Diel rhythms of basal and stress-induced corticosterone in a wild, seasonal vertebrate, Gambel's white-crowned sparrow

Glucocorticoids have a wide array of actions in vertebrates. Daily fluctuat ions in basal levels of glucocorticoids are thought to regulate homeostatic mechanisms. In contrast, elevated levels secreted in response to stress st imulate changes in physiology and behavior. These changes are thought to ai d an animal in avoiding chronic stress or death. Twenty-four-hour rhythms i n basal and stress-induced glucocorticoids have been detected in laboratory mammals, but studies in wild, seasonal vertebrates are rare. Identificatio n of plasticity in hormone secretion in wild vertebrates is critical to und erstanding the effects of hormones on physiology and behavior, and therefor e the success of an animal in its natural environment. In the present study , we characterized diel patterns of basal and stress-induced corticosterone (the avian glucocorticoid) under two photoperiods in Gambel's white-crowne d sparrow (Zonotrichia leucophrys gambelii). In contrast to previous findin gs in the white-crowned sparrow, we demonstrated a robust rhythm in basal c orticosterone secretion, in which corticosterone reaches peak levels at the end of the inactive period, and has returned to trough levels just after t he active period has begun. We also demonstrated a diel rhythm in secretion of corticosterone in response to a stressor, showing the greatest response at the beginning of the active period. Patterns of CORT secretion were sim ilar under both photoperiods. These patterns show interesting similarities and differences to classical mammalian rhythms. (C) 1999 Wiley-Liss, Inc.