Effect of an immune challenge on the functional performance of male weaponry

• I immune challenged male and female Wellington tree weta (Hemideina crassidens).
• I measured changes in body mass and bite force performance over 24 h.
• Both sexes lost significant body mass with females losing more than males.
• Females exhibited a significant decrease in bite force performance 8 h post-challenge whereas no alternative male morph suffered a reduction in performance.
• Females but not males appear to trade-off bite force performance for immunity.

Theories of parasite-mediated sexual selection predict a positive association between immune function and the expression of sexually selected ornaments. Few studies, however, have investigated how an immune challenge affects the performance of sexually selected weaponry. Male Wellington tree weta (Hemideina crassidens) (Orthoptera: Anostostomatidae) possess enlarged mandibles that are used as weapons in fights for access to females residing in tree galleries. Intense sexual competition appears to have favoured the evolution of alternative male mating strategies in this species as males have a trimorphic phenotype in which weapon size varies across morphotype: 8th instar males have the smallest jaws, 10th instar males have the largest and 9th instar males being intermediate to the other two. After injecting males and females with either lipopolysaccharide (LPS; immune challenge) or saline (control) I measured over a 24 h period each weta's body mass to assess whether they responded immunologically to the LPS and their bite force to assess the functional performance of their jaws. Both sexes responded immunologically to the immune-challenge as LPS-injected individuals lost significantly more body mass than saline-injected controls with females losing more mass than males. Female bite force was significantly reduced 8 h after LPS-injection whereas male bite force did not significantly decline. Both sexes regained pre-injection functional performance of their jaws 24 h after the immune challenge. My results suggest that females trade-off bite force for immune function whereas males do not.This article is part of a Special Issue entitled: insert SI title.