Sex differences in leukocyte telomere length in a free-living mammal

Mounting evidence suggests that average telomere length reflects previous stress and predicts subsequent survival across vertebrate species. In humans, leukocyte telomere length (LTL) is consistently shorter during adulthood in males than females, although the causes of this sex difference and its generality to other mammals remain unknown. Here we measured LTL in a cross sectional sample of free-living Soay sheep and found shorter telomeres in males than females in later adulthood (>3 years of age), but not in early life. This observation was not related to sex differences in growth or parasite burden, but we did find evidence for reduced LTL associated with increased horn growth in early life in males. Variation in LTL was independent of variation in the proportions of different leukocyte cell types, which are known to differ in telomere length. Our results provide the first evidence of sex differences in LTL from a wild mammal, but longitudinal studies are now required to determine whether telomere attrition rates or selective disappearance are responsible for these observed differences.