Scientists have identified a substance in semen that triggers ovulation in some species and could also affect human fertility.
Many animals, humans included, are spontaneous ovulators, meaning that they produce eggs from their ovaries in a some sort of cycle. If mating occurs at the right time, they can become pregnant.
But other species, including koalas, camels, llamas and rabbits are "reflex ovulators". In them, the act of sex itself triggers the ovary to release fertilisable eggs.
The trigger for this wasn't previously known, although scientists had found that it probably involved a signal from the semen travelling via the bloodstream because injection of seminal fluid into a female camel's muscles could trigger the animal to ovulate. And even more interesting, it could also trigger ovulation in non-reflex ovulators like cows, mice and, presumably, us.
Now the identity of this chemical can been solved. Writing in PNAS, University of Saskatchewan scientist Gregg Adams and his team separated the different chemical constituents of llama semen and analysed them.
Amongst a group of proteins of a certain size that had previously been shown to contain the ovulation signal they found a chemical signature identical to a well-known substance called NGF - nerve growth factor.
Elsewhere in the body this nourishes nerve cells but, the team confirmed, injected in isolation into llamas, it stimulated the animals to ovulate.
Where NGF acts in the body to produce this effect has not yet been proven, but the ovaries have receptors for the signal, as does the brain's hypothalamus, which produces the ovulation-controlling hormone GnRH.
This has led Adams and his colleagues to speculate that NGF present in semen can travel via the bloodstream to the brain where it docks with the hypothalamus to influence fertility.
Although humans don't need NGF to ovulate, the protein could still play a role in human fertility. Llama seminal fluid, for instance, can shorten the ovulation cycles of cows and promote the development of the corpus luteum that forms after an egg has been released and produces the hormone progesterone to support pregnancy.
Like humans, cows are spontaneous ovulators. And if either a man failed to produce enough NGF in his semen or the woman lacked the receptors to respond to it, it could directly affect a woman's pregnancy success rate...