Soay, a small island in the Scottish archipelago of St. Kilda in one of the remotest part of the British Isles, is home to a population of wild sheep with a fascinating story to tell – if only they could. It is related on their behalf by the scientists who have been studying their evolution and, in particular, how their fertility is linked to vitamin D – also known as the “sunshine vitamin.”

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The study of Soay sheep found that ewes with higher levels of vitamin D in the summer had more lambs in the following spring.

In the journal Scientific Reports, a team led by the University of Edinburgh, UK, describes how their study of the Soay sheep adds to growing evidence that vitamin D – once thought only to be essential for healthy bones and teeth – is important for reproductive health.

The study found that high levels of vitamin D in wild sheep is linked to improved fecundity (number of lambs born) and reproductive success (number of lambs surviving to 1 year of age).

Senior author Dr. Richard Mellanby, head of small animal medicine at Edinburgh’s Royal (Dick) School of Veterinary Studies, says:

“Our study is the first to link vitamin D status and reproductive success in a wild animal population.”

He and his team hope the finding will lead to further research that investigates whether it is true of other mammals, including humans.

Mammals need exposure to sunlight to produce vitamin D – which is made in the skin. They can also get it from food, including some types of plants.

The important role that the sunshine vitamin plays in the development of healthy bones and teeth came to light nearly a century ago, when vitamin D deficiency was found to be the cause of rickets.

However, numerous studies since – and over the past 20 years in particular – have linked vitamin D deficiency to the development of a range of other disorders, including several inflammatory and autoimmune diseases.

For the study, the team measured blood levels of a marker linked to vitamin D in the Soay sheep.

They found that ewes with higher levels of vitamin D at the end of summer went on to have more lambs the following spring.

The result is significant because it is the first to show that an animal’s vitamin D status is associated with an evolutionary advantage.

The study is part of the long-term Soay Sheep Project – which has been running since 1985. The tiny sheep (they are about a third of the size of most domestic sheep) have lived wild on the islands of St. Kilda for thousands of years. The archipelago is a World Heritage site that is owned and managed by the National Trust for Scotland.

Dr. Mellanby explains why the Soay sheep make good subjects for a study of vitamin D:

Examining the non-skeletal health benefits of vitamin D in humans is challenging because people are exposed to different amounts of sunlight each day. Studying the relationship between skin and dietary sources of vitamin D – and long-term health outcomes – is more straightforward in sheep living on a small island.”

The study follows research led by the University of California-Santa Barbara that Medical News Today reported in November 2015, where it was found that infection with parasitic worms may influence women’s fertility – for better or worse, depending on the species of worm.