A newly-discovered protein in the semen of all mammals - including humans - prompts females to ovulate through a direct effect on the brain.
Surprisingly, it's the same molecule that regulates the growth, maintenance, and survival of nerve cells - nerve growth factor (NGF) which is found primarily in nerve cells throughout the body.
The scientists, though, say that large amounts of the protein are produced by the accessory sex glands that contribute seminal fluid to semen.
"To our surprise, it turns out they are the same molecule," says Gregg Adams of the University of Saskatchewan. "Even more surprising is that the effects of NGF in the female were not recognized earlier, since it's so abundant in seminal plasma."
NGF in the semen acts as a hormonal signal, working through the hypothalamus of the female brain and the pituitary gland. This triggers the release of other hormones that signal the ovaries to release an egg or eggs.
While the protein may function differently from animal to animal, the team found it in every species they checked, from llamas, cattle and koalas to pigs, rabbits, mice, and humans - implying it's pretty important.
Just how it works, though, its role in various species, and its clinical relevance to human infertility are still unclear.
In their study, the team looked at two species: llamas and cattle. Llamas are 'induced ovulators', ovulating only when they have been inseminated. Cows, like humans, are 'spontaneous ovulators', meaning that a regular buildup of hormones stimulates the release of an egg.
"The idea that a substance in mammalian semen has a direct effect on the female brain is a new one," says Adams. "This latest finding broadens our understanding of the mechanisms that regulate ovulation and raises some intriguing questions about fertility."