Exploring the role of carotenoid pigments in physiological function and color signals
Type of DegreePhD Dissertation
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The study of carotenoid-based coloration, particularly in bird species, is a large and growing literature that is increasingly defined by the framework of a single overarching concept, the resource tradeoff hypothesis. Contrary to widespread acceptance of resource tradeoffs in maintaining the condition-dependence of carotenoid-colored signals, methodological and theoretical challenges alike have complicated my ability to conclusively test the hypothesis. Here, I first review the existing empirical literature that has attempted to test the resource tradeoff hypothesis in bird species, and I highlight outstanding uncertainty in several key components of the hypothesis as well as important directions for future research. Given that one of the primary complications in testing for differential resource allocation of carotenoid resources is the ability to experimentally manipulate dietary intake, I next present a meta-analysis that quantitatively describes the relationships between dietary carotenoid supplementation and changes to circulating carotenoid levels or carotenoid-based coloration. A second main complication in testing the resource tradeoff hypothesis is the ability to use experimental challenges to physiological systems to assess whether carotenoid pigments play a role in the response. While immune challenges, such as pathogen infection or parasite administration, have been better studied, experimental techniques that induce increased oxidative stress remain new and relatively unexplored; I therefore present the first comprehensive review of a variety of methods that have been or could potentially be used to induce increased oxidative stress in animal bodies without confounding side-effects. Before applying one of these techniques to my own study system, I first describe that system—the domestic canary (Serinus canaria)—in a short comparative study that uses feather samples and museum specimens to establish the evolution of sexual monochromatism through domestication. Finally, I perform my own empirical test of the core assumption of the resource tradeoff hypothesis, that carotenoid pigments provide direct physiological benefits, by examining whether carotenoid-free mutant canaries are unable to perform as well as carotenoid-rich wild-type canaries on immune and oxidative tests. The results of my analyses are a conclusive demonstration that birds do not need internal carotenoid pigments to defend against pathogens or prevent oxidative stress, at least when the confounding effects of retinol are removed. Collectively, my dissertation presents a critical examination of all angles of the resource tradeoff hypothesis, from reviewing existing evidence to raising methodological concerns to finally challenging the hypothesis with my own empirical study. My research offers an important advancement to understanding the role of carotenoid pigments in physiological processes in animals and to how future researchers study carotenoids, those physiological processes, and coloration itself.