Variability in the Diet of Bwindi Gorillas

Studying the diet of gorillas can assist in our understanding of their foraging behavior, habitat utilization, population dynamics, and social behavior and it may also assist in conservation efforts. The diet of gorillas is likely to differ among gorilla populations because they are found in a diversity of habitats that vary in plant composition and availability (Doran & McNeilage 1998), so it is important to investigate their diets in a variety of habitats.
From the pioneering studies of mountain gorillas (Gorilla beringei beringei) at the Karisoke Research Center in Rwanda, gorillas were traditionally thought of as strict herbivores (Fossey & Harcourt 1977; Watts 1984). As researchers began investigating the diets of western gorillas (Gorilla gorilla) and Grauer's gorillas (Gorilla beringei graueri), it became apparent that gorillas also consume a significant amount of fruit, as well as many different species of herbaceous vegetation (Tutin & Fernandez 1985; Yamagiwa et al. 1996; Doran et al. 2002). While much has been learned about the variability of dietary patterns between gorilla populations in recent years, less is known about how gorilla diets may vary within a small population (but see McNeilage 2001).
Bwindi Impenetrable National Park, Uganda is home to almost half of the world's remaining mountain gorillas (the other population is found in the Virunga Volcanoes and the majority of our knowledge of gorillas is derived from over 30 years of research at the Karisoke Research Center). Bwindi is at a lower altitude (1160-2607 m) than Karisoke (~2700-3700 m); correspondingly the two areas differ greatly in habitat types and plant species composition (Butynski 1984). Furthermore, the Bwindi Forest spans a wide range of altitudes that correspond to differences in temperature, rainfall, and plant species composition within the park (Nkurunungi et al. 2004). Because of the differences in altitude/habitats between the areas surrounding Karisoke and Bwindi, and also the differences within Bwindi itself, mountain gorilla diets in Bwindi are expected to differ from those at Karisoke, and also between different areas of Bwindi.
Over a one year period between September 2001 and August 2002, we conducted a study to investigate differences in diet among three gorilla groups at two different locations in Bwindi (separated by ~17 km). Two gorilla groups, Mubare and Habinyanja, at a low altitude site (Buhoma, 1450-1800 m) were compared with the Kyagurilo group at a high altitude site (Ruhija, 2100-2500 m). We studied the gorillas' diets by using fecal samples to record frugivory (samples collected from night nests on an approximately daily basis were examined to identify fruit seed species and the number of seeds found), and by following the gorillas' trails to record herbivory (examining food remains left behind is an indication of which herb, shrub, and tree plant parts were eaten).
The results of our study demonstrate that the diets between gorilla groups at the two study sites in Bwindi are notably different. The groups at the low altitude site consumed more species of both fibrous foods (non-fruit food from herbs, shrubs, and trees) and fruit than the group at the high altitude site (fibrous foods: 140 vs. 62 species; fruit: 36 vs. 11 species). Furthermore, there was little overlap in the actual species of foods eaten; the Mubare (low altitude) and Kyagurilo (high altitude) groups shared only 24.4% of their important fibrous food species (those species consumed on more than 5% of days) and 16.7% of their important fruit species (those species found in feces on more than 1% of days), while the Habinyanja (low altitude) and Kyagurilo groups shared only 12.7% of their important fibrous food species and 16.7% of their important fruit species. A surprising result was that the Mubare and Habinyanja groups shared only 46.3% of their important fibrous food species and 62.5% of their important fruit species, despite having overlapping home ranges.
By examining the availability and distribution of the plants consumed by the gorillas at each site, we found that the majority of differences in the diets between the groups at the two sites could be explained by differences in food availability, but the difference in diet between the groups with overlapping home ranges could not be entirely accounted for by differences in availability. We suggest that differences between these two groups could be possibly due to small scale differences in patterns of habitat utilization by the two groups, or due to "group traditions" (gorillas learn differently as to what plants to incorporate into their diet), or that gorillas choose different plant species with similar nutrient contents which would make them equally profitable to the gorillas. We are currently investigating the nutrient and chemical content of foods consumed by the three gorilla groups to examine the latter possibility in more detail.
How does the diet of Bwindi gorillas compare with the diets of other eastern gorilla populations? Eastern gorillas (Grauer's gorillas and mountain gorillas) live in a particularly wide range of habitat types and altitudes in Uganda, Rwanda, and the Democratic Republic of the Congo. A comparison of the results of our study with those from the Virunga Volcanoes in Rwanda (Watts 1984; McNeilage 2001) and Kahuzi-Biega (Yamagiwa et al. 1996), shows that the gorillas in these three locations have very few species of plants in their diets in common. Furthermore, it is apparent that, as altitude increases, the number of plant species eaten and the degree of frugivory declines. Bwindi mountain gorillas are much more frugivorous than those in the Virungas, but less than gorillas of Kahuzi-Biega. Thus there is a great amount of variability in the dietary patterns of eastern gorillas.
Our study reveals new information about the dietary patterns of mountain gorillas in Bwindi and highlights the high degree of dietary flexibility within eastern gorillas. These results emphasize the importance of using a comparative approach across multiple sites and habitats when studying a species’ dietary patterns, social system, and when making conservation plans.

Jessica Ganas, John Bosco Nkurunungi, Martha Robbins
The full results of this study were published in the October 2004 issue of the International Journal of Primatology.

References
Doran, D. M. & McNeilage, A. (1998) Gorilla ecology and behavior. Evol. Anthropol. 6: 120–131
Doran, D. M. et al. (2002) Western lowland gorilla diet and resource availability: new evidence, cross-site comparisons, and reflections on indirect sampling methods. Amer. J. Primatol. 58: 91–116
Fossey, D. & Harcourt, A. H. (1977) Feeding ecology of free ranging mountain gorillas (Gorilla gorilla beringei). In: Primate ecology. Clutton-Brock, T. H. (ed.). London (Academic Press), pp. 539–556
McNeilage, A. 2001. Diet and habitat use of two mountain gorilla groups in contrasting habitats in the Virungas. In: Mountain Gorillas. Robbins, M. M. et al. (eds.). Cambridge (Cambridge University Press), pp. 265–292
Nkurunungi, J. B. (in press) A comparison of two mountain gorilla habitats in Bwindi Impenetrable National Park, Uganda. Afr. J. Ecol.
Tutin, C. E. G. & Fernandez, M. (1985) Foods consumed by sympatric populations of Gorilla g. gorilla and Pan t. troglodytes in Gabon: some preliminary data. Internat. J. Primatol. 6: 27–43
Watts, D. P. (1984) Composition and variability of mountain gorilla diets in the central Virungas. Amer. J. Primatol. 7: 323–365
Yamagiwa, J. et al. (1996) Dietary and ranging overlap in sympatric gorillas and chimpanzees in Kahuzi-Biega National Park, Zaire. In: Great Ape Societies. McGrew, W. C. et al. (eds.). Cambridge (Cambridge University Press), pp. 82–98

Dr. Jessica Ganas is a graduate student at the Max Planck Institute of Evolutionary Anthropology in Leipzig, Germany. She conducted field work for her PhD dissertation on Bwindi gorillas.
John Bosco Nkurunungi studied gastro-intestinal parasites of gorillas and humans in Bwindi and the feeding ecology of the gorillas. He is currently studying their foraging efficiency.
Dr. Martha Robbins, Max Planck Institute for Evolutionary Anthropology, has been studying the behavioral ecology of gorillas for 14 years, for the past 6 years in Bwindi.

Bwindi overview

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