title: Baseline and stress-induced glucocorticoid concentrations are not repeatable but covary within individual great tits (Parus major).
abstract: In evolutionary endocrinology, there is a growing interest in the extent and basis of individual variation in endocrine traits, especially circulating concentrations of hormones. This is important because if targeted by selection, such individual differences present the opportunity for an evolutionary response to selection. It is therefore necessary to examine whether hormone traits are repeatable in natural populations. However, research in this area is complicated by the fact that different hormone traits can be correlated. The nature of these trait correlations (i.e., phenotypic, within-, or among-individual) is critically relevant in terms of the evolutionary implications, and these in turn, depend on the repeatability of each hormone trait. By decomposing phenotypic correlations between hormone traits into their within- and among-individual components it is possible to describe the multivariate nature of endocrine traits and generate inferences about their evolution. In the present study, we repeatedly captured individual great tits (Parus major) from a wild population and measured plasma concentrations of corticosterone. Using a mixed-modeling approach, we estimated repeatabilities in both initial (cf. baseline; CORT0) and stress-induced concentrations (CORT30) and the correlations between those traits among- and within-individuals. We found a lack of repeatability in both CORT0 and CORT30. Moreover, we found a strong phenotypic correlation between CORT0 and CORT30, and due to the lack of repeatability for both traits, there was no among-individual correlation between these two traits-i.e., an individual's average concentration of CORT0 was not correlated with its average concentration of CORT30. Instead, the phenotypic correlation was the result of a strong within-individual correlation, which implies that an underlying environmental factor co-modulates changes in initial and stress-induced concentrations within the same individual over time. These results demonstrate that (i) a phenotypic correlation between two hormone traits does not imply that the traits are correlated among individuals; (ii) the importance of repeated sampling to partition within- and among-individual variances and correlations among labile physiological traits; and (iii) that environmental factors explain a considerable fraction of the variation and co-variation in hormone concentrations.
authors.affiliation: Department of Biology, Swarthmore College, 500 College Avenue, Swarthmore, PA 19081, USA; Evolutionary Physiology Group, Max Planck Institute for Ornithology, Am Obstberg 1, 78315 Radolfzell, Germany. Electronic address: abaugh1@swarthmore.edu.
authors.forename: Alexander T
authors.initials: AT
authors.lastname: Baugh
authors.affiliation: Department of Animal Ecology, Netherlands Institute of Ecology (NIOO-KNAW), P.O. Box 50, 6708 PB Wageningen, The Netherlands. Electronic address: k.vanoers@nioo.knaw.nl.
authors.forename: Kees
authors.initials: K
authors.lastname: van Oers
authors.affiliation: Evolutionary Ecology of Variation Research Group, Max Planck Institute for Ornithology, Eberhard-Gwinner-Str., 82319 Seewiesen, Germany; Behavioural Ecology, Department of Biology, Ludwig-Maximilians University of Munich, Planegg-Martinsried, Germany. Electronic address: ndingemanse@orn.mpg.de.
authors.forename: Niels J
authors.initials: NJ
authors.lastname: Dingemanse
authors.affiliation: Evolutionary Physiology Group, Max Planck Institute for Ornithology, Am Obstberg 1, 78315 Radolfzell, Germany; Evolutionary Physiology Group, Max Planck Institute for Ornithology, Eberhard-Gwinner-Str., 82319 Seewiesen, Germany; Department of Biology, University of Konstanz, Universitätsstrasse 10, Konstanz, Germany. Electronic address: mhau@orn.mpg.de.
authors.forename: Michaela
authors.initials: M
authors.lastname: Hau
mesh_terms: D000818:Animals; D003345:Corticosterone / Q000097:blood; D005938:Glucocorticoids / Q000097:blood*; D008954:Models, Biological; D046109:Passeriformes / Q000097:blood* / Q000502:physiology*; D010641:Phenotype; D015203:Reproducibility of Results; D013312:Stress, Physiological
chemical_list: D005938:Glucocorticoids; D003345:Corticosterone
keywords: Corticosterone; Endocrine stress response; Hypothalamic-pituitary-adrenal axis; Multi-level variation; Repeatability; Variance partitioning