Date of Award:


Document Type:


Degree Name:

Doctor of Philosophy (PhD)




Susannah S. French


Animals trade-off limited resources among competing demands. Trade-offs are difficult to quantify because it is challenging to measure investment into disparate physiological systems using a common scale. Additionally, biologists desire methods to more precisely measure energy status in wild animals. I used stable isotopes to help solve both of these problems. I examined natural spatial and temporal variation in stable isotope signatures of wild lizards and found significant variation. In the lab, I was able to demonstrate the utility of nitrogen stable isotope ratios of uric acid pellets for measuring nutritional stress. By tracing labeled amino acids through the bodies of gravid female lizards, I demonstrated that vitellogenesis and wound healing compete for amino acids and quantified the direction and magnitude of the trade-offs. I showed that reproductive-immune trade-offs vary based on reproductive stage and energy availability, have effects on metabolism and immune function, and are influenced by hormonal mechanisms. My findings shed light on the interconnectedness of stable isotope endpoints and key physiological systems in animals. I showed that isotopic signatures of physiological stress can be reflected at a large scale in natural populations, and I made novel measurements of the size and direction of trade-offs, which were formerly limited to physiological and performance outcomes.



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