The correlation between immunocompetence and an ornament trait changes over lifetime in Panorpa vulgaris scorpionflies

The immunocompetence-handicap hypothesis posits that costly male ornament traits might function to signal superior heritable immunocompetence to females. Quite a number of studies have aimed at testing this hypothesis. Yet the empirical data obtained so far are ambiguous. Many studies analysed the phenotypic correlation between handicap expression and immunocompetence at the same time point. However, since immunocompetence may change drastically over an individual's lifetime, such a singular measurement may not represent genetic differences among males and the benefits of choosing handicapped males for females might thus be weak. Here, I tested the correlation of a potential immunocompetence-handicap, the production of salivary secretions as nuptial gifts in a scorpionfly (Panorpa vulgaris), with immunocompetence at two different time points. I found a positive correlation with the handicap, but only if immunocompetence was measured shortly after expression of the handicap, i.e. briefly after mating in 2 weeks old scorpionflies. By contrast, there was no correlation with immunocompetence of the same flies at a younger age, i.e. shortly after adult emergence and a weak, insignificant trend for increased immunocompetence in offspring. These results are in agreement with positive phenotypic correlations between immunocompetence and handicap expression reported from other species, but advise caution when generalizing such one-time correlations.