Gregg Adams of the University of Saskatchewan said male mammals have accessory sex glands that contribute seminal fluid to semen, but the role of this fluid and the glands that produce has not been 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, a professor of veterinary biomedical sciences, said in a statement.
The team said the protein OIF was found in the semen of all species of mammal they have looked at so far -- llamas, cattle and koalas to pigs, rabbits, mice and humans.
The research team compared OIF to thousands of other proteins, including nerve growth factor, which is found primarily in nerve cells throughout the body.
"To our surprise, it turns out they are the same molecule," Adams said. "Even more surprising is that the effects of nerve growth factor in the female were not recognized earlier, since it's so abundant in seminal plasma."
The molecule in the semen acts as a hormonal signal, working through the hypothalamus of the female brain and the pituitary gland, which triggers the release of other hormones that signal the ovaries to release an egg -- or eggs.
The findings were published in the Proceedings of the National Academy of Sciences.