Investigating the turkey's ‘snood’ as a morphological marker of heritable disease resistance

Summary The sexual ornamentation of many wild species, including the ancestors of domestic poultry, are reliable indicators of parasite burden, suggesting that they may serve as morphological markers of heritable disease resistance. Measures of male ornamentation might be a culturally and economically feasible method of identifying conservation‐worthy genotypes from indigenous poultry flocks. Here we investigate the relationship between the sexually selected, parasite‐burden‐indicating ‘snood’ of male game farm turkeys, and their immunocompetence and immunogenetic [major histocompatibility complex (MHC) class II B restriction fragment‐length polymorphism (RFLP)] variation. We find that specific restriction fragments appear to be favourable to both immunocompetence and ornament size, but that turkey snood length is not a valuable predictor of wing web swelling due to phytohemagglutinin (PHA) injection. Immunocompetence increased with composite band sharing, a measure of absolute variation at the MHC. Snood length, however, was greatest in males that showed intermediate measures of relative band sharing. We conclude that tradeoffs among the conflicting demands on the energy reserves of free‐living poultry, might ensure that long‐snooded indigenous turkeys are best adapted to local disease conditions without wasting resources on hyperimmunity.