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Stress hormone levels in aging wild chimpanzees mirrors that of humans

Wild chimpanzees, which have a potential lifespan of about 60 years, have an increase in levels of stress-related hormones as they age just as humans do, reported NIA-supported researchers in the Proceedings of the National Academy of Sciences. These findings from a 20-year study of animals in their natural environment suggest that increases in stress hormones are a normal part of aging, rather than a consequence of other factors, such as the environment where humans are studied or certain lifestyle factors such as poor nutrition or insufficient physical activity.

Chimpanzee in forest
Image courtesy of Ronan Donovan.

The primary product of the stress response in humans and chimpanzees is cortisol, a steroid hormone that has many important functions throughout the body. Cortisol levels follow a 24-hour circadian rhythm, reaching their highest peak just before awakening and their lowest point at night before sleep, with fluctuations throughout the day. The natural daily rhythms of cortisol and other hormone levels are controlled by the brain.

In developed countries, as humans age, overall cortisol levels generally rise. The up-and-down daily fluctuation for older adults flattens because cortisol levels remain higher at night. At the same time, stress-induced cortisol levels reach higher levels than they did at younger ages.

Because prolonged exposure to abnormally high levels of cortisol can cause health problems, a team of researchers set out to learn whether older adult wild chimpanzees show the same age-related patterns of rising levels of cortisol as humans in developed nations. Wild chimpanzees were selected because their lifespans are nearly as long as humans, who tend to live 70 or 80 years. The research team measured cortisol levels in urine samples collected during 1997 to 2017 from wild chimpanzees in the Kanyawara community of Kibale National Park in Uganda. Over that period, the field staff observed the chimpanzees on more than 6,000 days for more than 75,000 hours and collected nearly 16,000 free-falling urine samples from 20 adult males and 39 adult, nonpregnant females.

The team found that cortisol levels increased significantly with age for wild chimpanzees, just as they do for humans. The stressors of poor dietary quality, as defined by no access to fresh fruit; a high rank in the dominance hierarchy for male chimpanzees; and an increased reproductive effort, defined on the basis of mating competition for males and breastfeeding for females, did not contribute to the rising levels of cortisol seen with age. Instead, and just as in humans, older chimpanzees had higher cortisol levels at the end of the day, making the overall daily up-and-down fluctuation flatter than for younger animals.

A system of three glands — the hypothalamus, the pituitary gland, and the adrenal gland — carefully controls the release of cortisol from cells. The study showed that this system is similar in chimpanzees and humans. As humans and chimpanzees age, the system becomes dysregulated and cortisol increases.

Because the same cortisol increase happens to aging chimpanzees in the natural environment of Kibale National Park as with humans, the higher cortisol levels observed in older adult humans is probably a natural part of aging, rather than a side effect of living an exceptionally long time, the specific environment in which humans are studied, or lifestyle factors, the researchers concluded.

This research was funded in part by NIA grant R01AG049395.

Reference: Emery Thompson M, et al. Wild chimpanzees exhibit humanlike aging of glucocorticoid regulation. Proceedings of the National Academy of Sciences of the United States of America. 2020;117(15):8424-8430. doi: 10.1073/pnas.1920593117. Epub 2020 Mar 30. PMID: 32229565.