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Temporal variation in early-life conditions impacts on later-life levels of infection in sex specific ways. Associated data and code

Ravenswater, Hannah1; Burthe, Sarah2; Reed, Thomas3; Newell, Mark2; Daunt, Francis2; Carravieri, Alice4; Dunn, Ruth45; Granroth-Wilding, Hanna1; Gunn, Carrie2; Hicks, Olivia4; Cunningham, Emma1

Published Oct 17, 2025 on Dryad. https://doi.org/10.5061/dryad.pnvx0k70d

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Abstract

Parasites are a fundamental component of wild animal populations, often inducing sub-lethal chronic effects that impact host fitness and demography. However, the factors determining variation in infection burden are often poorly understood in wild systems. Environmental conditions can determine exposure to infection and the resources required to respond, but exhibit strong temporal variation. As these are predicted to become more variable, understanding how they shape infection is key to predicting the impact of environmental change on host populations. Responses to infection are shaped in early life potentially leading to lagged effects of environmental variation on fitness. The extent to which these are mediated by resources and later-life conditions remains unclear and may vary between the sexes who often differ in exposure and resource requirements. Here, we examine how intra- and inter-annual differences in hatching and breeding conditions influence infection levels throughout life. 

We utilise data from a long-term population study of European shags (Gulosus aristotelis) on the Isle of May, Scotland, in which there is substantial variation in the timing of breeding within and between years and nematode parasite burden can be measured in vivo using endoscopy. We used two data sets; the first containing information on parasite burden collected in chicks, matched with early life monitoring data, and the second containing adult parasite burden data matched with early life and current breeding conditions. Sex, age and the age of the bird at the time of parasite burden quantification were also present. For adults multiple measurements of parasite burden were taken across life in some individuals.

We show that adult parasite burden, is influenced by seasonal and annual differences in current and early life conditions, but different patterns were observed in adult males and females. Burdens increased across the season in chicks and adult females but not in adult males. Instead, early life effects better explained burden in adult males with those hatching later and in productive years displaying lower burdens. This suggests that early life may shape behaviour or physiology, impacting subsequent infection. Our findings reveal complex temporal effects on parasitism in species breeding in fluctuating environments. Incorporating seasonal and sex specific responses to parasitism is crucial to understanding how predicted environmental shifts could impact disease dynamics.