Abstract
Hormones are central mediators of genotype-phenotype and organism-environment interactions. Despite these important functions, the role of selection in shaping hormonal mediators of phenotype remains poorly understood. Thanks to decades of work by endocrinologists, circulating hormone levels have been measured in a diversity of organisms. Variation in other endocrine traits and mediators (e.g., receptor expression and binding globulins), and the hormonal response to standardized challenges (e.g., restraint, pharmacological challenges) are also increasingly measured in both captive and free-living populations. Large-scale comparative analyses of the multitude of available endocrine data represent a particularly promising approach to addressing the function and evolution of these key phenotypic mediators, and their potential to serve as indicators of disturbance. Several recent phylogenetic comparative analyses and meta-analyses have begun to reveal the power and potential of these approaches to address key questions in integrative biology. Here we highlight two recent developments that are facilitating such analyses: increasingly powerful and flexible phylogenetic comparative methods, and the release of two endocrine trait databases-HormoneBase (currently 474 species) and the Wildlife Endocrinology Information Network (currently 25 species)-that contain compiled measures of endocrine traits across vertebrates. Increasingly comprehensive comparative analyses of endocrine data could provide insight into many interesting questions, including how rapidly changing environments are impacting phenotypes, why endocrine traits differ so remarkably within and across populations, and the evolution of plasticity. The endocrine system mediates interactions between genotypes and phenotypes, and between organisms and their environment. Environmentally induced hormonal responses regulate phenotypic flexibility across timescales by altering physiological state, gene expression, and epigenetic marks. A staggering diversity of phenotypic traits are mediated by hormones from early development through senescence. Through their actions on behavior, hormones also exert widespread influence over how organisms interact with their biotic and abiotic environments. Because hormones are responsive to the environment, there has long been interest in their use as biomarkers of exposure to challenges. More recently, increasing attention has been paid to the potential for within and among-population variation in endocrine regulation or responsiveness to serve as indicators of resistance or resilience to future challenges, or measures of evolutionary potential.