just a sensory bribe. In women, at least, it seems to be an important factor in the scramble among sperm to fertilise the egg. Recent evidence suggests that the timing of female orgasm relative to the male partner's ejaculation may influence sperm retention and allow the woman to manipulate the chances of the sperm from a particular male fertilising her egg. Orgasms thus become an important means of exercising mating preferences. If this is the case, the achievement and timing of female orgasm should reflect the desirability of the male partner. Studies by Randy Thornhill and his colleagues at the University of New Mexico suggest that it does, and that what makes a male a real earth-mover is the evenness of his features.

Evolutionary biologists have recently discovered that a form of bodily asymmetry may be an important factor in the mating preferences of several species. Human beings are bilaterally symmetrical - the left-hand side of our bodies is, at least outwardly, a mirror image of the right-hand side. But this symmetry is not exact. There are, for example, left-right differences in the size of hands, feet, ankles and ears. This is an example of what is known as fluctuating asymmetry because it varies randomly within populations.

In bilaterally symmetrical species, females seem to prefer males whose paired features show little difference between one side of their body and the other. There is a good biological reason for this. Fluctuating asymmetry appears to be a symptom of disturbance during early stages of the male's physical development, perhaps as a result of nutritional stress, disease or simply low genetic quality. If susceptibility to these problems is heritable, lop-sided males will make a poor choice of father for the female's offspring.

Evidence from Thornhill and co-workers' studies suggests that asymmetry in humans (here measured as left-right differences in hand, foot, wrist, ankle and ear size) has a marked effect on male mating success. Not only are relatively symmetrical men more likely to be judged attractive, but they also have more sexual partners and sexual relationships outside their long-term pair bond, and achieve first copulation with a new partner after shorter periods of courtship. Most recently, in research just published in the scientific journal Animal Behaviour, Thornhill's group has found that the tendency for women to orgasm during intercourse is influenced by the asymmetry of their male partner.

Even when other features of male and female partners and difference in the nature of individual relationships were taken into account, women in Thornhill's study turned out to have more orgasms during intercourse when their partners showed low fluctuating asymmetry. The fact that asymmetry affected orgasm during copulation, rather than foreplay, is important. Earlier work by Robin Baker and Mark Bellis has shown that orgasms at this time tend to result in a high degree of sperm retention, possibly through a muscular "upsuck" effect. Orgasms during foreplay tend to increase loss of sperm through later flowback from the vagina.

Few other features of men predicted female copulatory orgasm, although facial attractiveness and body weight had some effect independently of asymmetry. The upshot thus seems to be that copulatory orgasm in women is designed to retain the sperm of men with a good developmental pedigree, and also those that are facially attractive and large. Facial attractiveness and body weight both correlate positively with symmetry in a number of studies, so it may be that they act as easy indices of the developmental quality of the male. In a species in which females frequently mate with more than one male and pitch their sperm into a competitive play-off, orgasm seems to be a fun way of loading the dice in the female's favour.

CHRIS BARNARD

The writer is in the Behaviour and Ecology Research Group at Nottingham University.

Start your day with The Independent, sign up for daily news emails
News
news
Arts and Entertainment
tvSPOILER ALERT: Like a mash-up of 28 Days Later, Braveheart, The Killing and Lord of the Rings, this GoT episode was a belter
Life and Style
Angel Di Maria is shown the red card
tech
Sport
Roger Federer after his win over Tomas Berdych
sport
News
ebooksAn unforgettable anthology of contemporary reportage
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

Recruitment Genius: Business Development Executive / Digital Marketing Executive

£26000 - £35000 per annum: Recruitment Genius: A luxury beauty house with a nu...

Recruitment Genius: Housekeepers - Immediate Start

£8 per hour: Recruitment Genius: This company are currently recruiting new exp...

Recruitment Genius: Head Concierge

£25000 - £28000 per annum: Recruitment Genius: This award winning Property Man...

Recruitment Genius: Content, SEO and PPC Executive

£20000 - £25000 per annum: Recruitment Genius: A fantastic opportunity has ari...

Day In a Page

On your feet! Spending at least two hours a day standing reduces the risk of heart attacks, cancer and diabetes, according to new research

On your feet!

Spending half the day standing 'reduces risk of heart attacks and cancer'
Liverpool close in on Milner signing

Liverpool close in on Milner signing

Reds baulk at Christian Benteke £32.5m release clause
With scores of surgeries closing, what hope is there for the David Cameron's promise of 5,000 more GPs and a 24/7 NHS?

The big NHS question

Why are there so few new GPs when so many want to study medicine?
Big knickers are back: Thongs ain't what they used to be

Thongs ain't what they used to be

Big knickers are back
Thurston Moore interview

Thurston Moore interview

On living in London, Sonic Youth and musical memoirs
In full bloom

In full bloom

Floral print womenswear
From leading man to Elephant Man, Bradley Cooper is terrific

From leading man to Elephant Man

Bradley Cooper is terrific
In this the person to restore our trust in the banks?

In this the person to restore our trust in the banks?

Dame Colette Bowe - interview
When do the creative juices dry up?

When do the creative juices dry up?

David Lodge thinks he knows
The 'Cher moment' happening across fashion just now

Fashion's Cher moment

Ageing beauty will always be more classy than all that booty
Thousands of teenage girls enduring debilitating illnesses after routine school cancer vaccination

Health fears over school cancer jab

Shock new Freedom of Information figures show how thousands of girls have suffered serious symptoms after routine HPV injection
Fifa President Sepp Blatter warns his opponents: 'I forgive everyone, but I don't forget'

'I forgive everyone, but I don't forget'

Fifa president Sepp Blatter issues defiant warning to opponents
Extreme summer temperatures will soon cause deaths of up to 1,700 more Britons a year, says government report

Weather warning

Extreme summer temperatures will soon cause deaths of up to 1,700 more Britons a year, says government report
LSD: Speaking to volunteer users of the drug as trials get underway to see if it cures depression and addiction

High hopes for LSD

Meet the volunteer users helping to see if it cures depression and addiction
German soldier who died fighting for UK in Battle of Waterloo should be removed from museum display and given dignified funeral, say historians

Saving Private Brandt

A Belgian museum's display of the skeleton of a soldier killed at Waterloo prompts calls for him to be given a dignified funeral