Hair cortisol levels track phylogenetic and age related differences in hypothalamic-pituitary-adrenal (HPA) axis activity in non-human primates.

Hair has been shown to archive a uniquely time averaged signal of endocrine activity, and holds attractive advantages for both laboratory and field research. Prior research has explored the potential of hair hormone analysis to examine hormone-behavior relationships. To date, no research has focused on the potential of the technique to investigate age-related changes or taxon differences in endocrine function. It is known that non-human primate infants of many taxa exhibit high cortisol levels after parturition, which rapidly decline with age. It has also been shown that hypercortisolism generally characterizes platyrrhine (New World monkey) endocrine function. These endocrine trends have been characterized using cortisol levels determined from serum, plasma, and feces. Here we test whether cortisol levels determined from hair recover similar phylogenetic and age related patterns in endocrine function in non-human primates. In order to test whether hair cortisol reflect infant hypercortisolism with significant age-related decline, hair cortisol levels are measured in samples from wild vervet monkeys (Chlorocebus aethiops) and captive Guinea baboons (Papio hamadryas papio), ranging in age from infants through juveniles. Further, in order to test whether platyrrhines exhibit significantly higher hair cortisol levels compared to strepsirrhines and catarrhines, and therefore faithfully recover similar signals as more traditionally used substrates (e.g. serum), hair cortisol levels are quantified in adult female hair samples collected from a broad range of non-human primate taxa. Results confirm that hair cortisol levels accurately reflect known phylogenetic and age related patterns of circulating cortisol levels. Therefore, these results suggest that hair may be an ideal hormone bearing substrate for research focused on the examination of population endocrine profiles, cross-sectional studies of endocrine function and taxon variation in hormone levels, as well as stable behavioral trends.