The Tokai Baboon Sociality Project

In many primate societies, individuals form strong, long-lasting social bonds. It is now known that these bonds can be adaptive, for example by increasing longevity, offspring survival, and, ultimately, reproductive success. What is still poorly understood are the mechanisms by which social bonds achieve their effects. One possibility is that because of their known function in helping individuals cope with stressors, social bonds mitigate negative effects of chronic stress on health and reproduction. But whether – and how – social stress in wild social groups of primates influences female reproduction is also poorly understood.

The Tokai Baboon Sociality Project addresses these gaps in our knowledge by testing three potential pathways that could connect variation in social bonds with measures of health and reproductive function: 1) the lack of strong social bonds leads to elevated stress hormone levels that in turn mediate increased susceptibility to intestinal parasite infections, with potential effects on energy availability; 2) social stress influences reproductive physiology directly; 3) social bonds mediate success in feeding competition; and 4) social bonds (particularly with males) reduce exposure to aggression and infanticide risk.

Data for this project were collected from two troops of chacma baboons (Papio ursinus) on the Cape Peninsula of South Africa. Methods included (1) behavioral observations to document sources of stress and social bonds; (2) measurement of stress and reproductive hormone levels in fecal samples; (3) examination of intestinal parasites from fecal samples; (4) measurement of energy balance via urinary C-peptide, a byproduct of insulin production; and (5) a novel field experiment in which intestinal parasites are eliminated from a random selection of females to assess, for the first time in a wild primate, the factors associated with the timing and intensity of re-infection.

Analyses are currently underway and will continue into 2016. Ultimately, this project will generate new insights into the mechanisms linking social behavior, stress physiology, health, and reproduction and help elucidate the function of social bonds in primate societies.

Background

Glucocorticoids 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.

Non-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.

To 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.