Seasonal Rhythms
Abstract
© 2017 Elsevier Inc. All rights reserved. Many free-living species show seasonal rhythms in baseline and stress-induced glucocorticoid release. Glucocorticoid titers, as well as glucocorticoid-binding proteins (CBG) and intracellular glucocorticoid receptors, vary with different life history stages over the course of the year. Concentrations are often highest during breeding. Seasonal glucocorticoid rhythms appear to be regulated by a mixture of hypothalamic, pituitary, adrenal, and glucocorticoid feedback mechanisms. Three hypotheses have been proposed to explain why seasonal glucocorticoid rhythms exist: to mediate seasonally different energetic needs, to initiate seasonally appropriate stress-induced behaviors, and/or to prepare for subsequent stressors whose frequency varies seasonally. Future work distinguishing between these hypotheses will help us understand how seasonal glucocorticoid rhythms likely aid the short-term survival of free-living animals. Finally, biomedical studies that ignore seasonal variation in glucocorticoid responses miss the rich variation in these responses that likely affect humans as well.