Male horn dimorphism, phylogeny and systematics of rhinoceros beetles of the genus Xylotrupes (Scarabaeidae : Coleoptera)

Abstract

Male horns in several groups of beetles represent a special class of secondary sexual characters in which condition-dependent, alternate developmental programs produce not only a bimodal horn-size distribution, but also discrete male mating behaviours correlated with these alternate phenotypes. While these intrasexual dimorphisms have recently received theoretical and experimental attention concerning how they are produced and modified, less has been focussed on the macroevolutionary behaviour of the ontogenetic mechanism that produces them. The developmental program that produces alternate male morphologies is manifested by a non-linear horn-size allometry that has been noted to vary within and among various taxa according to its shape and position. The purpose of the present study is to produce a preliminary measure of the macroevolutionary behaviour of these allometric characters as a function of defined phylogenetic scale among the rhinoceros beetles of the widespread genus Xylotrupes.

A phylogenetic analysis performed herein suggests that Xylotrupes is monophyletic and is composed of six lineages, which are treated as discrete species. The taxon Xylotrupes gideon of previous literature is shown to constitute five species. Explicit rationale, including morphological diagnoses and evidence of reproductive isolation, supports a new, readily testable taxonomic scheme that recognises the following species: Xylotrupes florensis in the Lesser Sunda and Tanimbar Islands, Indonesia; X. meridionalis in Sri Lanka and India; X. ulysses in Sulawesi, Moluccas, Australia, Papua New Guinea and Melanesia; X. pubescens in the Philippines, Sumatra and Sulawesi; X. mniszechi in south-central and south-east Asia and China; and X. gideon in west Malaysia, Borneo and the Indonesian archipelago from Sumatra through the Lesser Sunda Islands. Subspecies are recognised in some of these taxa and are based upon geographic and phylogenetic partitioning.

As in other groups of beetles, the sigmoidal allometric relationship of horn size to body size produces bimodal horn-size distributions in the males of all the species of Xylotrupes in which adequate samples were obtained. The present data show that there is more variation in allometric shape but less variation in allometric position in Xylotrupes than in dung beetles of the genus Onthophagus. Moreover, the phylogenetic patterns of variation in horn allometry among the taxa of Xylotrupes indicate that as much variation in allometric position and shape occurs among the subspecies of a single species, X. ulysses, as occurs among the remainder of the species in this genus.

Evidence is provided that allometric position in Xylotrupes is responsive to interspecific competition inasmuch as character displacement of body size relative to horn size occurs in newly discerned sympatric populations of X. gideon and X. pubescens zideki. Further, major evolutionary modifications in allometric shape in two subspecies of X. ulysses have apparently occurred independently and involve fundamentally different adaptive mechanisms.

These results suggest that modifications in the developmental program that controls male horn dimorphism are a principal feature of diversification in the beetles of the genus Xylotrupes.