If you're trying to avoid getting pregnant, here's another reason to mistrust the rhythm method of birth control. New research confirms the fluid in semen, long dismissed as primarily a vehicle for sperm, contains a substance that can trigger ovulation and other pregnancy-supporting hormonal responses in female mammals.
The find could lead to new fertility treatments in humans.
Like most female animals, women are spontaneous ovulators, meaning they release eggs on a fairly regular basis regardless of their sexual activity.
A few animal species, however, such as camels and rabbits, release viable eggs only in response to sex. These animals are called ''induced ovulators''.
For decades, scientific dogma has held that in induced ovulators, the physical stimulation of sex triggers hormonal responses within the female that lead to the production and release of eggs.
In 1985, however, Chinese researchers suggested there might be an ovulation-inducing factor (OIF) in semen itself.
The reproductive biologist Gregg Adams, of the University of Saskatchewan, Canada, says the hypothesis ran so counter to common wisdom that ''people just ignored it - me included''.
When he and his colleagues finally tested the idea decades later, the results took them aback.
In 2005, the team injected the seminal fluid of male llamas into the hind legs of females llamas to determine whether they would ovulate without genital stimulation. To their surprise, he says, it had a ''potent ovulatory effect''.
That sparked a seven-year search for OIF in semen. Now, in a study published in the Proceedings of the National Academy of Sciences, this week, Adams and his colleagues say they have found it.
The researchers took samples of llama and bull semen to determine whether OIF could be found in the semen of both induced ovulating species and spontaneously ovulating species. First they separated the seminal fluid from the sperm. Sperm, says Adams, makes up only about 5 per cent of semen.
Then, the team used heat, various protein-digesting enzymes, and size filters to try to winnow out the effective molecule.
After each treatment, they went through a ''thorough process of elimination'', Adams says, injecting the altered seminal fluid into the female llamas' hindquarters then determining whether the molecule had survived and effectively induced ovulation, or been destroyed.
To Adams's surprise, the mystery substance turned out to be a protein crucial to the development and survival of sensory neurons: neural growth factor. ''We were looking for an unknown protein,'' Adams says, but in fact OIF/neural growth factor is a molecule found throughout the bodies of many species.
Neural growth factor was discovered in bull semen in the early 1980s, Adams says, but ''it was one of those dangling facts that no one knew what to do with''. Now, he says, ''we can connect the dots''.
He and his team found the molecule in abundance in the semen of every species they've studied-including humans.
As the first evidence neural growth factor, or OIF, from semen travels throughout the female body, acting as a hormone, Adams says ''this is really new for us". The team now plans to study how neural growth factor affects human fertility. ''We're really interested to know the relationship between OIF and infertility. Are males with high concentrations of OIF in their semen more fertile?''
For couples experiencing difficulties in becoming pregnant, he says, perhaps OIF treatments could be developed.