The importance of underground foods in female gelada (Theropithecus gelada) socioecology

Food plays a foundational role in animals’ lives, particularly during periods of food scarcity. Foods consumed only during times of food scarcity, “fallback foods”, exert strong selective forces in shaping behavior and life history. However, the role of fallback foods has not been examined in many species. Geladas (Theropithecus gelada) present a particular conundrum. While geladas primarily eat grasses blades, alternative diet items, such as underground plant storage organs, are a large part of the diet, especially in the dry season, when green grasses are scarce. In this thesis, I investigate the role that these alternative diet items have in gelada foraging and behavioral ecology in a population of wild geladas in the Simien Mountains National Park, Ethiopia, from January, 2015 January, 2016. I asked the following questions: (1) How much of the gelada diet (across seasons) is comprised of grass? And, (2) to what extent do geladas utilize “fallback foods”? I quantified monthly diet profiles of adult geladas from eight study units (n=37 females, 17 males) using instantaneous scan samples (n=7,533 feeding scans), and seasonal aboveand underground food availability. I compiled data from published gelada feeding studies and compare the relative importance of underground foods in the gelada diet. Geladas preferred green grasses year-round (up to 93.4% of monthly foraging time), but spent a large amount of time consuming underground foods in the dry season (up to 58% of monthly foraging time). Grass consumption was positively correlated with grass availability, and underground food consumption was negatively correlated with green grass availability. In contrast with grasses, underground food availability did not vary seasonally, which supports the hypothesis that underground foods are important fallback foods for geladas. Next, I examined the role that underground foods play in shaping gelada social relationships among females. Socioecological models predict that female competition should increase when food resources are clumped and defensible. Female geladas thus represent a puzzle for socioecology; they feed on dispersed resources that cannot be monopolized, yet they exhibit aggressive competition and a strict dominance hierarchy. I hypothesized that female geladas must routinely contest underground resources during times of scarcity. To test this hypothesis, I used behavioral data from adult females (n=32 females; 1,424 observation hours) collected in the dry and wet seasons to assess the effect of behavioral context, season, and on the probability of receiving aggression. I found that females were more likely to receive aggression from within-unit females than from females outside of their unit, that aggression was highest during the dry season when geladas were not feeding on a widely-dispersed resource (grass) but rather were feeding on underground foods, that aggression was higher during a feeding context than a non-feeding context, and even more so when the feeding was on underground food items, and that low-ranking females were the most likely to receive aggression in these contexts. Therefore, the results of this study support the hypothesis that female aggression (and the dominance hierarchy that ensues) functions to usurp lower-ranking females from their underground excavations that expose valuable underground foods. These excavations represent an investment of time, energy, and a level of uncertainty as to whether the effort will uncover a valuable food resource, all of which can be eliminated or reduced for a high-ranking individual that can acquire the “patch” without investing any of the effort. In sum, the results presented here support the hypothesis that fallback foods can be contestable resources, and can be influential in shaping social relationships among female geladas.