Individual variation in glucocorticoid levels: a field study of repeatability
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We readily appreciate the wide phenotypic diversity that distinguishes animal species; however, even within a population of the same species, individuals can express significant phenotypic variation in their behavior, morphology, and physiology. Natural selection acts on among-individual phenotypic variation, linking an individual’s phenotype to its fitness. A necessary first step toward understanding some evolutionary and ecological processes, which are driven by the relative fitness of certain phenotypes over others, is to accurately characterize consistent individual differences in traits that influence survival and/or reproduction. Circulating concentrations of glucocorticoid (GC) hormones are plastic (i.e., capable of changing across a range of temporal scales in response to internal or external conditions) and involved in numerous processes related to fitness. We assessed the repeatability (or, trait consistency) of corticosterone (CORT; the primary avian GC) titers in free-living black-capped chickadees (Poecile atricapillus) during the non-breeding season. We also investigated internal and external factors that could explain the degree of repeatability/plasticity we observed. We found that initial CORT concentration had a significant agreement repeatability of 0.284. On average, CORT concentrations were higher during the second capture compared to the first. Furthermore, initial CORT was lower among individuals that experienced lower temperatures and higher wind speeds prior to capture, and among individuals with relatively poorer oxidative balance. Overall, our results suggest that individuals respond plastically to harsh winter conditions, while also maintaining a degree of within- individual consistency that may reflect the influence of a combination of genetic differences among individuals and stability in aspects of their environment. These findings add to the accumulating evidence that suggests that individual variation in GC concentrations within populations can often reflect short-term, dynamic changes in the environment more so than fixed differences among individuals.