Bimodal frequency distribution as a criterion for dimorphism in male rhinoceros beetles (Coleoptera: Scarabaeidae: Dynastinae)

Abstract

AbstractBateson & Brindley (1892) reported that males of the rhinoceros beetle Xylotrupes gideon are dimorphic (i.e. occurring in two distinct forms) with respect to size of the prominent cephalic horn. This claim was based on a comparison of frequency distributions for size of the horn (bimodal) and size of the body (unimodal). Bateson and Brindley did not realize, however, that a bivariate plot of horn length against body size follows a steep, S-shaped trajectory, or that steepness of such a curve—and not dimorphism—underlies the bimodal frequency distribution for size of the horn. The bivariate distribution for horn size vs. body size is continuous and without breaks or interruptions, so the beetles merely exhibit a pattern of allometric variation that went unrecognized at the time. Other species of dynastine beetle also display patterns of sigmoid allometry, but only those with the steepest slopes for the allometric curve also have bimodal frequency distributions for size of the horn. The concept of male dimorphism in X. gideon and other dynastine beetles needs to be reconsidered in the light of findings reported here and elsewhere.