Date of Award

Spring 1-1-2012

Document Type


Degree Name

Master of Arts (MA)


Ecology & Evolutionary Biology

First Advisor

Rebecca J. Safran

Second Advisor

Michael D. Breed

Third Advisor

Christopher A. Lowry


In natural populations, reproductive success and survival largely depend on phenotypic traits that are functionally relevant in a given context. It is also important for individuals to be able to respond adaptively to a fluctuating environment and to provide reliable information about this ability to conspecifics. There is remarkable variation in phenotypic traits such as sexual signals and behavior that stems from variation in the physiological mechanisms mediating trait expression, and in turn, physiological systems are shown to greatly influence fitness outcomes. In order for evolutionary processes to shape sexual signals and behavior to be both informative and adaptive, variation in the underlying physiology that regulates phenotypic trait expression should be explained by the variation in the expressed trait and genetic variation that selection can act upon.

The avian glucocorticoid hormone corticosterone (CORT) is a steroid hormone involved in many important physiological and behavioral processes at baseline levels and also at heighted concentrations in response to stressors. Within populations, inter-individual variability in both baseline and stress-induced CORT concentrations is often substantial, and can predict reproductive success and survival in free-living populations. Sexual signal traits have been proposed to convey information about individual variation in the response to stressors, but these links are not well understood. Likewise, while previous studies have found that patterns of CORT secretion are likely shaped by evolutionary processes, few studies have attempted to estimate the relative roles of genes and environment on variation in CORT profiles in natural populations. The aim of my thesis was to address these gaps in our knowledge about the relationship between a sexually selected melanin-based plumage trait and the stress response as well as the heritability of CORT secretion, which will ultimately serve to provide information on whether selection can shape phenotypic traits important to reproduction and survival.