Year of Award


Document Type


Degree Type

Doctor of Philosophy (PhD)

Degree Name

Organismal Biology, Ecology, and Evolution

Department or School/College

Division of Biological Sciences

Committee Chair

Douglas J. Emlen

Commitee Members

Mark Briffa, John P. McCutcheon, Bret W. Tobalske, H. Arthur Woods


Animal Weapons, Behavior, Entomology, Evolution, Sexual Selection


University of Montana


Sexually selected weapons represent some of the most spectacular morphologies in the animal world. They grow out of proportion with body size or other, more typically proportioned structures, and are some of the largest traits in both absolute and relative size. It is therefore unsurprising that animal weapons are some of the most intensely studied structures in biology. Yet, despite this interest, surprisingly little is known about the expression and evolution of these traits. In particular, four questions remain unanswered: How does selection act on weapons in the wild? Do the costs of large weapons ever outweigh the benefits? How are these patterns of cost and benefit reflected in the morphology and development of modern weaponed species? Can we use these patterns to infer the strength and direction of selection when natural observation is unattainable? My dissertation aims to answer these questions by describing the costs and benefits surrounding sexually selected weapons in the wild. I use the frog legged leaf beetle (Sagra femorata) as my primary study system.

In Chapter 1, I provide the first description of S. femorata mating behavior in the wild and provide an explicit measure of selection acting on the their hindleg weapons. In Chapters 2 and 3, I investigate factors that may shape patterns of selection observed in Chapter 1 – specifically, biomechanical and metabolic cost. In Chapter 4, I explore broad trends in morphological scaling that result from patterns of selection described in earlier chapters. I review the literature surrounding weapon evolution and propose a new method for characterizing selective history through measures of static morphological scaling. Collectively, this work provides a comprehensive analysis of weapons within and across taxa, expanding our understanding of sexually selected morphology and setting the stage for future studies of sexual selection and morphological evolution.



© Copyright 2018 Devin Mackenzie O'Brien