1School of Natural Resources and the Environment, University of Michigan, 2Department of Psychology and Department of Ecology and Evolutionary Biology, University of Michigan, 3Department of Psychology and Department of Anthropology, University of Michigan
April 14, 2016 2:15, A 602
Socioecological theory predicts that aggression among females should accompany the presence of a clumped and defensible resource. One of the many frustrating exceptions to this rule are female geladas (Theropithecus gelada), who exhibit high rates of aggression among females yet feed on one of the most dispersed and non-defensible resources of any primate - montane grasses. However, below-ground food comprises a substantial proportion of the gelada diet in the dry season, when the availability of green grass declines. Although grasses are an abundant and evenly distributed food with low handling time, below-ground food (e.g., roots, corms, and tubers), which contain large reserves of water and carbohydrates, require substantially more time to excavate. Here, we investigated the seasonal patterns of female aggression in wild geladas in the Simien Mountains National Park, Ethiopia (2014-2015). We used behavioral data collected in both the wet and dry season to quantify monthly aggression rates and time budgets. We have two main results. First, aggression rates among females are higher in the dry season than in the wet season. Second, the aggression rates when geladas are feeding below-ground are disproportionately high. These results indicate that dominant females may be using aggression to gain access to below-ground food by supplanting subordinate females from sites where subordinates have invested in digging a hole to extract below-ground resources. In other words, we argue that dominant females are “sub-contracting” their digging to subordinate females; and thus the primary defensible resource that leads to aggression among female geladas is excavating time.
This research was supported by the NSF Graduate Fellowship Program to JCJ. The long-term gelada reserach in the Simien Mountains is supported by an NSF-LTREB (IOS-1255974) to JCB and TJB.