MEN may contribute more than just sperm to the babymaking process - their semen may coax things along as well.

Seminal fluid is regarded as primarily a carrier agent for sperm. It is thought to increase sperm motility and strength but not actually affect the woman's body. Indeed, IVF clinics rinse the semen from sperm and discard it. But now Rebecca Burch, at the State University of New York at Oswego, has found some intriguing compounds in semen.

In particular, hormones. Some found in semen, such as follicle stimulating hormone, luteinising hormone and estradiol, are known to induce ovulation. FSH actually causes the egg to ripen and burst out of the ovary. Others, such as human chorionic gonadotropin and human placental lactogen have a role in maintaining pregnancy.

Burch thinks human males have evolved the concoction as a counter-strategy to concealed ovulation in women. Although there are subtle chemical cues that a woman is ovulating, there are no overt signs. So at any given copulation, a male wanting to reproduce could be wasting his time. Semen that could help induce ovulation while his sperm are in her reproductive tract would be a great advantage, she says, as would compounds that could bolster a new conception and help the fertilised egg implant.

If this was an evolved strategy, Burch reasoned, species where ovulation is not concealed would not need it. To test her theory, she asked colleagues at Yerkes National Primate Research Center in Atlanta, Georgia, to test the semen of six chimpanzees. Unlike humans, chimp females advertise their fertility with bright red swollen behinds.

Sure enough, compared to humans, the semen of chimp males had significantly lower levels of FSH and no luteinising hormone whatsoever. Burch presented her results earlier this week at the meeting of the International Society for Human Ethology in Detroit, Michigan. IVF clinics should consider not rinsing away the semen, she told New Scientist.

Roger Gosden, at the Center for Reproductive Medicine and Infertility in New York, says he doubts that enough of these compounds could get into the female bloodstream to make a difference, but says he and others remain "mystified" by semen's composition.

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Author: Alison Motluk, Toronto.

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