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Age\u2010related 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\u2010history trait that shapes lifelong infection dynamics through trade\u2010offs 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\u2010specific infection trajectories across adulthood, yet whether early\u2010life reproductive strategies are linked to adult parasite dynamics in a sex\u2010specific manner remains largely unexplored. Using 10\u2009years of individual\u2010based monitoring in European shags (\n                    <jats:italic>Gulosus aristotelis<\/jats:italic>\n                    ), we tested whether age at first reproduction predicts within\u2010individual changes in gastrointestinal nematode burden across adulthood and whether these trajectories differ between sexes. Parasite burdens were quantified by repeated endoscopy, and a within\u2010subject centring approach was used to separate within\u2010 from between\u2010individual age effects and explicitly account for selective disappearance. Age at first reproduction predicted sex\u2010specific infection trajectories in male but not female shags. Early\u2010 and modal\u2010recruiting males showed within\u2010individual declines in parasite burden with age, whereas late\u2010recruiting males exhibited progressive increases. A marginal trend in early\u2010recruiting males suggests possible selective disappearance in this group. Females showed consistent within\u2010individual 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\u2010specific parasite infection trajectories in a long\u2010lived bird, revealing how early\u2010life reproductive strategies are linked to adult host\u2013parasite dynamics. Integrating life\u2010history variation with longitudinal, within\u2010individual approaches exposes mechanisms that cross\u2010sectional analyses cannot detect and highlights the population\u2010level consequences of sex\u2010specific infection trajectories.\n                  <\/jats:p>","DOI":"10.1002\/ece3.74092","type":"journal-article","created":{"date-parts":[[2026,8,14]],"date-time":"2026-08-14T01:43:32Z","timestamp":1786671812000},"update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Age at First Reproduction Predicts Sex\u2010Specific Parasite Infection Trajectories Through Adulthood"],"prefix":"10.1002","volume":"16","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-4646-1975","authenticated-orcid":false,"given":"Francisco","family":"Ruiz\u2010Raya","sequence":"first","affiliation":[{"name":"Ashworth Laboratories, School of Biological Sciences, Institute of Ecology and Evolution University of Edinburgh  Edinburgh UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Sarah J.","family":"Burthe","sequence":"additional","affiliation":[{"name":"UK Centre for Ecology and Hydrology  Penicuik UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-1273-6595","authenticated-orcid":false,"given":"Hannah","family":"Ravenswater","sequence":"additional","affiliation":[{"name":"Ashworth Laboratories, School of Biological Sciences, Institute of Ecology and Evolution University of Edinburgh  Edinburgh UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Fiona","family":"Greco","sequence":"additional","affiliation":[{"name":"Ashworth Laboratories, School of Biological Sciences, Institute of Ecology and Evolution University of Edinburgh  Edinburgh UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Olivia","family":"Hicks","sequence":"additional","affiliation":[{"name":"School of Ocean Sciences Bangor University  Menai Bridge UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Mark","family":"Newell","sequence":"additional","affiliation":[{"name":"UK Centre for Ecology and Hydrology  Penicuik UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Carrie","family":"Gunn","sequence":"additional","affiliation":[{"name":"UK Centre for Ecology and Hydrology  Penicuik UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Francis","family":"Daunt","sequence":"additional","affiliation":[{"name":"UK Centre for Ecology and Hydrology  Penicuik UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Emma J. 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