45.6 Saturday, Jan. 5 Fitness consequences of individual variation in stress hormone levels: why repeatability and plasticity of physiological traits matter MADLIGER, C.L.*; LOVE, O.P.; University of Windsor, Ontario; University of Windsor, Ontario email@example.com
Physiological measures can provide insight into how organisms respond mechanistically to changes in their environment. Baseline stress hormones (glucocorticoids – GCs) have garnered considerable attention due to their essential role in the maintenance of energetic balance. However, to understand the evolutionary implications of individual variation in GCs and interpret concentrations as population-level indicators of environmental change, GCs must display two characteristics: i) high repeatability (consistency); ii) a predictable relationship with fitness. Results pertaining to both have been markedly mixed and investigations often lack a consideration of ecological or demographic contexts. We investigated the repeatability of baseline GCs in a free-living population of tree swallows (Tachycineta bicolor) within and across breeding seasons. In addition, we incorporated a feather-clipping manipulation to examine the influence of changing energetic cost (i.e., environmental quality). We find high repeatability within, but not across, years. However, our results indicate that this high within-season repeatability is dependent on age and energetic constraints, providing evidence for individually-specific plasticity in the response to environmental fluctuations. We further investigate whether plasticity in GCs represents a better predictor of fitness than static measures of the trait. Our results call to attention the importance of considering the contexts of environmental quality and age when examining repeatability, caution the interpretation of individual baseline GC levels as population-level indicators of environmental disturbance, and indicate that an investigation of plasticity can provide insight into the evolutionary consequences of variation in physiological traits.