Sexually dimorphic immune response in the harem polygynous Wellington tree weta Hemideina crassidens

Adult males are often less immunocompetent than females. One explanation for this is that intense sexual selection causes males to trade-off investment in immunity with traits that increase mating success. This hypothesis is tested in the Wellington tree weta (Hemideina crassidens), a large, sexually dimorphic orthopteran insect in which males possess enormous mandibular weaponry used during fights for access to female mates. Field-collected males have a significantly greater immune response (greater melanotic encapsulation) than females, suggesting that body condition, longevity or an allied trait is important to male fitness, or that females require materials for egg production that would otherwise be used to boost immunity. Although immunity is expected to trade-off against reproductive traits in both sexes, there is no significant relationship between immune response and weapon or testes size in males, nor fecundity in females.