A recent study published by the journal Scientific Reports 6 suggests that high vitamin D status is associated with improved fertility in Soay sheep.
Vitamin D status varies considerably among human populations. Several factors contribute to this wide distribution, including but not limited to sun exposure, skin pigmentation, location, dietary habits and genetic factors. In fact, some researchers theorize that the evolution of skin pigmentation in modern humans is the result of a dual need to both protect the body from ultraviolet radiation and allow for optimal vitamin D production.
Previous studies have also provided evidence suggesting vitamin D may influence both survival and reproductive success in humans, two major components of Darwinian fitness. However, it has been difficult to find supporting evidence due to the lack of research pertaining to vitamin D and natural selection in the wild.
Therefore, researchers recently aimed to determine whether variation in vitamin D status is linked with reproductive success in a wild population. They measured the serum 25(OH)D of approximately 350 female Soay sheep. The Soay sheep are descendants of domestic sheep which have been living in the archipelago of St Kilda, Scotland for about 3,000–4,000 years. This population is particularly interesting as they are subject to strong natural selection, with up to a 60% mortality rate as a result of food limitation, climate conditions and parasite infestations. To date, the Rare Breeds Survival Trust has categorized the Soay sheep at risk for extinction, with only 900-1,500 breeding ewes left.
The Soay sheep may obtain their vitamin D in two ways: from the diet (vitamin D2) and from sun exposure (vitamin D3). However, some researchers believe the sheep ingest their vitamin D by licking their coat. Lanolin, the oil obtained from sheep’s wool, is used as the main source of vitamin D3 supplements. Others speculate that vitamin D is produced in the skin when exposed to sunlight, like humans.
The researchers evaluated the relationship between 25(OH)D2 (dietary vitamin D) and 25(OH)D3 (vitamin D via sun exposure) and age, coat color, immunological measures, parasitic infection and body mass. They also observed the role of vitamin D in indicators of reproductive fitness, including fertility, birth weight and survival of offspring within the first year.
Here is what they found:
- Serum 25(OH)D3 levels were substantially higher than 25(OH)D2 levels, indicating a greater sunlight contribution to an individual’s total 25(OH)D status.
- Adult and geriatric sheep had significantly higher 25(OH)D3 levels than lambs (p < 0.05). Though, geriatrics showed a significant decline in their 25(OH)D2 levels compared to adult sheep (p < 0.01).
- Light coated sheep had significantly higher vitamin D status than dark sheep (p = 0.014). Since this color variation is due to the mutation of a single point of a gene, this result suggest vitamin D status is inherited in this population.
- Females with higher vitamin D status in the summer experienced improved fertility the following spring (p = 0.007), but lamb birth weight or survival was not significantly affected by vitamin D.
- Due to improved fertility, those with higher vitamin D levels experienced greater annual reproductive success (p = 0.045).
The researchers concluded,
“This study offers the first insight into the predictors of vitamin D status within an unmanaged animal population, and the first direct evidence of natural selection acting on circulating levels of vitamin D metabolites in the wild.”
Sturges, M. & Cannell, JJ. Closer look: High vitamin D levels linked to improved fertility in sheep. The Vitamin D Council Blog & Newsletter, February, 2016.