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

Abstract

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.

Keywords: diet diversity; diet overlap; dietary evenness; food plant nutrition; habitat use; park restoration.

FIGURE 1

Vegetation zones in the Virunga Massif (McNeilage 1995) and home range locations of study groups in the northeast (NE) groups and the southwest (SW) group of the Volcanoes National Park in Rwanda.

FIGURE 2

Rarefaction and extrapolation sampling curves for diet richness (q = 0) and exponential Shannon diversity (q = 1) computed from feeding data collected using focal sampling and group scan sampling.

FIGURE 3

Pie charts indicate the percentage of time each study group (top and middle row: Southwest (SW) groups; bottom row: Northeast (NE) groups) spent in each vegetation zone during the study period.

FIGURE 4

Percentage of time study groups in the southwest (SW) and northeast (NE) of the VNP spent in the “bamboo/mixed bamboo zone” each month of the year (2020–2022). SEG was not observed from January to April.

FIGURE 5

Rarefaction and extrapolation sampling curves for Hill‐Shannon diversity (q = 1) computed from feeding observations of food type‐items consumed (sample‐based) by study groups (northeast groups represented by the four upper curves: Kwisanga, Kwitonda, Mutobo, and Sabyinyo).

FIGURE 6

Observed (red dots) and expected (black dots; set at 2500 observations) diet diversity (Hill‐Shannon: Q = 1) with 95% upper and lower confidence limits by group (Northeast groups SAB, MUH, KSA, KWI; others belong to SW groups). Confidence limits between groups without overlap indicate distinct diet diversities.

FIGURE 7

Between‐group variation in observed diet diversity (Hill‐Shannon: Q = 1) within vegetation types that were used by multiple groups.

FIGURE 8

Boxplots showing median, mean (cross symbol), interquartile range, and min/max of metabolic energy in kcal/g in key food type‐items making up 80% of the diet by study group in the southwest (SW) and the northeast (NE) of the park. Note that foods included in the diet of KSA only make up 77.5% because Acacia mearnsii was not sampled for nutritional analysis.

FIGURE 9

Boxplots showing median, mean (cross symbol), interquartile range, and min/max of the mean proportion of macronutrients (ADF, acid detergent fiber; ASH, total ash; CP, crude protein; L, lipids; NDF, neutral detergent fiber; TNC, total nonstructural carbohydrates) in key food type items making up 80% of the diet by study groups in the Southwest (SW) and Northeast (NE) of the park. Note that foods included in the diet of KSA only make up 77.5% because Acacia mearnsii was not sampled for nutritional analysis.