Corticosterone as a Physiological Biomarker: Decoding the Environment-Cort-Energy Paradigm.

), using 14 years of repeated physiological, morphological, and environmental measurements. We applied structural equation modeling to compare three conceptual frameworks describing plasma glucocorticoid physiology: models based on total circulating GCs, free GCs (unbound to CBG), and both total GCs with CBG variation. Across models, we identified links between environmental conditions, energetic state, and GC/CBG physiology, but the strength and structure of these relationships differed by sex and by the hormonal measure considered. Models examining relationships with free hormone included a greater number of supported explanatory variables than models based on total hormone alone, while models incorporating CBG dynamics provided additional explanatory structure and clarified sex-specific patterns not fully captured by either measure individually. Our results indicate that inferences about glucocorticoid-environment relationships depend strongly on how hormone physiology is characterized, highlighting the importance of integrating binding dynamics and sex-specific physiology when interpreting glucocorticoid variation.