Samuel Urlacher

Objectives The hypothalamic–pituitary–adrenal (HPA) axis and its primary end product, the glucocorticoid cortisol, are major components of the evolved human stress response. However, most studies have examined these systems among populations in high‐income settings, which differ from the high pathogen and limited resource contexts in which the HPA axis functioned for most of human evolution. Methods We investigated variability in diurnal salivary cortisol patterns among 298 Indigenous Shuar from Amazonian Ecuador (147 males, 151 females; age 2–86 years), focusing on the effects of age, biological sex, and body mass index (BMI) in shaping differences in diurnal cortisol production. Saliva samples were collected three times daily (waking, 30 minutes post‐waking, evening) for three consecutive days to measure key cortisol parameters: levels at waking, the cortisol awakening response, the diurnal slope, and total daily output. Results Age was positively associated with waking levels and total daily output, with Shuar juveniles and adolescents displaying significantly lower levels than adults ( p < .05). Sex was not a significant predictor of cortisol levels ( p > .05), as Shuar males and females displayed similar patterns of diurnal cortisol production across the life course. Moreover, age, sex, and BMI significantly interacted to moderate the rate of diurnal cortisol decline ( p = .027). Overall, Shuar demonstrated relatively lower cortisol concentrations than high‐income populations. Conclusions This study expands the documented range of global variation in HPA axis activity and diurnal cortisol production and provides important insights into the plasticity of human stress physiology across diverse developmental and socioecological settings.