"Why do sheep get frisky in winter?" isn't the beginning of a bad joke, but rather the question that jump-started a joint study by researchers at the University of Nottingham and the University of Bristol. The Guardian reports that scientists may now have an answer, one that could help farmers better control the breeding season. That answer, laid out in the journal PNAS, is a bit complicated. It's been known for some time that sheep's fertility is tied to production of the hormone melatonin, but how melatonin communicates with the pituitary gland, which shoots off sex hormones, has been a mystery. "No one has been able to find what the link is," says co-author David Bates. Until now, perhaps.
Press releases from Nottingham and Bristol explain what may solve this 30-year-old puzzle: a protein made in the pituitary gland called vascular endothelial growth factor. It's made in two different forms, and scientists say melatonin controls which one surfaces. In the summer (when melatonin is in shorter supply), VEGF makes blood vessels grow and serves as a sheep-sex buzzkill of sorts. But in the winter, VEGF blocks blood vessel growth—and also appears to trigger a sex hormone spike. “Now we know what that link is we can start to understand how it can be controlled,” says Bates. In addition to helping farmers, the study could lead to developments in studying hormonal and seasonal patterns in humans in regard to endocrine diseases. (Using your smartphone right before you turn in could mess with your melatonin levels.)