Data from: Parasitism in early life: environmental conditions shape intra-brood variation in responses to infection

Parasites play key ecological and evolutionary roles through the costs
they impose on their host. In wild populations, the effect of parasitism
is likely to vary considerably with environmental conditions, which may
affect the availability of resources to hosts for defense. However, the
interaction between parasitism and prevailing conditions is rarely
quantified. In addition to environmental variation acting on hosts,
individuals are likely to vary in their response to parasitism, and the
combined effect of both may increase heterogeneity in host responses.
Offspring hierarchies, established by parents in response to uncertain
rearing conditions, may be an important source of variation between
individuals. Here, we use experimental antiparasite treatment across 5
years of variable conditions to test how annual population productivity (a
proxy for environmental conditions) and parasitism interact to affect
growth and survival of different brood members in juvenile European shags
(Phalacrocorax aristotelis). In control broods, last-hatched chicks had
more plastic growth rates, growing faster in more productive years. Older
siblings grew at a similar rate in all years. Treatment removed the effect
of environment on last-hatched chicks, such that all siblings in treated
broods grew at a similar rate across environmental conditions. There were
no differences in nematode burden between years or siblings, suggesting
that variation in responses arose from intrinsic differences between
chicks. Whole-brood growth rate was not affected by treatment, indicating
that within-brood differences were driven by a change in resource
allocation between siblings rather than a change in overall parental
provisioning. We show that gastrointestinal parasites can be a key
component of offspring's developmental environment. Our results also
demonstrate the value of considering prevailing conditions for our
understanding of parasite effects on host life-history traits.
Establishing how environmental conditions shape responses to parasitism is
important as environmental variability is predicted to increase.