The 85th Annual Meeting of the American Association of Physical Anthropologists (2016)


Stabilizing selection and optimal group size in geladas

ELIZABETH TINSLEY JOHNSON1, NOAH SNYDER-MACKLER2, THORE J. BERGMAN3,4 and JACINTA C. BEEHNER1,3.

1Department of Anthropology, University of Michigan, 2Department of Biology, Duke University, 3Department of Psychology, University of Michigan, 4Department of Ecology and Evolutionary Biology, University of Michigan

April 14, 2016 3:00, A 602 Add to calendar

Group size is a fundamental component of primate socioecology and is thought to reflect the costs and benefits of female sociality. However, empirical data to test hypotheses about the causes and consequences of group size within a species are rare. Here we take advantage of the unusual social structure of geladas (Theropithecus gelada) to study variation in group size. The core groups in gelada society are one-male units that aggregate into larger groups (bands) allowing us to simultaneously study multiple social groups ranging in size from 1-12 adult females. We analyzed 9.5 years of reproductive data from a population of geladas living in the Simien Mountains National Park, Ethiopia (33 one-male units, 187 adult females). When we examined reproductive success, we found strong evidence for stabilizing selection. Specifically, females in mid-sized units (5-7 females) had significantly higher reproductive success than females in both large (8+ females) and small (1-4 females) units. Variation in success was largely due to differences in infant survival, with infanticide causing a majority of deaths. Several lines of evidence suggest that females prefer, and actively seek, an optimal group size. First, many of our largest units were observed to fission. Second, a few of the smallest units were observed to fuse with another small unit. Third, females in mid-sized units had the lowest levels of glucocorticoids. Fourth, females in mid-sized units exhibited the strongest grooming relationships. Together these results represent some of the first intra-specific data on how and why group size varies in primates.

Funding provided by the Leakey Foundation, National Geographic Society (Gr. #8989-11, Gr. #8100-06), National Science Foundation (IOS-1255974, BCS-1340911, BCS-0715179), University of Michigan, and the Wildlife Conservation Society (Gr. #67250).