Functional roles of multiple male weapons in the flower beetle <i>Dicronocephalus wallichii</i>

Males of various species of Scarabaeoidea with horns or enlarged mandibles also have elongated forelegs. The co-occurrence of these male traits raises the question of whether the forelegs and mandibles/horns are functionally related and thus examples of correlated evolution. Few studies have examined in detail how these multiple weapons are used in contests. In this paper, I explain the function of elongated forelegs and horns in male?male competition of the Taiwanese flower beetle, Dicronocephalus wallichii bourgoini (Coleoptera: Scarabaeidae: Cetoniinae). Individuals of this species aggregate on bamboo shoots, where they mate and injure the shoots to feed on sap. Males mount the females after copulation for several hours. The sex ratio at feeding sites is male biased, and competition between the mate-guarding (i.e., owners) and unpaired males (i.e., intruders) frequently occurs. Analyses of the sequence of competition indicated that an owner would touch an intruder's body with its forelegs and then intensely move the forelegs, probably to assess the body size or fighting ability of the opponent. In escalated contests, both opponents tried to drag the other away from the female or substrate mainly using horns. This suggests that their multiple weapons are specialized for specific phases of contests. Males with larger weapons (or bodies) were more successful in defending ownership of their mates. Analyses of horn and foreleg allometry also suggested that these traits are products of sexual selection. Furthermore, I tested whether elongated forelegs impede the maximum sprint speed on bamboo branches. There was no negative relationship between relative foreleg length and sprint speed. Additionally, males with longer forelegs were found to have longer midlegs and hindlegs, independent of body size. Thus, elongated midlegs and hindlegs in males may enhance balance, stabilize running on bamboo branches, and compensate for the locomotor costs of possessing enlarged forelegs.