Early-life environmental and genetic factors play key roles in shaping the fitness of individuals by influencing life-history traits. The impact of early life conditions is hypothesized to be reflected at the cellular level on telomere length. In many animal species, telomeres shorten with age and their length early in life is often related to individual life expectancy. It is therefore essential to understand the relative importance of environmental and genetic factors influencing telomere length during this early developmental period. In this study we determine the relative influence of environmental and genetic components on telomere length in Tree swallows (Tachycineta bicolor) across various environments. To do so, we used data from 518 nestlings across 176 broods, collected over 2 years in a population breeding in southern Québec (Canada). We found no effect of environmental conditions on telomere length but found that nestlings from larger broods tended to have slightly longer telomeres. Parent-offspring regressions revealed a significant and positive association between nestling rTL and that of their mothers. While the association was also positive for fathers-offspring, it was slightly weaker and not statistically significant. Heritability of telomere length estimated using an animal model ranged between 0.34 and 0.55. Our results suggest that telomere length of Tree swallow nestlings is influenced by the proximal environment (i.e., within a brood) and by a genetic component. These findings suggest that broader-scale environmental conditions might not directly affect nestlings at this stage.
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