Size Does Not Matter: Same-Sex Sexual Behavior Occurred Regardless of Mandible Size in Male Stag Beetle Aegus chelifer chelifer (Coleoptera: Lucanidae)

Abstract

Male Aegus chelifer chelifer stag beetles are normally equipped with long mandibles as a weapon to compete with rival males over females, where fighting or aggressive behavior is generally expected during the encounter between two male stag beetles. However, three main patterns were observed between 164 pairings of male stag beetles. Besides the predominant aggressive or fighting behavior (107 pairings), male stag beetles also exhibited disregarding behavior (29 pairings) and same-sex sexual behavior (28 pairings). For sexual interactions, active males exhibited courting and mating behavior as found in male-female interactions, but the insemination process was unsuccessful. Responses of passive males varied from totally passive to resisting and running away. Visual confusion of short-mandible males for females by active males was not likely to be responsible for the same-sex sexual behavior because the correlation between body size and the sexual behavior or sexual role between male-male pairs was not significant. Rather, sexual recognition via chemicals on the cuticle surface might be responsible for the same-sex sexual behavior.