What makes an individual successful? Individual variation in hormones, behavior, and fitness

Abstract

Understanding the mechanisms underlying life-history diversity is a major goal in evolutionary biology. Hormones participate in organizing life-stage transitions and can provide the mechanistic basis of individual differences in reproductive behavior. Although there is substantial variation, reflected in each individual's facultative decisions, given its energy state and the local environmental conditions, we currently have few studies that characterize individual endocrine variation in relation to direct measures of fitness, especially in wild populations. Previous studies have documented the co-variation between phenotypic traits and hormone levels, but questions remain open as to whether hormonal variation affects fitness and if it is affected by selection. Studying the magnitude and patterns of endocrine variation can uncover the mechanisms regulating the evolution of complex traits. For my thesis, I characterized intra- and inter-individual variation in hormone levels, behavior, reproductive success, and survival in relation to the external environment for a detailed investigation of the endocrine traits that underlie life-history evolution.

In my work, I showed that concentrations of corticosterone, a hormone that regulates metabolism, are highly plastic and not a characteristic of an individual, as concentrations varied across reproductive stages. This plasticity was reflected during a year with low food availability, in which high stress-induced corticosterone levels of male great tits (Parus major), induced them to abandon their low-quality broods. I also showed that absolute concentrations of hormones during the pre-breeding season can predict yearly reproductive success in both house sparrows, (Passer domesticus), and great tits, supporting the preparative function of hormonal mechanisms. Specifically, prolactin concentrations before breeding indicated proximity to laying, and baseline corticosterone levels predicted reproductive effort during breeding, thus affecting yearly fledgling production. In great tits, plasticity in stress-induced corticosterone concentrations reflected food availability during the three years of study. Lastly, I showed that an exogenous increase in baseline corticosterone concentrations increased individual reproductive behaviors, which corroborates the correlative relationships found earlier. My results further our understanding of the evolutionary dynamics of hormonal traits in wild avian species, demonstrate that endocrine axes respond plastically to the external environment, and establish the functional significance of individual hormonal variation.