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It Takes Guts to Go Green: Identifying the Factors That Limit the Evolution of Herbivory in Reptiles Background

The evolution of herbivory in reptiles is rare and poorly understood. Researchers have speculated why some lineages of lizards have undergone evolutionary shifts from omnivory to herbivory, but no one has studied the mechanistic basis of this form of diet evolution. I will test whether induced shifts in diet (i.e., omnivory to herbivory) adhere to the popular paradigm that formfollows-function in evolution. This hypothesis predicts that major evolutionary changes occur in a predictable pattern: behavior evolving first (e.g., recognizing plants as food), followed by changes in physiology (e.g., gut enzyme activity), and finally morphology (e.g., gut size and proportion). To test this hypothesis on a proximate level, I will compare several physiological and morphological characteristics of the alimentary tracts of omnivorous lizards ( Pogona vitticeps) raised on one of three diets (carnivore, omnivore, or herbivore). The digestive tract is the focus of this investigation because, unlike other traits associated with specific diets (e.g., teeth), the gut exhibits both flexibility and plasticity in response to changes in diet, thus providing the necessary variation upon which selection could act. I will measure rates of growth, digestive efficiency, and passage rate as whole-organism measures of digestive performance. Gut enzyme levels and nutrient transport rates will provide measures of tissue-level function. Gross and histological measures of gut anatomy will quantify morphological plasticity. My study should elucidate whether physiological and/or morphological characteristics preclude this omnivore from exploiting a strictly herbivorous diet. This is the first study to attempt to elucidate the mechanisms constraining the evolution of herbivory from omnivory, while simultaneously testing the paradigm that major evolutionary shifts evolve in a form-follows-function manner.