Behaviour 2019
Alpha females have the lowest glucocorticoid levels: A dominance rank metric comparison in wild baboons
Emily J Levy1, Laurence R Gesquiere1, Emily McLean2, Mathias Franz3, Kinyua Warutere4, Serah N Sayialel4, Raphael S Mututua4, Tim L Wango4,5, Vivian K Oudu4, Jeanne Altmann6,7, Elizabeth A Archie7,8, Susan C Alberts1,7. 1Duke University, Durham, NC, United States; 2Oxford College of Emory University, Oxford, GA, United States; 3Leibniz Institute for Zoo and Wildlife Research, Berlin, Brandenburg, Germany; 4Amboseli Baboon Research Project, Amboseli, Kajiado, Kenya; 5University of Nairobi, Nairobi, Nairobi, Kenya; 6Princeton University, Princeton, NJ, United States; 7Institute of Primate Research, Nairobi, Nairobi, Kenya; 8University of Notre Dame, Notre Dame, IN, United States

In group-living animals, dominance hierarchies are a common cause of unequal access to fitness-related resources. There are many different methods to measure dominance rank, including ordinal rank, proportional rank (i.e., standardized or relative rank), high-middle-low categories, alpha versus non-alpha status, and cardinal rank measures (e.g., Elo rating). We hypothesize that each rank metric makes a different assumption about resource availability and within-group competition. Using a large dataset of fecal samples from wild adult female baboons, we test the ability of five common rank metrics to predict fecal glucocorticoid concentrations as a proxy for psychosocial and energetic stressors. Surprisingly, alpha status was the best predictor of fecal glucocorticoids, with lower levels in alpha females relative to non-alphas, indicating fewer stressors experienced by alpha females relative to other adult females. We also observed a weak effect of proportional rank, with lower fecal glucocorticoids in high-ranking females than low-ranking females. Our work introduces a new and easy approach to gain insight into the competitive landscapes occurring in animal societies.