Aggression, glucocorticoids, and the chronic costs of status competition for wild male chimpanzees.

Across vertebrates, high social status affords preferential access to resources, and is expected to correlate positively with health and longevity. Increasing evidence, however, suggests that although dominant females generally enjoy reduced exposure to physiological and psychosocial stressors, dominant males do not. Here we test the hypothesis that costly mating competition by high-ranking males results in chronic, potentially harmful elevations in glucocorticoid production. We examined urinary glucocorticoids (n = 8029 samples) in a 20-year longitudinal study of wild male chimpanzees (n = 20 adults) in the Kanyawara community of Kibale National Park, Uganda. We tested whether glucocorticoid production was associated with dominance rank in the long term, and with mating competition and dominance instability in the short term. Using mixed models, we found that both male aggression and glucocorticoid excretion increased when the dominance hierarchy was unstable, and when parous females were sexually available. Glucocorticoid excretion was positively associated with male rank in stable and unstable hierarchies, and in mating and non-mating contexts. Glucorticoids increased with both giving and receiving aggression, but giving aggression was the primary mechanism linking elevated glucocorticoids with high rank. Glucocorticoids also increased with age. Together these results show that investment in male-male competition increases cumulative exposure to glucocorticoids, suggesting a long-term tradeoff with health that may constrain the ability to maintain high status across the life course. Our data suggest that the relationship between social rank and glucocorticoid production often differs in males and females owing to sex differences in the operation of sexual selection.

Wood and meat as complementary sources of sodium for Kanyawara chimpanzees (Pan troglodytes).

OBJECTIVES: Sodium, a vital micronutrient that is often in scarce supply for tropical herbivores, is sometimes found at high concentration in decaying wood. We tested two hypotheses for chimpanzees: first, that wood-eating facilitates acquisition of sodium; second, that wood-eating occurs in response to the low availability of sodium from other dietary sources. MATERIALS AND METHODS: We studied the behavior of more than 50 chimpanzees of all age-sex classes in the Kanyawara community of Kibale National Park, Uganda. We quantified the sodium content of dietary items, including wood samples from tree species that chimpanzees consumed or did not consume. To assess variation in sodium intake, we used 7 years of data on time spent feeding on plant foods, 18 months of data on rates of food intake by adult females, and 20 years of data on meat-eating. RESULTS: Major dietary sources of sodium were wood, fruits and meat. Chimpanzees consumed wood primarily from decaying trees of Neoboutonia macrocalyx (Euphorbiaceae), which had substantially higher sodium content than all other dietary items tested. Wood-eating was negatively correlated with fruit-eating. Females ate wood more often than males, while males had a greater probability of consuming meat at predation events. DISCUSSION: We propose that females ate wood more often than males because females had reduced access to meat, their preferred source of sodium. This hypothesis suggests that the need for sodium is a motivating reason for chimpanzees to consume both meat and wood.

Validating Urinary Neopterin as a Biomarker of Immune Response in Captive and Wild Capuchin Monkeys.

Non-invasive health monitoring is advantageous for wild and captive primate populations because it reduces the need for traditional invasive techniques (i.e., anesthetization) that can be stressful and potentially harmful for individuals. The biomarker neopterin is an emerging tool in primatology to measure immune activation and immunosenescence, however, most neopterin studies have focused on catarrhine species with little comparative work examining neopterin and health in platyrrhines. To address this gap, we validated a commercially available enzyme-linked immunosorbent assay (ELISA) to measure urinary neopterin in two types of capuchin monkeys, a wild population of white-faced capuchins (Cebus imitator) and a socially housed captive colony of tufted capuchins (Sapajus apella). We analytically validated methods for measuring urinary neopterin in two capuchin populations and demonstrated that two commonly-used methods to control for urine concentration-creatinine and specific gravity (SG)-produced highly concordant results. We also biologically validated these methods by examining variation in neopterin levels based on environment (captive and wild) and age, and changes in levels associated with immune-response. We found that neopterin increased after immune perturbation (rabies vaccine booster), varied by environmental condition, and mirrored expected trends in immune system ontogeny. Our results improve understanding of the innate immune system in platyrrhine species and suggest neopterin may be useful for non-invasive health monitoring in both captive and wild primates.