A team of researchers led by Gregg Adams at the University of Saskatchewan found the protein dubbed the 'ovulation-inducing factor' (OIF) in the sexual fluid acts as a hormonal signal on the female brain, the Daily Mail reported.
This triggers the release of other hormones that signal the ovaries to release an egg.
The protein is found in many mammals from llamas to rabbits to humans, implying that it plays an important role in reproduction in all mammals.
"The idea that a substance in mammalian semen has a direct effect on the female brain is a new one," Adams said.
Male mammals have accessory sex glands that contribute seminal fluid to semen, but the role of this fluid and the glands that produce it are not well understood.
"From the results of our research, we now know that these glands produce large amounts of a protein that has a direct effect on the female," Adams said.
The protein works through the hypothalamus of the female brain. This part of the brain links the nervous system to the endocrine system (a system of glands that secrete hormones into the blood stream) via the pituitary gland.
The scientists discovered it is the same molecule that regulates the growth, maintenance, and survival of nerve cells.
The study looked at two species: llamas and cattle.
Llamas are 'induced ovulators,' that is, they ovulate only when they have been inseminated. Cows - and humans - are 'spontaneous ovulators,' meaning that a regular buildup of hormones stimulates the release of an egg.
Using a variety of techniques, the researchers found OIF present in semen samples taken from both animals.
However, while injecting the llama with this protein caused them to ovulate, the same effect wasn't seen in cattle.
But while the molecule doesn't appear to induce ovulation in spontaneous ovulators, it did affect fertility in different ways. The protein was found to effect the timing of when cattle developed the fluid-filled sacs in the ovary which carry the eggs.
It also promoted the development of a temporary endocrine structure needed to sustain pregnancy.
"This latest finding broadens our understanding of the mechanisms that regulate ovulation and raises some intriguing questions about fertility," Adams said.
The study was published in the Proceedings of the National Academy of Sciences.
