Abstract
Vitamin D deficiency has been associated with the development of many human diseases, and with poor reproductive performance in laboratory rodents. We currently have no idea how natural selection directly acts on variation in vitamin D metabolism due to a total lack of studies in wild animals. Here, we measured serum 25 hydroxyvitamin D (25(OH)D) concentrations in female Soay sheep that were part of a long-term field study on St Kilda. We found that total 25(OH)D was strongly influenced by age, and that light coloured sheep had higher 25(OH)D3 (but not 25(OH)D2) concentrations than dark sheep. The coat colour polymorphism in Soay sheep is controlled by a single locus, suggesting vitamin D status is heritable in this population. We also observed a very strong relationship between total 25(OH)D concentrations in summer and a ewe’s fecundity the following spring. This resulted in a positive association between total 25(OH)D and the number of lambs produced that survived their first year of life, an important component of female reproductive fitness. Our study provides the first insight into naturally-occurring variation in vitamin D metabolites, and offers the first evidence that vitamin D status is both heritable and under natural selection in the wild.
Highlights
The main physiological role of vitamin D has been considered to be in the development and maintenance of skeletal health
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
There was no evidence that vitamin D predicted lamb birth weight or over-winter survival, but the fecundity effect alone was sufficient to result in significantly improved annual reproductive success in females with higher vitamin D concentrations (Fig. 6)
Summary
The main physiological role of vitamin D has been considered to be in the development and maintenance of skeletal health. Considerable variation has typically been observed in circulating levels of vitamin D metabolites within healthy human populations[5,6,7] This variation has been linked to environmental and phenotypic influences including diet, exposure to sunlight and skin colour, and has a heritable component[8,9,10]. Sheep resident to Village Bay are captured and marked at birth and subsequently closely monitored and regularly captured, providing exceptionally detailed reproductive life history and survival data They are subject to variable and often very strong natural selection, characterized in particular by irregular ‘crash’ winters when upwards of 60% of the population can perish due to interactions among food limitation, climate conditions and parasite infection (Fig. 2)[20]. We present the first documentation of an association between colouration and serum 25(OH)D3 concentrations from the wild, and the first evidence that higher levels of either 25(OH)D3 or 25(OH)D3 predict improved female fecundity and reproductive fitness under natural conditions
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