Many species of animals have developed seasonal fertility patterns so that their offspring are born when there is most access to food sources. For example, most breeds of sheep are most fertile in the fall so that lambs are born in the spring when ewes can graze on fresh grass.

The seasonality of sheep fertility is strong, although some breeds have been developed to breed out of season. The biological mechanisms of this seasonality hasn't been completely clear although research has shown that day length — via natural daylight or artificial lighting systems — can influence breeding season.

Research from South Dakota State University has shown that even strongly seasonal sheep breeds can be selected to lamb out of season with the aid of an artificial lighting regimen.

Now, U.K. scientists have discovered how animals link the change in seasons to their fertility.

A study, led by academics at the Universities of Nottingham and Bristol in the U.K. and published in the journal Proceedings of the National Academy of Sciences, reveals the link between breeding season and the hormone melatonin, made in the pineal gland in the brain during long winter nights.

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.

“The length of the day is recognized in most vertebrate animals by the pineal gland in the brain, which produces melatonin. 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,” Bates said.

The team of scientists, which also included doctoral 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). This happens within a specific region of the pituitary gland, away from the area where sex hormones are made.

In winter, sheep make forms of VEGF that stop blood vessels growing. In summer, they make a different VEGF that makes vessels grow between the two areas of the pituitary. This allows VEGF to act as a messenger to control sex hormone-producing cells and fertility — the first time that the molecule linking long winter nights to sex hormones has been found.

The search for the missing link between the different parts of the pituitary has been going on for nearly 30 years, because knowing how this works could, for instance, help control when sheep start lambing. The findings also have wider implications for how animals, and potentially humans, use this system in health and disease.