It takes two to tango: Functional roles, sexual selection and allometry of multiple male weapons in the flower beetle Dicronocephalus wallichii bourgoini

Males of many insect species exhibit multiple sexually selected weapons, but the functional roles and evolution of them are poorly understood. The males of the flower beetle, Dicronocephalus wallichii bourgoini are equipped with exaggerated horns and elongated forelegs. To explore the selective forces acting on these two weapons, we investigated their functions by examining male–male competition and allometric relationships between the weapons and body size. We found that competition between mate-guarding (i.e. owners) and unpaired males (i.e. intruders) frequently occurred in the field. In the early phase of the contests, males mainly used their forelegs, likely, to assess the body size of their opponents. If the foreleg length of the owner was shorter than that of the intruder, the intruder approached to take over the owner’s mate. Escalated contests occurred, in which both opponents tried to drag each other away from the females or the substrate using their horns. This suggests that the multiple weapons in this species are specialized for specific phases of contests. The males with larger bodies and weapons were more successful in defending their ownership of mates or taking over guarded females. The allometric slope of horns was positive in small males, but it decreased in large males. In contrast, male forelegs exhibited isometry without a switch point, and the slope was significantly steeper than that of female forelegs. Our findings suggest that sexual selection acts on both male weapon traits in D. w. bourgoini but that antagonistic natural selection constrains the further exaggeration of these traits in different ways.