Sexual Selection and Sexual Dimorphism in the Harlequin Beetle Acrocinus longimanus

Abstract

To investigate the causes of striking sexual dimorphism in the harlequin beetle (Acrocinus longimanus), we carried out a study of the behavior and morphology of two widely separated populations (in French Guiana and Panama). Males of this species possess greatly elongated forelegs which exhibit strong positive allometry with body size (elytra length). Males use their forelegs in fights with other males and in guarding females and oviposition sites. Field data on fighting and mating success suggest that sexual selection favors large size in males. However, the direct targets of selection were masked by high correlations between traits of the elytra, forelegs, and antennae. Sexual selection appears to be driven by intense male competition to monopolize suitable sites for egg deposition. Female harlequin beetles are highly selective in choosing only recently dead or dying trees for oviposition (Moraceae and Apocynaceae). Despite marked geographic variation in coloration, host trees, and climate, the two populations did not differ in mean size and extent of sexual dimorphism. We suggest that sexual selection in this species is sufficiently intense to override any effects of differing ecological factors.