Abstract:
The immune system provides vital protection against pathogens, but extensive evidence
suggests that mounting immune responses can entail survival and fecundity costs. The
physiological mechanisms that underpin these costs remain poorly understood, despite
their potentially important role in shaping life-histories. Recent studies involving laboratory
models highlight the possibility that oxidative stress could mediate these costs, as immuneactivation
can increase the production of reactive oxygen species leading to oxidative
stress. However, this hypothesis has rarely been tested in free-ranging wild populations,
where natural oxidative statuses and compensatory strategies may moderate immune responses
and their impacts on oxidative status. Furthermore, the possibility that individuals
scale their immune responses according to their oxidative status, conceivably to mitigate
such costs, remains virtually unexplored. Here, we experimentally investigate the effects of
a phytohaemagglutinin (PHA) immune-challenge on oxidative status in wild male and female
white-browed sparrow weavers, Plocepasser mahali.We also establish whether baseline
oxidative status prior to challenge predicts the scale of the immune responses.
Contrary to previous work on captive animals, our findings suggest that PHA-induced immune-
activation does not elicit oxidative stress. Compared with controls (n = 25 birds),
PHA-injected birds (n = 27 birds) showed no evidence of a differential change in markers of
oxidative damage or enzymatic and non-enzymatic antioxidant protection 24 hours after
challenge. We did, however, find that the activity of a key antioxidant enzyme (superoxide
dismutase, SOD) prior to immune-activation predicted the scale of the resulting swelling: birds with stronger initial SOD activity subsequently produced smaller swellings. Our findings
(i) suggest that wild birds can mount immune responses without suffering from systemic
oxidative stress, and (ii) lend support to biomedical evidence that baseline oxidative
status can impact the scale of immune responses; a possibility not yet recognised in ecological
studies of immunity.
