Stress responsiveness in a wild primate predicts survival across an extreme El Niño drought

We know more about the costs of chronic stress than the benefits of the acute stress response – an adaptive response that buffers organisms from life-threatening challenges. Yet, no primate study has empirically identified how the stress response adaptively impacts evolutionary fitness. Here, we take advantage of a natural experiment – an El Niño drought – that produced unprecedented mortality for wild white-faced capuchins. Using a reaction norm approach, we provide evidence from primates that a more robust stress response to a stressor, measured using fecal glucocorticoids, predicts a greater likelihood of survival. We show that individuals with greater stress responsiveness to previous droughts later had higher survival across a severe El Niño drought. Evolutionary models need empirical data on how stress responsivity varies in adaptive ways. While we cannot buffer subjects from catastrophic events, we can use them to understand which aspects of the stress response help animals to “weather the storm”.

Methods
These are observational data: Demographic data and noninvasively obtained fecal samples were collected from a wild population of white-faced capuchin monkeys that are subjects of a long-term study at Lomas Barbudal Biological Reserve, Costa Rica. The rawest form of the demographic data and information about fecal samples are housed in a MySQL database at UCLA, which was queried to produce the data used in this study. Fecal samples were dried in the Lomas Barbudal Monkey Project field lab, and then extracted and assayed in a University of Michigan-Ann Arbor core facility lab.