The battle of the sexes takes on a whole new meaning with theories suggesting that humans subconsciously adjust their sexual response to control the likelihood of conception.
Women, for example, can modify their physical response to sex in subtle ways which diminish or improve the chances of pregnancy. Men, unbeknown to themselves, automatically adjust the number of sperm delivered, depending on the probability that their sperm will have to compete with those from another man.
These components of the human sexual response were described yesterday by Dr Robin Baker of the University of Manchester to the British Association meeting.
The theories involved were driven by evolution and the need for "reproductive success", he explained. All aspects of human sexual behaviour had been shaped by millions of years of evolution, with each physiological response designed to influence an individual's reproductive success, the ability to pass on genes to successive generations, he argued.
His theories were outlined in Sperm Wars, "a scientific interpretation of why people behave the way they do". The "human sexual strategy" described was in turn backed up by human and animal studies which demonstrated a purpose behind a given response.
He stressed, however, that the physiological response was controlled at a subconscious level only. The female response was far more complex and had a greater impact on potential fertilisation, he suggested.
The acidity of mucus in the womb could partially control the access of sperm and this acidity was modified by the response to a given partner.
There was a strong correlation between orgasm and sperm retention, he said. "Not having an orgasm is just as strategic as having one" in reproductive terms. He also described a response called "flowback", the discarding of ejaculate some time after intercourse. "We see it as one of the major elements of female sexuality and health," Dr Baker said.
Analysis of flowback indicated great variability in the amount retained and lost and this had an impact on fertilisation.
He described studies of infidelity amongst women which threw up apparent contradictions between a woman's intentions and actions. The studies indicated that women most likely to have sex with another partner were older rather than younger, in a long-term relationship and with several children.
While pregnancy was usually not consciously wanted, most of these woman had unprotected sex when they were most likely to become pregnant - and retained two to three times more of the sperm delivered by their lovers than their partners, Dr Baker explained.
Men were found to subconsciously vary the amount of sperm released during ejaculation - and by a surprising degree. His group discovered that a man who remained in close proximity to his partner for some time before intercourse released less sperm than if the two had been apart.
He theorised that this behaviour delivered advantages. Less sperm was better for conception if competing sperm from another male were not present, while more sperm improved the chance of his fertilisation if there were competing sperm.
The sperm then battled it out "for the prize of fertilising any eggs that might be available". This change in sperm output was noted in studies of men and in behavioural studies of rats, he said.
The much greater subconscious control exerted over conception by the woman conflicted with the perception of women as vessels awaiting the intervention of men, Dr Baker said.
