Semen doesn’t just ferry sperm. It also bears a mystery ingredient that turns on ovulation in some animals and may even pump up fertility in humans as well. The molecule, nerve growth factor, kick-starts egg release and revs up pregnancy-protective hormones in llamas, researchers report online in the Proceedings of the National Academy of Sciences. Llama semen is loaded with NGF, says study coauthor and veterinarian Gregg Adams of the University of Saskatchewan in Saskatoon, Canada. The protein is also found in the semen of bulls and humans.