Sexually selected shields: male–male combat can promote the evolution of damage-reducing structures.

Empirical and theoretical evidence suggests that getting injured during combat decreases an individual's chances of winning. Thus, it has been hypothesized that species that engage in injurious fights may evolve armour. However, much remains unknown about the role that male–male combat has in promoting the evolution of such damage-reducing structures. Here, we tested the hypothesis that male–male combat can increase the damage resistance of structures using the giant mesquite bug, Pachylis neocalifornicus. When males in this species fight over access to mates, they use their weaponized hindlegs to puncture the dorsal side, but not the ventral side, of their rivals. We found that structures on the dorsal side of males could generally withstand the forces of combat, while structures on the dorsal side of females could not. The sexual dimorphism in damage resistance was location dependent because structures on the ventral side of both males and females also could not withstand the forces of combat. Thus, males appear to only increase their defensive capacity in the locations where strikes from rivals normally land. These results provide evidence that male–male combat can promote the evolution of damage-reducing structures (i.e. sexually selected shields). Moreover, the relatively inconspicuous nature of the sexually selected shields in this species suggests that defensive traits related to combat may be more common than currently considered. • We tested damage resistance of structures in the fighting insect P. neocalifornicus. • Male cuticle was harder to puncture than female cuticle. • Sexual dimorphism in damage resistance was location dependent. • Male armour can withstand some, but not all, fighting-related forces. [ABSTRACT FROM AUTHOR]