Developmental patterns of hair cortisol in male and female nonhuman primates: lower hair cortisol levels in vervet males emerge at puberty.

Studies have yielded inconsistent results with regard to effects of age and sex on short-term markers of hypothalamic pituitary adrenal (HPA) activity. Hair cortisol provides a retrospective proxy measure of the cumulative activity of the HPA axis over the preceding 3- to 4-month period. In order to describe potential developmental trends in this biomarker, we assessed hair cortisol levels between 1 and 12 years of age in a cross-sectional study of 350 vervets (222 females and 128 males). Monkeys were grouped according to age as 1 (young juvenile), 2 (juvenile), 3 (early adolescent), 4 (late adolescent-young adult), and 5-12 (adult) years of age such that fully mature animals were included in the 5-12 year old age group. We observed that hair cortisol level was higher among the younger monkeys and declined with age (p<.001). More importantly the effect of age significantly interacted with sex (p=.02), such that hair cortisol was consistently lower in males than females beginning at age 3 (p<.05 or better). The developmental decline began one year earlier in females than males suggesting an influence of the earlier maturational processes typical in both human and nonhuman primates. The advantage of lower cortisol levels in the males may be related to social group patterns of male emigration during adolescence in many nonhuman primate species.