Joint senior author David Bates, Professor of Oncology in the School of Medicine at The University of Nottingham, said: “Changes during the year in sex hormones made in the pituitary gland control when mammals start reproducing, and other changes like growing new coats or developing antlers. A study, led by academics at the Universities of Nottingham and Bristol and published in the journal Proceedings of the National Academy of Sciences (PNAS), reveals the link between breeding season and the hormone melatonin, made in the pineal gland in the brain during long winter nights. Firstly, it causes remodelling of the blood vessels that connect the brain with the pituitary gland, and secondly, it is used as a messenger signal to act on the cells of the pituitary that produce hormones that control seasonal fertility, which are located in a different part of the gland.” 
 
 The study has unravelled the mechanism through which animals measure time of year to allow their physiological adaptation to a changing environment. The region of the pituitary gland that detects melatonin is separate to the region that produces sex hormones. Scientists have discovered for the first time how animals link the change in seasons to their fertility. However, until now, it has not been known how melatonin, which is produced at night, signals to the area of the pituitary gland that controls sex hormones.” 
 
 The team of scientists, which also included PhD student Dr Jennifer Castle-Miller and Dr Domingo Tortonese, in the Faculty of Health Sciences at the University of Bristol, found that in sheep, melatonin controls production of two different types of a protein called vascular endothelial growth factor (VEGF). “The protein is made in two different types; it is made in a form that blocks blood vessel growth or a form that stimulates blood vessel growth,” said Bates. “Melatonin definitely controls which type [of the protein] you make.” 
 
 The team have found that the form of the protein which blocks the growth of blood vessels, reducing the connection between different regions of the pituitary gland, is triggered during long periods of melatonin production – typical of the winter.
– "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.)