Monkey Poop Reveals How Stress Boosts Survival

There’s no question that chronic stress can be harmful: Tens of thousands of studies in both humans and model organisms decry its effects, demonstrating its role in the development of depression, heart disease, and even cancer.^1–3 So why would evolution saddle humans and other animals with such a seemingly detrimental stress response system?

Evolutionary biologists have long hypothesized that the short-term physiological responses to stress help animals survive in life-threatening situations. Indeed, research shows that the canonical stress hormone cortisol increases blood pressure and blood glucose levels to supply energy to the brain and other tissues needed to escape or respond to a challenge, while suppressing nonessential functions like digestion.⁴ But whether the acute stress response truly improves evolutionary fitness—via enhanced survival or reproduction—has been difficult to demonstrate in wild populations.

In a paper published in Science Advances, a research team led by Jacinta Beehner and Susan Perry, anthropologists at the University of Michigan and the University of California, Los Angeles, respectively, provided support for the adaptive nature of the stress response.⁵ They found that a robust stress response—assessed using profiles of fecal glucocorticoids, which are stress-associated hormones—predicted greater survival of wild white-faced capuchin monkeys during a subsequent period of severe and prolonged drought. Improving scientific understanding of the advantages, in addition to the disadvantages, of a strong stress response may help researchers predict how individuals and populations of animals will cope with environmental change.

In the lab, researchers can test how different individuals respond to the same stimulus and explore how stress reactivity is related to other physiological parameters and health outcomes. In nature, however, finding a stressor that equally impacts many individuals in a population is much more difficult. Moreover, to truly test evolutionary fitness, researchers need to determine how an individual’s stress response impacts the probability of surviving this stressor, but there are serious ethical considerations when it comes to manufacturing these life-and-death situations—especially in the case of primates.

To avoid these issues, the researchers took advantage of a “natural experiment”—a two-year period of intense drought that struck Costa Rica in 2014. Using data from the Lomas Barbudal Monkey Project, a long-term monitoring program founded by Perry in 1990, the researchers were able to assess stress and mortality before and during this devastating event. They found that monkey mortality rates increased dramatically, with more deaths recorded than any other year in the project’s history. Next, the researchers analyzed fecal samples collected over several years, measuring the individual capuchins’ glucocorticoid responses to minor droughts that had occurred prior to the extreme event.

Then, they compared the stress responses of the monkeys that did or did not survive the 2014-2016 drought. In this case, the ability to mount a robust stress response appeared to be adaptive: Monkeys with stronger stress responses—at least one standard deviation above the mean—during previous minor droughts were about twice as likely to survive as those with weaker-than-average stress responses.

The study authors noted that their analyses were confined to one capuchin population experiencing a single challenge and that further research will be needed to determine whether these same patterns apply to other groups and other stressors. As climate change is expected to increase the frequency and severity of many types of extreme weather events, including droughts, fires, and floods, it is important to understand how animals respond and what factors may increase their odds of survival in the face of these challenges.⁶