Enhanced male coloration after immune challenge increases reproductive potential

In many animal species, females select a mate on the basis of the expression of secondary sexual traits. A prevalent theory suggests that male ornaments are reliable indicators of immunocompetence, because the cost of immune function prevents cheating. However, sexual signalling is a component of males’ reproductive effort, and an immune challenge may also alter the organism’s perceived future prospects and hence their signalling effort. In this study, to investigate the consequences of mounting an immune response on signalling effort, blue-footed booby males (Sula nebouxii) during courtship were inoculated with diphtheria–tetanus vaccine. We found that, after this immune challenge, on average males increased their signalling effort but lost more body mass compared to control males. Importantly, vaccination affected the partner’s reproductive decisions: compared to control females, females paired with vaccinated males laid eggs earlier, and increased clutch volume in early pairs. Overal...