Across the tree of life, species diversity and trait divergence abounds. Ants are no exception. Within this single family of Insecta, evolution has produced a stunning array of species boasting behaviors and morphologies ranging from obligate mutualisms with Hemipterans and plants, high-speed trapjaws exhibiting some of the fastest movements of any animal, flattened heads serving as shields to defend against invaders, massive nomadic colonial migratory behavior, fungus farming, ovipositors transformed into a sting or acidopore capable of delivering a number of different venoms, and a host of other traits whose simple description alone could easily fill the space of an entire dissertation. Faced with such a remarkable display of variation, my doctoral work is driven by the question: What drives the evolution of variable defensive traits in ants, and what constraining forces might limit morphological disparity? Across the three chapters of my dissertation, I approach this question from both macroevolutionary and ecological perspectives. In my first chapter, I investigate the potential role of evolutionary trade-offs in constraining the evolution of morphological defensive traits in ants and assess the impact of these traits on diversification. For my second chapter, I target the hyperdiverse spiny ant genus Polyrhachis, and conduct a number of lab trials with live colonies to test the impact of cuticular spines on resource discovery rate, foraging effort, competitive ability, and anti-predator defense. Building upon my first two chapters, in my third chapter I probe the consequences of apparent ecological benefits of spines for species diversification and geographic range size in Polyrhachis. Overall, my dissertation work establishes defensive traits, and especially spines, as important factors in ant evolution. While the first two chapters support spines as drivers of evolutionary and ecological success, the third highlights potential complexities of this association while additionally suggesting that spines promote increased geographic range size as well. Future work on defensive traits in ants is likely to produce informative tests of trait-based diversification and the impact of functional traits on ant ecological interactions.




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