Variation of hair cortisol concentrations among wild populations of two baboon species (Papio anubis, P. hamadryas) and a population of their natural hybrids.

Male olive (Papio anubis) and hamadryas (P. hamadryas) baboons have distinctive sociobehavioral and physical characteristics. In the Awash National Park, Ethiopia, a hybrid population at the contact zone between these two species, exhibits heterogeneous sociobehavioral and physical characteristics. The ambiguity of the hybrid social environment and disruption of parental stress genotypes may be sources of physiological stress for hybrids. We examined levels of chronic stress among males of the three populations and tested the prediction that chronic cortisol levels would be higher among the hybrids. Animals were captured, sampled, and released during the wet season, and a hair sample was taken for assay. Cortisol was extracted from 182 hair samples with methanol and quantified by ELISA. We included age, age class, rainfall variation, and species affiliation in models examining variation in hair cortisol levels. Species and age significantly contributed to models explaining variation in hair cortisol. Infant hypercortisolism was observed in all three groups, and a decline in cortisol through juvenile and adolescent stages, with a subsequent rise in adulthood. This rise occurred earliest in hamadryas, corroborating other evidence of the precocious development of hamadryas baboons. As expected, hybrids had significantly elevated hair cortisol compared with olive baboons and hamadryas, irrespective of age, except for very young animals. Infant hypercortisolism was also less pronounced among hybrids. Species differences and age-related differences in cortisol levels suggest a dysregulated cortisol phenotype in hybrids, and possibly reflect some form of hybrid disadvantage. More work will be required to disentangle the effects of genetic factors and the social environment.