Variation in the Acquisition of Dietary Independence in Wild Virunga Mountain Gorillas
Open AccessFeeding ecology plays an important role in understanding primate behavior, life history and population dynamics. Among primates, folivores’ diet consists of abundant foods available year-round and evenly distributed in their habitats, which makes researchers associate them with less feeding competition and other dietary challenges. However, accumulating evidence suggests that primate folivores are in fact subject to more dietary challenges than previously thought (Chapman et al., 2014; Harris et al., 2010; Snaith & Chapman, 2007). While most research on primate folivores has focused on adults, the juvenile period has been less studied. This period is extended in primates compared to other mammals, and this is presumed to relate at least in part to the acquisition of foraging competence. The latter may be constrained by the time needed to learn foraging skills, by the mechanical properties of the foods and/or by effects of feeding competition, which may in turn influence other aspects of behavioral and physical development. This dissertation examined this matter in the Endangered wild Virunga mountain gorillas (Gorilla beringei beringei) in Volcanoes National Park (VNP) Rwanda, a population widely regarded as the most folivorous of all ape populations. Virunga mountain gorillas occupy habitats with ecological heterogeneity and some foods in their diet may impose mechanical challenges and/or require the acquisition of foraging skills. Most feeding ecology studies of this population come from habitats represented in the home ranges of social groups monitored by the Dian Fossey Gorilla Fund’s (DFGF) Karisoke Research Center in VNP over the past five decades. However, gorilla groups in habitat zones outside the historical Karisoke study area consume different foods. Further, mountain gorillas generally live in cohesive social groups and immature gorillas may be physically inferior, and thus subject to effects of feeding competition. I integrated novel and long-term data on food nutritional properties and development of feeding behavior, sampled across a greater diversity of habitat types in VNP than studied to date, to assess the following objectives, each forming a dissertation chapter: In chapter 2, I Characterize spatiotemporal variability in the nutritional composition both between and within Virunga mountain gorilla foods in VNP. I explored variability in the nutritional composition of foods consumed across a wider range of habitats within VNP including habitats outside of the Karisoke study area towards the eastern side of VNP. Further, I tested the hypotheses that the environmental factors such as ecological habitat, altitude and season influence between and within-species variation in plant nutritional properties. To do this, we collected samples of plant foods consumed by mountain gorillas ranging in different vegetation zones and over different seasons as well as across an altitudinal gradient within VNP, and analyzed them for protein, fat, fiber and ash and estimated the total non-structural carbohydrates and energy content. Our findings showed pronounced between and within-species nutritional variation and this was more driven by spatial rather than temporal factors. Foods consumed by mountain gorillas at high altitude habitats were more likely to be energy rich food resources than foods in other habitats. Our results of spatial nutritional variability may provide more insights into the variation of mountain gorillas life history and behavioral patterns reported for this population. In chapter 3, I investigated the development of solid food diet acquisition and adult-like foraging competence in infant and juvenile mountain gorillas. I hypothesized that infants and juveniles acquire adult-like diet composition gradually with age. I further hypothesized that the development of foraging competence with age is constrained by the need to learn technical skills and/or gain physical abilities to process specific foods. To test these hypotheses, I used detailed food intake data from different age classes, which covered most consumed plant foods. Further, I used data on food processing and toughness from published studies (Food processing: Byrne & Byrne, 1991; Food toughness: Glowacka et al., 2017) to classify foods according to their complexity, and tested the effect of food properties on foraging competence. Results indicated different patterns of diet composition throughout development between mountain gorillas ranging in different habitats within VNP, which is suggested to relate to different foods consumed in those habitats. Across all habitats investigated, immatures have been shown to have lower absolute energy concentrations in diet relative to adults, suggesting that they may be limited to accessing energy dense foods possibly due to plant harvesting and processing challenges. Next, our results indicated variation in foraging competence, though we failed to provide evidence that immatures mountain gorillas are constrained by the need to learn technical skills. Further, while the food mechanical properties partially explained patterns in foraging competence, our results imply that mountain gorilla foods do not seem to be tough enough to constrain immatures’ foraging competence relative to adults. The acquisition of foraging competence in immatures may also be constrained by other factors including feeding competition with older individuals. In chapter 4, I investigated whether infant and juvenile mountain gorillas experience competitive interactions, especially given the cohesive social system characteristic of this population. Using long-term datasets from the DFGF, I assessed how mechanisms of feeding competition such as rates of agonistic interactions changed with age and feeding versus non-feeding contexts. Findings revealed that immature mountain gorillas experience effects of feeding competition. Specifically, rates of competitive interactions increased with the age of immatures, and this occurred faster in a feeding context than non-feeding context. Further, I showed that rates of agonism received in feeding contexts increase earlier (at a younger age) than do rates of agonism given. When I explored the relationship between these mechanisms of feeding competition and group size, our results failed to provide evidence of effect of group size on competitive interactions experienced by immature mountain gorillas. Overall, results from this project contributes to recent evidence showing that primate folivores in particular mountain gorillas, experience more ecological variation in terms of food quality and the generated data can inform conservation efforts. Findings from this study improve our understanding of how local ecology may shape feeding development of gorillas and, have broader implications for spatial variation in population growth and behavioral patterns within VNP. This study also complements knowledge in socioecological models, which have tended to focus exclusively on adults. But this study provides more insight about the juvenile period.
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