Wild chimpanzees exhibit human-like aging of glucocorticoid regulation

Cortisol, a key product of the stress response, has critical influences on
degenerative aging in humans. In turn, cortisol production is affected by
senescence of the hypothalamic-pituitary-adrenal (HPA) axis, leading to
progressive dysregulation and increased cortisol exposure. These processes
have been studied extensively in industrialized settings, but few
comparative data are available from humans and closely-related species
living in natural environments, where stressors are very different. Here,
we examine age-related changes in urinary cortisol in a 20-year
longitudinal study of wild chimpanzees (N = 59 adults) in the Kanyawara
community of Kibale National Park, Uganda. We tested for three key
features of HPA aging identified in many human studies: increased average
levels, a blunted diurnal rhythm, and enhanced response to stressors.
Using linear mixed models, we found that aging was associated with a
blunting of the diurnal rhythm and a significant linear increase in
cortisol, even after controlling for changes in dominance rank. These
effects did not differ by sex. Aging did not increase sensitivity to
energetic stress or social status. Female chimpanzees experienced their
highest levels of cortisol during cycling (versus lactation), and this
effect increased with age. Male chimpanzees experienced their highest
levels when exposed to sexually-attractive females, but this effect was
diminished by age. Our results indicate that chimpanzees share some key
features of HPA aging with humans. These findings suggest that impairments
of HPA regulation are intrinsic to the aging process in hominids and are
neither side effects of extended human lifespan nor of atypical
environments.