A long-term study of baboons found that high levels of stress hormones were strongly linked with shorter lifespan, providing the first evidence that stress influences the survival of wild animals.

In the study, published April 21 in Science Advances, researchers compared the lifespans of 242 wild female baboons with glucocorticoid stress hormone levels measured from feces collected over two decades.

Stress responses, mediated by hormones such as glucocorticoids, help an individual respond to stressful situations. But long-term or chronic activation of this response may be bad for health. While elevated glucocorticoids have been linked with heart disease, diabetes and poor immune function in humans and other animals, it's less clear how stress is associated with life expectancy.

"A question that people have been curious about for a long time is, what's the relationship between having an elevated glucocorticoid profile and your life outcome?" said lead author Fernando Campos, an assistant professor of anthropology at the University of Texas at San Antonio who started the project as a postdoctoral researcher at Duke University.

To address this question, Campos and his colleagues measured glucocorticoid levels in more than 14,000 fecal samples collected over 20 years from wild baboons in Kenya's Amboseli basin.

"This baboon population has been studied continuously since 1971 and it's one of the largest data sets in the world on a wild primate. So, we can ask questions in this population that just aren't possible for most other study systems," he explained. "We can continuously get these hormone readings from the baboons just by collecting their feces."

According to the researchers, poop is ideal for this type of analysis because it's easy to collect without handling the animals, and hormone concentrations in feces are less prone to short-term fluctuations than those in blood.

In addition to individual baboons' hormone profiles, the researchers gathered detailed information on how long they lived, their social lives, day-to-day lives and environmental conditions. Analyzing these data, the team found a strong association between elevated glucocorticoid levels and shorter lifespan in the clever primates.

To assess the magnitude of the association between stress hormones and survival in baboons, the team developed a model to predict the lifespan of two animals at opposite extremes of glucocorticoid levels. An individual with very high glucocorticoid levels was predicted to live 5.4 fewer years than one with very low levels. The difference equates to about one-quarter of a female baboon's life expectancy.

But according to Campos, the research cannot conclude that high stress hormone levels were necessarily the cause of lower life expectancy in baboons. It's possible that high glucocorticoid levels represent an efficient stress response and that individuals with elevated stress hormones fare better than those with lower hormone levels under the same environmental conditions, he explained. 

Another explanation is that some other environmental variable is responsible for reducing baboon life expectancy, and that elevated glucocorticoids are just a symptom of their body's response to this stress.

Other studies have suggested that stress could be linked with lowered survival in people, but there is a lack of data to support this hypothesis because it's challenging to repeatedly sample humans over long lifespans, according to Campos.

"This [study] suggests — it doesn't show — that it's likely that this sort of relationship might also be true in humans," he said.

The study, "Glucocorticoid exposure predicts survival in female baboons," published April 21 in Science Advances, was authored by Fernando A. Campos, University of Texas at San Antonio; Elizabeth A. Archie, University of Notre Dame; Laurence R. Gesquiere and Susan C. Alberts, Duke University; Jenny Tung, Duke University and Canadian Institute for Advanced Research; and Jeanne Altmann, Princeton University.