Sex and population mediate the ‘fast growth–short telomere’ association in two species of large herbivores monitored in the wild

Fast growth is expected to incur physiological costs, notably through accelerated telomere shortening driven by high cell turnover and reactive oxygen species. However, through a systematic literature synthesis, we show that empirical studies exploring this relationship yield mixed results. We investigated the association between early growth and telomere length (TL) in two wild herbivores: the Soay sheep (Ovis aries) and the European roe deer (Capreolus capreolus). Instead of the expected negative relationship, early-life growth and TL associations were sex- and population-dependent. In Soay sheep, TL correlated negatively with postnatal mass gain in males but not in females. Moreover, the heaviest males at birth appeared buffered against the telomere-shortening associated with rapid postnatal growth. In roe deer, the relationship between reaching a heavy mass at the onset of the first winter and TL was population-specific. Shorter telomeres were only observed in high-quality habitats where fawns reach much heavier body masses, being absent in resource-limited habitats. Additional traits, including secondary sexual characteristics in sheep (e.g. horn type) and male vulnerability to harsh environments in roe deer were further associated with TL. Ultimately, our findings support TL as a relevant somatic marker of the physiological cost of growth, yet underscore its sensitivity to individual attributes and environmental context. This article is part of the theme issue 'Developmental circuits in ageing'.