Departmental News

Posted:  Oct 30, 2023 - 11:00am

Megan Cole, PhD candidate in Evolutionary Anthropology, was awarded the student prize for her talk,Trait-level Cortisol Predicts Gooming Partner Diversity in Wild Chimpanzees, at the Southwest Association of Biological Anthropologists meeting in Tucson, AZ.

Abstract

Longitudinal human studies find that individual variation in baseline glucocorticoid production, identified in controlled experiments, predicts social behavior. Among wild primates, it is difficult to evaluate baseline cortisol using opportunistic sampling, as individuals with higher average cortisol may experience more frequent and/or severe stressors. However, studies that seek to identify social and environmental stressors frequently discover significant individual variation. Here we apply linear mixed models to a large, longitudinal sample of urinary cortisol results (N=19,713 samples) from wild chimpanzees in the Kanyawara community of Kibale National Park, Uganda (48 adults, 23 years). We identify significant random effects of individual identity for both males (σ²=0.04, p<0.001) and females (σ²=0.02, p<0.001), even after controlling for numerous temporal, demographic, and socioecological covariates. We subsequently test two alternative hypotheses about the relationship between cortisol and social behavior: 1) if moderate glucocorticoid activity facilitates social tolerance, lower cortisol will be associated with more grooming partners; but 2) if heightened glucocorticoid activity prompts individuals to seek social support, lower cortisol will be associated with fewer partners. To this end, we introduce a novel approach for quantifying individual-varying cortisol as the local maxima (modes) of frequency distributions – and show that modal cortisol better captures trait-level variation than the mean. Modal, but not mean, cortisol was positively correlated with grooming partner diversity (males, p=0.02; females, p=0.04). Our findings suggest that individual variation in adrenal function may explain some dimensions of social behavior in wild chimpanzees, and further help reconstruct the evolution of glucocorticoid regulation in the human lineage. 

This study was funded by the National Institute on Aging and the Office for Research on Women’s Health of the NIH (R37-AG049395), the National Science Foundation (BCS-2141766, GRF), and UNM. 

Megan is a 6^th year PhD student in Evolutionary Anthropology, an NSF Graduate Research Fellow, and a graduate student researcher with the Kibale Chimpanzee Project. Her dissertation focuses on individual differences in stress physiology and social tolerance in wild chimpanzees.