1. Sexually selected traits often honestly advertise aspects of individual quality, such as immune function. Such traits have traditionally been thought to reveal real-time information (e.g. current health state), but they may also reflect immunological conditions experienced during ontogeny, which can fundamentally shape survival prospects, adult immune function, and reproductive performance.
2. We tested the effects of immune challenges (injections of sheep red blood cells) during neonatal development on adult immune function and ornamental coloration in male mallard ducks (Anas platyrhynchos). Mallards have a carotenoid-pigmented beak that plays a role in mate choice, and because carotenoids are also associated with immune responsiveness, carotenoid accumulation in body tissues may change as a function of immunological state during development and into adulthood.
3. We found that mallards that received immune challenges at the completion of growth (8–10 weeks old) had a reduced proinflammatory response (as measured by the local swelling response to phytohemagglutinin; PHA) as adults, compared to control birds and those receiving immune challenges at other developmental time points. Mallards that received immune challenges during the acquisition of nuptial plumage (13–15 weeks old) had an increased humoral immune response as adults, as assessed by antibody production to keyhole limpet hemocyanin (KLH). Body mass, adult beak colour, and carotenoid circulation were not affected by immune challenges issued at any point during development.
4. Circulating carotenoid levels prior to adult immune challenge and change in carotenoid titre over the course of the adult immune assessment period were correlated with degree of response to PHA and KLH at adulthood, as well as with nitric oxide production, which is associated with innate immunity. However, the direction of these correlations varied by type of immune assessment. Surprisingly, unlike in several previous studies, we found no link between beak coloration and adult immune function.
5. These findings demonstrate the importance of developmental immune challenges in shaping adult immune response, although the precise mechanisms remain unidentified. Moreover, we found support for the hypothesis that carotenoids are linked to immune function, because of the relationship between multiple immune responses and both initial and change in circulating carotenoid levels.
6. However, our study does not support the hypothesis that sexual signal expression reveals developmental immune history. Taken together, our findings suggest that the relationships between carotenoid physiology, carotenoid-based ornamentation, and immune function are complex.