Understanding the selective pressures that drive morphological diversity is one of the most exciting challenges facing evolutionary biologists. Mate Choice and male-male competition, in particular, are potent drivers in the evolution of male morphology and has given rise to an array of flashy ornaments that are attractive to females, and an arsenal of enlarged weapons that aid in male-male combat. Although previous studies have focused extensively on the process of female choice and the evolution and diversification of male ornaments, surprisingly little work has been done on the evolution of male weapons, and the diversity of selected weapons to engage males and attract females is still poorly understood. The research takes a multidisciplinary and integrative approach to explore the evolution and diversification of a class of particularly extravagant male weapons: rhinoceros beetle horns. Specifically, the project examines how 1) selection to minimize the physiological costs of horns, and 2) selection to maximize the performance of horns during combat may help explain the patterns of divergence in weapon form among rhinoceros beetles. One exciting and unexpected explanation for why beetle horns are so large and diverse is already emerging from this research: horns are not especially costly to produce or carry, and therefore may be particularly evolutionarily labile. The experiments outlined here expand on this research by incorporating several novel techniques, including micro-CT scanning and finite element analysis, to explicitly test how variation in horn morphology affects performance during combat. This dissertation project therefore encompasses many fields of biology, including evolution, behavioral ecology, physiology, and biomechanics, and applies new engineering tools for analyzing and interpreting variation in animal body form. Combined, the research promises new insight into the interplay between organismal form, function, and diversity.
Surface reconstructions of the horns will be posted at http://dbs.umt.edu/emlenlab/
