Semen contains a protein that triggers ovulation in female mammals, a new study suggests. The protein is the same molecule that controls growth and survival of nerve cells.

The study shows that proteins from semen travel through the female's bloodstream to reach the brain where they trigger the release of pregnancy related hormones, said Sergio Ojeda, a neuroscientist at Oregon Health & Science University in Portland, Science reported.

Certain animals like cows, and humans, have what is known as spontaneous ovulation meaning that these animals ovulate during a fixed time of their biological cycles. There are other animals like llama that don't follow this fixed time ovulating pattern and have induced ovulations meaning that they ovulate when they are inseminated.

Science reports that in 1985, a group of Chinese researchers had challenged the idea that induced ovulation, in female mammals, occurs by physical stimulation. They had said that the semen has a protein that can induce ovulation in females. However, many in the scientific community had rejected the idea.

The present study examined the effects of the ovulation inducing factor (OIF) protein in llamas. They separated the OIF from the llama serum and transferred the OIF in the females. The female llama showed signs of ovulation even without any genital stimulation. They also found that OIF extracted from semen of bulls, stallions and boars could induce ovulation in female llamas while OIF from male llamas could induce ovulation in prepubescent mice, according to Science.

“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,” said Gregg Adams, a professor of veterinary biomedical sciences at the Western College of Veterinary Medicine at the University of Saskatchewan.

The research team found the OIF protein in all the animals they have studied so far. They have compared this protein to thousands of other proteins including nerve growth factor which is also found in human semen, leading researchers to believe that it has an important role in fertility.

They found that OIF and the Nerve Growth Factor (NGF) protein were similar in structure.

“To our surprise, it turns out they are the same molecule. Even more surprising is that the effects of NGF in the female were not recognized earlier, since it’s so abundant in seminal plasma," Adams said in a press release.

According to Warren Foster, a reproductive biologist at McMaster University in Hamilton, Canada, NGF can explain why some couples can't conceive, Nature reports.

“The idea that a substance in mammalian semen has a direct effect on the female brain is a new one. This latest finding broadens our understanding of the mechanisms that regulate ovulation and raises some intriguing questions about fertility," Adams added.

The study was published in the Proceedings of the National Academy of Sciences.