Dietary Variability Among Mountain Gorilla Groups Across Volcanoes National Park, Rwanda

Gaining a more complete understanding of a species' dietary variability is crucial to properly discern distribution, population growth trends, and conservation actions. Endangered mountain gorillas live in topographically complex forests covering a wide elevational range and diverse habitat matrices. Since 1967, mountain gorillas have been studied at high elevations in the southwest of the Volcanoes National Park (VNP) in Rwanda, where groups use different compositions of habitats and have been growing at higher rates than groups in the northeast VNP region, which is characterized by lower elevations. Building on previous efforts, we describe dietary variability among VNP mountain gorilla groups by integrating data from groups ranging in the northeast VNP. We assessed and compared nutritional components of key foods (making up 80% of the diet) to better understand whether variation in diet quality could be linked to within-population growth differences. Feeding and ranging data were collected between November 2019 and December 2022, using long-term monitoring data, group scans, and focal animal sampling. To compare diet quality, we combined nutritional values from newly collected food plants and previously collected and assessed food plant samples using comparable field and laboratory methods. We recorded 57 new foods for the study population. Groups in the southwest (N = 8) and the northeast (N = 4) regions of VNP used different vegetation zones, and there was high dietary variability with low diet overlap among these regions. Although northeast groups rely on more diverse diets, key foods (making up ~80% of the diet) had comparable nutrient concentrations to southwest groups. This suggests that diet quality is unlikely to be a main driver of observed heterogeneous population growth. For follow-up research, we discuss alternative explanations linked to food distribution, biomass, and energy expenditure to access foods. Our findings add important information for future habitat suitability assessments essential for mountain gorilla conservation management and habitat restoration and expansion efforts.