Meeting Abstract
114.1 Tuesday, Jan. 7 10:30 The variance within: stress hormone levels vary and co-vary within but not among individual wild great tits Parus major BAUGH, A.T.*; VAN OERS, K.; DINGEMANSE, N.; HAU, M.; Swarthmore College; Netherlands Institute for Ecology; Max Planck Institute for Ornithology; Max Planck Institute for Ornithology abaugh1@swarthmore.edu
Circulating steroid hormones, such as glucocorticoids, can have diverse and enduring effects on the phenotype and might serve as a basis for individual differences in hormone-mediated traits. To understand the extent and basis of variation in the circulating concentrations of hormones themselves it is important to accurately decompose within- and among-individual variance components. Previous research in songbirds suggests that concentrations of glucocorticoids exhibit individual consistency, but the variance and covariance of plasma hormones in free-living animals remains poorly understood. In the present study, we repeatedly captured individual great tits Parus major from a wild population and measured their initial and stress-induced plasma corticosterone (CORT) levels. We evaluated variances and covariances in these concentrations using a mixed-modeling (character-state) approach. First, we examined within- and among-individual variances in initial (CORT0) and stress-induced levels (CORT30) and found little evidence of repeatability in either measure. Next, we examined the covariance between CORT0 and CORT30. As predicted, given the lack of repeatability, we found no among-individual covariance in these two measures—i.e. average initial levels did not correlate with average stress-induced levels. We did find, however, strong within-individual correlations suggesting that an underlying environmental factor(s) simultaneously modulates changes in initial and stress-induced levels within the same individual. Together, these results suggest that plasma glucocorticoid concentrations are determined principally by acute environmental or state-dependent factors.
