WINGFIELD, J.C.: Endocrine Responses to Unpredictable Environmental Events: Stress or Anti-stress Hormones?

In addition to seasonal changes in morphology, physiology and behavior that occur in predictable annual cycles, there are facultative responses to unpredictable events known as labile (i.e. short-lived) perturbation factors (LPFs). These rapid behavioral and physiological changes have been termed the “emergency” life history stage (ELHS) and serve to enhance life time fitness. Glucocorticosteroids interacting with other hormones in the hypothalamo-pituitary-adrenal (HPA) cascade, initiate and orchestrate the ELHS within minutes to hours. Components of the ELHS include: redirection of behavior from a normal life history stage to increased foraging, irruptive-type migration during the day, enhanced restfulness at night, elevated gluconeogenesis and recovery once the perturbation passes. These physiological and behavioral changes allow an individual to avoid potential deleterious effects of stress that may result from chronically elevated levels of circulating glucocorticosteroids over days and weeks. Several field studies in diverse habitats indicate that free-living populations have elevated circulating levels of corticosteroids when in an ELHS. However, expression of an ELHS may not always be advantageous and there is accumulating evidence from birds that the adrenocortical responses to LPFs are modulated both on seasonal and individual levels. These changes in responsiveness to LPFs may have ecological bases such as reproductive state, body condition etc., that in turn may indicate different control mechanisms in the HPA cascade. Some simple models based on food available in the habitat, body condition, social status and life history stage, may allow predictions of sensitivity of the HPA axis to LPFs.