Age at first reproduction predicts sex-specific parasite infection trajectories through adulthood

Parasites can impose substantial fitness costs on hosts, yet infection intensity varies markedly among individuals and across life stages. Age‐related variation in parasite burden may reflect shifts in behaviour or physiology that alter exposure and susceptibility, or arise from selective disappearance of heavily infected individuals. Distinguishing between these processes is essential for understanding the ecological and evolutionary consequences of parasitism in wild populations. Age at first reproduction (AFR) is a fundamental life‐history trait that shapes lifelong infection dynamics through trade‐offs between reproductive investment, somatic maintenance, immune defence and survival. Because reproductive investment and timing often differ between sexes, age at first reproduction may generate sex‐specific infection trajectories across adulthood, yet whether early‐life reproductive strategies are linked to adult parasite dynamics in a sex‐specific manner remains largely unexplored. Using 10 years of individual‐based monitoring in European shags (Gulosus aristotelis), we tested whether age at first reproduction predicts within‐individual changes in gastrointestinal nematode burden across adulthood and whether these trajectories differ between sexes. Parasite burdens were quantified by repeated endoscopy, and a within‐subject centring approach was used to separate within‐ from between‐individual age effects and explicitly account for selective disappearance. Age at first reproduction predicted sex‐specific infection trajectories in male but not female shags. Early‐ and modal‐recruiting males showed within‐individual declines in parasite burden with age, whereas late‐recruiting males exhibited progressive increases. A marginal trend in early‐recruiting males suggests possible selective disappearance in this group. Females showed consistent within‐individual declines regardless of age at first reproduction, with no evidence of selective disappearance. These findings provide the first evidence that age at first reproduction is associated with sex‐specific parasite infection trajectories in a long‐lived bird, revealing how early‐life reproductive strategies are linked to adult host–parasite dynamics. Integrating life‐history variation with longitudinal, within‐individual approaches exposes mechanisms that cross‐sectional analyses cannot detect and highlights the population‐level consequences of sex‐specific infection trajectories.