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

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.