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The Bwindi-Impenetrable Great Ape Project (BIGAPE) was begun in 1996 in Bwindi-Impenetrable National Park in southwestern Uganda. The research project’s central goal is a better understanding of the ecological relationship between the park’s populations of mountain gorillas (Gorilla gorilla beringei) and chimpanzees (Pan troglodytes schweinfurthii). The world population of mountain gorillas is currently estimated at 600, half of which live in the 331 square kilometers of Bwindi. The Bwindi chimpanzee population size is unknown but roughly estimated at 350-400. The nearby Virunga Volcanoes Conservation Area has a population of 300 mountain gorillas but no chimpanzees, making Bwindi the only forest in Africa in which these two apes occur together.

Bwindi is an African montane rainforest with exceptionally high levels of species endemism. The park is on UNESCO’s list of world heritage sites due to its ecological uniqueness and natural beauty. Bwindi is home to a number of other primate species, including black-and-white colobus (Colobus guereza), L’Hoest’s guenon (Cercopithecus l’hoesti), red-tailed guenon (C. ascanius schmidti), blue guenon (C. mitis mitis), vervet (C. aethiops), baboon (Papio anubis) plus nocturnal prosimians. In addition, there are duiker (Cephalophus nigrifrons), elephant (Loxodonta africana), bushbuck (Tragelaphus scriptus), bushpig (Potamochoerus larvatus), giant forest hog (Hylochoerus meinertzhageni) and several small cat species. More than 350 species of birds have been recorded, of which many are regional endemics.

The Project

The project was begun after a 1995 visit to the park showed that it had the potential for conducting research on both chimpanzees and gorillas that shared a habitat and the resources therein, with important implications for conservation management policies.

Key questions that we are addressing include:

  • What is the degree of dietary overlap between Bwindi chimpanzees and gorillas ?
  • Is there food competition, either direct or indirect, between the two species ?
  • Do the two species ever use, or compete for, the same nesting sites ?
  • What aspects of Bwindi gorilla habitat use indicate guidelines for gorilla conservation management?
  • What does chimpanzee/gorilla sympatry in Bwindi suggest about the evolutionary ecology of ancient communities of extinct hominoids?


Gorilla research in Bwindi has lagged far behind that in the Virunga Volcanoes, with the result that in spite of a major ecotourism program, even the most basic details of the ecology and behavior of the 300 gorillas that inhabit the park are only now being learned. The project goals have included:

  1. Collection of data on one habituated gorilla group (the Ruhija “research” group) to investigate diet, ranging, and nesting behavior from dung and food samples and by direct observation. This part of the project is conducted by John Bosco Nkurunungi, a doctoral candidate at Makerere University who has also had training at the University of Southern California. Since August 1997, Nkurunungi has spent three years in the field collecting behavioral and ecological data on the gorillas. His doctoral thesis is in preparation, as are several journal articles detailing his results. These articles will be submitted to UWA upon publication.
  2. Collection of the same sort of ecological data on the chimpanzee population. The chimpanzees cannot be followed at close range, but repeated encounters with them in the forest have made them observable from distances of 30-40 meters through binoculars, and allows us to identify individuals. This identification is essential to learning the size of the chimpanzee community and also the home range it occupies. We collect dung to obtain information on diet, and record nesting data such as height, tree species, and location to better understand chimpanzee habitat use. Using GIS, we expect to generate a seasonal and annual map of habitat use, diet, and other ecological variables for both chimpanzees and gorillas.
  3. Phenological monitoring and vegetation analysis within the study site. The study site is an area of approximately 15 sq.km. in the Ruhija section of the park, and encompasses the home ranges of several gorilla groups and at least one chimpanzee community. Within this area there are at least four unhabituated gorilla groups, one habituated gorilla group, and a chimpanzee community of unknown but considerable size. Two transects have been established comprising approximately 200 trees, and we plan to expand this sample in future years.

Gorilla behavioral ecology

For the past three and a half years, Nkurunungi collected ecological data on gorilla nests, ranging and diet in the upper elevation Ruhija section of the park. Simultaneously, Stanford and a field assistant (M. Keiver) collected similar data on a gorilla group in the lower elevation, Nkuringo section the park. Preliminary results have shown the following (data are still under analysis and a first journal article on the results is in preparation):

  1. Bwindi gorilla diet is, at least seasonally, markedly higher in fruit than is that of the nearby Virungas gorillas. More than 20% of dung samples of Ruhija gorillas contain seeds of ripe fruits during some months. In other months, however, the gorilla diet is essentially 0% fruit, similar to that in the Virungas.
  2. Bwindi gorillas make more use of arboreal substrates, climbing to feed on foliage, fruits, and epiphytes. Silverbacks also climb, at times as high as 40 m above the ground. This behavior is extremely rare among Virungas gorillas.
  3. Bwindi gorilla diet is quite similar to that of Bwindi chimpanzees in some months. Seasonally, the two species’ dung both has contained largely the seeds of the same three fruit species, Chrysophyllum sp., Cassina aethiopica, Podocarpus sp. and Syzigium guineense.
  4. Bwindi gorillas travel further per day than do their Virungas counterparts; they travel further on days when feeding heavily on fruit than when they are feeding on fibrous foods.
  5. Perhaps in keeping with their greater arboreality, Bwindi gorillas are much more likely to construct nests in trees than their Virungas counterparts, who nest entirely on the ground. When tree-nesting, Bwindi gorillas nearly always choose the same tree species, Echizogwa, a small understory tree.

Chimpanzee behavioral ecology

From June-August 1997, Stanford conducted a preliminary study of the local chimpanzee population in the Nkuringo area. Chimpanzee parties were contacted daily; party size was greater than or equal to 15 for one 7-day period, during which time they fed almost exclusively on fruits of two tree species (Cassina aethiopica and Chrysophyllum gorungosanum [Sapotaceae]). Using a formula for community size estimation from Yamagiwa et al., we estimate the Nkuringo chimpanzee community to contain at least 40 members. Based on estimates from the Buhoma area and extrapolation to the rest of the park provides a very rough estimate of approximately 350-400 chimpanzees in Bwindi, i.e. this species is more abundant locally than gorillas are. Follows of the gorillas in the same valley during this time showed they also fed on the same two fruit species. >From June 1998 through June 1999 we collected daily data on chimpanzee nesting, ranging, and diet. This, combined with GPS mapping of the Nkuringo area, has allowed us to put together a detailed map of the habitat use of both the chimpanzees and gorillas. In January 2000 we moved the project to Ruhija and have since collected 18 months data on on chimpanzees and gorillas there. Preliminary observations suggested that chimpanzees occur at higher density near Ruhija, based on nest densities, than in other sections of the park where we have surveyed them. In May, 2001, we witnessed for the first time a gorillas group feeding in the same tree (a Ficus natalensis) with members of our chimpanzee community. They did not interact, but approached to within three meters of each other and fed on nearby branches. Analysis of chimpanzee fecal samples has demonstrated that many of their foods (mainly figs) were not important food items for gorillas during this month.

A second recent finding is that Bwindi chimpanzees are more bipedal in posture, at least when feeding in trees, than other documented chimpanzee populations. During May, 2001, while feeding in Ficus sp., the Ruhija chimpanzees foraged in the tree bipedally (standing on the largest terminal branches) more often in a ten day period than other studies have recorded in a year or more. A total of 38.5 hours of observational data were collected on 20 individuals, during which 78 instances of bipedalism by nine individuals were recorded, an average of 0.49 bouts per observation hour for the entire community. All bipedalism occurred arboreally on the larger limbs of the Ficus natalensis, and all instances occurred in a fig-fruit feeding context. Individuals varied widely in their tendency to be bipedal, as well as the average duration of their bipedal bouts. One male was bipedal at least 24 times in approximately 20 contact hours. The sample included four adult males and four adult females; males were bipedal significantly more often than females. A manuscript about this behavior is currently in preparation.

A third recent finding is that chimpanzees in the northern, “Kayonza” sector (which we are calling the Byumba chimpanzees) show a tendency to sometimes nest on the ground. These are believed to be night nests, and apparently are a local cultural tradition dating back at least forty years. A paper on this funding in submitted for publication (Maughan and Stanford, under review).

Chimpanzee Tool Use

Bwindi chimpanzees use two types of stick tools to extract honey of two bee species. Tools were found to fall into two size categories. Nine tools found at the base of three trees containing nests of the stinging Apis mellifera had a mean length of 60.2 cm (range 25.0 – 85.0 cm), and a mean mid-point diameter of approximately 1.6 cm. Twelve tools found at the base of three trees containing nests of a stingless bee known locally as Obuhuru maranga (Meliponula brocaudei [Trigonidae]) had a mean length of 27.2 cm (range 14.0 – 70.5 cm) and a mean mid-point diameter of approximately 0.5 cm. Only one tool from each size category fell within the size range of the other category. All sticks had been cleared of attached leaves and twigs. Sticks of both types were missing bark from one end and may have been peeled; the bare end on most was frayed and smelled strongly of honey. Full results can be found in Stanford et al. (2000).

The significance of the tool discoveries is that other local chimpanzee populations that have been studies, in Kibale National Park and Budongo Forest Reserve to the north, do not typically use tools of any sort. Bwindi chimpanzees therefore represent a local culture of tool use that may be unique. We believe that this will be an area of fruitful future research.

Phenological Sampling

Two phenology transects have been established in the Ruhija area. A total of about 200 trees known to be used by either gorillas or chimpanzee, or both, have been tagged and monitored for the past year. We plan to increase the number of trees to be monitored to about 350, on separate transects that sample the elevational gradient in the site. The transects are planned to eventually consist of ten individuals of each known food species. The use of handheld GPS units allows us to map stands of fruit trees and their seasonal use by gorillas and chimpanzees onto digitized GIS maps of the study site that have been prepared by the GIS laboratory at Makerere University in Kampala. In this way we hope to create a graphical as well as quantitative picture of monthly changes in habitat use by the two ape species.

The study began in 1996, and in 1998 a research camp was constructed along the remote southeastern perimeter of the park. However, recent terrorist activities by Hutu rebel militia from neighboring Congo have necessitated relocating the project to Ruhija, the highest elevational region of the park. There, we are studying an unhabituated chimpanzee population and also collecting ecological data on one habituated gorilla group. The study is being carried out under the auspices of the Ugandan Wildife Authority, the Ugandan National Council for Science and Technology, and the Institute for Tropical Forest Conservation.