The hormonal and immunological correlates of social dominance in wild male chimpanzees
Negrey, Jacob Douglas
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In social primates, the acquisition and maintenance of social dominance may augment reproductive success while incurring immunological costs. This trade-off is hypothetically facilitated by hormones that modulate both status-enhancing behavior and immune function. In the three studies comprising this dissertation, I investigated hormonal mechanisms by which social dominance may reflect immune health, testing relationships between behavioral correlates of dominance rank, steroid hormone secretion, and immune activity in wild adult male chimpanzees (Pan troglodytes schweinfurthii). Between January 2016 and July 2017, I collected behavioral observations and urine samples from adult males at Ngogo in Kibale National Park, Uganda, home to the largest community of habituated wild chimpanzees yet described. In the first study, I assessed behavioral and anatomical mechanisms that may link dominance rank to the secretion of testosterone, the primary male sex hormone. Testosterone was positively correlated with dominance rank and creatinine, a product of muscle catabolism and noninvasive proxy for lean muscle mass. Contrary to expectations, testosterone was negatively correlated with overall rates of aggression, indicating that aggressiveness does not itself account for positive linear correlations between dominance and testosterone in this species. In the second study, I analyzed reproductively salient correlates of cortisol, a glucocorticoid hormone secreted in response to psychosocial and metabolic demands. Urinary measures of reproductive effort and immune challenge were positively correlated with cortisol, indicating adaptive energy allocation. Furthermore, dominance rank was positively correlated with urinary cortisol when c-peptide of insulin, as a measure of energy intake, was low. This indicates that high ranking males deprioritize energy intake in certain social contexts, including competition for sexually receptive mates. In the third study, I found that although urinary testosterone seemingly diminished immune function, high ranking males were less likely to die from severe acute immune challenge than low ranking conspecifics. My results provide evidence that mating effort increases immune challenge both by increasing testosterone secretion and reallocating energy away from immune function. However, despite the increased mating effort exhibited by high ranking males, social dominance does not incur notable immunological costs. On the contrary, social dominance likely reflects immunocompetence and male quality in nonhuman primates.