Foerster-2014-001-IMG_7861Glucocorticoids or GCs (stress hormones) such as cortisol are the primary mediators of physiological responses to physical or psychological perturbations from homeostasis in all vertebrates. During acute stress responses, these hormones optimize the availability of energy needed to cope with a stressor and activate important aspects of the immune system. When exposure to a stressor exceeds the coping capacity of the organism, however, the beneficial actions of stress hormones turn into maladaptive responses that contribute to the depletion of bodily resources, increase disease risk and progression, and may even lead to death.

20140209_061240Non-invasive techniques to quantify GC metabolites in urine or feces have resulted in a growing number of studies that use GCs to assess “costly” aspects of behavior and life history in wild vertebrates. The interpretation of naturally occurring hormonal variation is rendered difficult, however, by the dearth of studies documenting effects of variable stress levels on reproduction in non-human primates and humans. Much of what we know about the health effects of social stress comes from studies conducted on captive non-human primates, or from modern human societies where natural social coping mechanisms may be impaired and individuals subject to high levels of life and work stress. Given its partly adaptive function, some observed variation in stress levels is likely tied to fitness-maximizing strategies; this may explain why a recent review of vertebrate studies found conflicting evidence for a relationship between GCs and indicators of fitness.

TK1600_P1120543o interpret stress responses in wild primates using non-invasive measures and to understand their evolutionary significance, it is necessary to (a) study tangible fitness correlates and how they are modulated by, or modulate, GCs, (b) collect longitudinal data on individual stress responses and measures of reproductive function, (c) study free-ranging populations, because artificially high levels of psychological stress that often occur in captivity are unlikely to reflect daily life experiences in an evolutionary context, and (d) disentangle the influence of social stressors from that of energetic stressors.

The Tokai Baboon Project tries to meet these requirements by (1) studying social groups that are free-ranging but relatively well buffered against energetic stressors due to partial reliance on anthropogenic food sources; (2) collecting a large number of biological samples to arrive at representative estimates of hormone metabolites; (3) evaluating multiple potential stressors and controlling for confounds of stress responses using modern statistical tools, and (4) quantifying the association of GC variation with two potential fitness correlates: reproductive function and the number and relative intensity of gastro-intestinal (GIT) parasite infections. GIT parasite infections are a potential correlate of fitness because they can have effects on host nutrition, immunology, and reproduction, and are known to influence health and morbidity in humans.