Diurnal and annual variation of adrenocortical activity in the squirrel monkey

Diurnal changes in basal cortisol levels and stress reactivity were assessed in male and female squirrel monkeys. Blood samples were collected at four-hour intervals throughout the day and night in the mating and nonmating seasons. Basal cortisol levels in females were similar in both seasons, but males tended to have higher cortisol levels during the mating season, especially at night. For both sexes, cortisol secretion was highest between 0400 and 0800 preceding the onset of the diurnal activity period. Assessment of cortisol responses following brief handling and anesthesia indicated that stress responses were relatively stable across the year, but cortisol increments were slightly higher in the nonmating season. Cortisol levels post-stress were generally related to prior baseline values. Thus, a knowledge of biorhythmic changes in basal hormone levels was important for predicting hormone levels after acute stressors. Males also underwent marked seasonal variation in their basal testosterone levels, which markedly altered the nature of their testosterone responses 30-minutes poststress. © 1995 Wiley-Liss, Inc.