Study may reveal why melatonin affects breeding, horn growth and coat thickness – and allow farmers to change the timing of lambing season

Why do sheep get horny in winter? Because the light is baaad, says study

The mystery of why sheep get horny in the winter might have been solved, according to new research.

Scientists say they have uncovered the key to the mechanism by which changes in the length of the day prompt certain animals to begin breeding, trigger the growth of horns and even change the thickness of their coat.

The findings, the team add, could help farmers tinker with the timing of the lambing season.

“Now we know what that link is we can start to understand how it can be controlled,” said David Bates, professor of oncology at the University of Nottingham and co-author of the research.

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It has long been known that changes in animals’ fertility over the seasons is linked to melatonin – a hormone released at night from the pineal gland in the brain. This hormone acts on another gland, the pituitary, affecting the levels of various sex hormones it produces.



With the onset of fertility in sheep linked to longer periods of melatonin production, winter is the season for ovine Casanovas. But there is a puzzle.

The region of the pituitary gland that detects melatonin is separate to the region that produces sex hormones. As a result, scientists had been baffled as to how melatonin ends up affecting the onset of fertility. “No-one has been able to find what the link is,” said Bates.

Now Bates and colleagues from the University of Bristol say they have the answer.

Writing in the journal PNAS, the team reveal the missing link is a protein, known as vascular endothelial growth factor, which is made in the region of the pituitary gland that detects melatonin.

“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. In addition, this form of the protein was found to stimulate the production of sex hormones which prompt sheep to eye up the opposite sex.

By contrast, a shorter duration of melatonin production – typical of summer – was linked to the formation of the alternative form of the protein, which increases the growth of blood vessels and acts as an anaphrodisiac.

The revelation, says Bates, could help farmers. “As climate change occurs and you start to get harsher winters, for instance, you might want to delay the breeding so the lambs are born slightly later,” he said. “So you might be able to actually increase survival of the lambs.”

Mysteries remain, however, not least the question of why the breeding season of horses, for example, falls during the spring and summer when the production of melatonin is at its lowest.

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Andrew Loudon, a professor of animal biology at the University of Manchester who was not involved in the research, welcomed the study, pointing out that it highlights the importance of the region of the pituitary gland that detects long-term seasonal changes in melatonin. “[This] tissue contains specialised ‘calendar cells’ that act as the master regulator of seasonal cycles – so it is terribly important,” he said. “This new study offers an important insight into how the seasonal clock in this structure works to control the production of hormones in the pituitary gland.”



With similar melatonin-detecting regions present in the pituitary glands of humans, Loudon believes it is important to probe whether the newly discovered mechanism could also play a role in human biology. “Patterns of hormone secretion and also immune function are known to be seasonal in humans, although we are unusual as a species in being far less seasonal in our biology than wild animals,” he said. “However, I think it is highly likely that malfunction of these seasonal hormone-regulating pathways may well turn out to be important in the development of some endocrine diseases in the pituitary gland.”