Seasonal and altitudinal effects on glucocorticoid metabolites in a wild primate (Theropithecus gelada)

Behavioral ecologists are increasingly adopting sophisticated non-invasive methods for measuring glucocorticoids from "excreta" because samples are easy to collect, animals are left undisturbed, and measures may be more reflective of external events than serum samples. Some of the most common predictors for hormone profiles in wild animals are seasonal changes in ecology and behavior. For example, we might expect baseline glucocorticoid levels to track changes in food availability or other seasonal stressors such as unusually high or low temperatures. Geladas (Theropithecus gelada) are one of the few non-human primates that live at extremely high altitudes where nighttime temperatures often dip below freezing. However, the physiological effects of this relatively inhospitable environment have never been examined in this species. Here we validate a "field-friendly" method of hormone extraction from gelada feces and demonstrate that this method can be used to detect seasonal and altitudinal differences in glucocorticoid metabolites for this species. We use two years of climatological and hormonal data from a wild population of geladas to test the hypothesis that geladas exhibit elevated glucocorticoids under environmentally "challenging" conditions - mainly, when temperatures and rainfall are low and altitude is high. Our results indicate that cold temperatures and high altitude predicted elevated glucocorticoids, but low rainfall did not. Therefore, we suggest a metabolic hypothesis (as opposed to a nutritional hypothesis) to explain this result. However, at the present time, we cannot rule out a behavioral stress hypothesis.