Telomere dynamics in relation to sexual selection, longevity and fitness: a case study on a small migratory passerine

Unraveling the molecular mechanisms driving individual variation in longevity, reproductive performance, and sexually selected traits is key to understanding life-history evolution. Telomere length (TL) and its rate of change (ΔTL) have been seldomly suggested as potential biomarkers of physiological condition and cellular maintenance. However, most studies have focused on short-term associations between telomeres, survival, and fitness, while their long-term effects and role in sexual selection remain unclear. We examined TL and ΔTL in nestling and adult barn swallows (Hirundo rustica) to assess: (1) environmental and parental influences on early-life TL; (2) annual survival and lifespan, as well as seasonal and lifetime reproductive success in relation to nestling and adult TL and ΔTL; (3) the link between TL and a sexually selected trait (i.e., outermost tail feathers length). Nestling TL declined with increasing brood size and paternal age, indicating developmental constraints and paternal aging effects. Individuals with longer TL early in life had greater longevity, supporting the hypothesis that selection favors longer telomeres early in life. In adults, TL declined with age, but neither TL nor ΔTL predicted annual survival or reproductive success. Similarly, TL at one year was unrelated to lifespan or lifetime reproductive output. However, in females, shorter TL correlated with longer tail feathers, suggesting a sex-specific reproductive cost on telomeres. Our results indicate that TL may predict survival early in life but not in adulthood, where it may instead reflect trade-offs between reproduction and physiological maintenance.