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, a group of Chinese researchers challenged this idea by suggesting that there might be an ovulation-inducing factor (OIF) in semen itself. According to veterinarian and reproductive biologist Gregg Adams of the University of Saskatchewan in Saskatoon, Canada, the hypothesis ran so counter to common wisdom that "people just ignored it.
When Adams and his colleagues finally tested the idea decades later, they were taken aback by their results. In 2005, the team injected the seminal fluid of male llamas—closely related to camels—into the hind legs of female llamas to see if the llamas would ovulate without genital stimulation. To their surprise, he says, injecting seminal fluid into the female llamas' bloodstream had "a very potent ovulatory effect."
That sparked a 7-year search for OIF in semen. Now, in a recent study, Adams and his colleagues say they've finally found it. The researchers took samples of llama and bull semen to see if OIF could be found in the semen of both induced ovulating species and spontaneously ovulating species. They spun the samples several times in centrifuges to separate the seminal fluid from the sperm. Sperm make up only about 5% of semen, Adams says. 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 very thorough process of elimination," Adams says, injecting the altered seminal fluid into the female llamas' hindquarters to see if the molecule had survived and effectively induced ovulation, or been destroyed. To Adams's surprise, the mystery substance turned out to be a protein that's crucial to the development and survival of sensory neurons: neural growth factor, or NGF. "We were looking for an unknown protein," Adams says, but in fact OIF/NGF is a molecule found throughout the bodies of many species.
Via Dr. Stefan Gruenwald