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

Area of Science:

• Endocrinology
• Ecology
• Animal Physiology

Background:

• Glucocorticoid hormones (GCs) are vital for assessing wild animal stress responses to environmental changes.
• Total plasma GCs are commonly measured, but free GCs, unbound to corticosteroid binding globulin (CBG), may better reflect tissue-level activity.
• Understanding GC dynamics is crucial for accurate interpretation of physiological stress in wildlife.

Purpose of the Study:

• To compare different frameworks for measuring plasma glucocorticoid physiology in wild animals.
• To investigate how environmental factors and energetic state influence GC and CBG dynamics.
• To determine the best approach for characterizing glucocorticoid stress responses, considering sex-specific differences.

Main Methods:

• Utilized 14 years of physiological, morphological, and environmental data from mountain white-crowned sparrows (Zonotrichia leucophrys oriantha).
• Applied structural equation modeling to compare models based on total GCs, free GCs, and GCs with CBG variation.
• Analyzed sex-specific patterns in hormone-environment relationships.

Main Results:

• Links between environmental conditions, energetic state, and GC/CBG physiology were identified, varying by sex and hormonal measure.
• Models incorporating free hormone and CBG dynamics explained more variables than models using total GCs alone.
• CBG dynamics clarified sex-specific physiological patterns not evident when considering total or free GCs separately.

Conclusions:

• Inferences on glucocorticoid-environment relationships are highly dependent on the chosen measurement method.
• Integrating corticosteroid binding globulin (CBG) dynamics and sex-specific physiology is essential for accurate interpretation of glucocorticoid variation.
• This study underscores the importance of a nuanced approach to measuring stress hormones in wildlife research.