Gorilla Studies in the Nouabalé-Ndoki National Park
Field studies by the Japanese team in the Nouabalé-Ndoki area, in Northern
Congo, began in November 1987, and ecological surveys of primates in the
area were initiated in 1988 by Masazumi Mitani, Suehisa Kuroda and Tomoaki
Nishihara. Beginning in 1989, a WCS (Wildlife Conservation Society)
team initiated forest surveys in the area and in November 1993, with the
great effort of WCS, the Congolese government established this area of
3,865.92 km² as a national park. The park is covered by primary forest,
composed of 3 types of vegetation: evergreen monospecific forest (Gilbertiodendron
dewevrei), semi-deciduous (mixed species) forest, and swamp forest.
The annual rainfall is about 1,500 mm and is divided into a major dry
season (December-February), minor rainy season (March-June), minor dry
season (July-August) and a major rainy season (September-November).
Primates have been found in high densities within the park, with western
lowland gorillas, chimpanzees, and nine other species of diurnal primates
living sympatrically, without large human impact for at least 50 years.
The Japanese research team has focused mainly on the feeding ecology of
western lowland gorillas. As the gorillas were difficult to observe at
the first study site (Njinji), data on gorilla behaviour was gathered
indirectly and the main study methods used were dung and trail analyses,
plus nutritional analysis of certain food items in the gorillas' diet.
The phenology of fruit production was also studied to examine its effects
on the gorillas' diet and Nishihara spent a full year in the forest from
1991 to 1992 collecting data on gorilla diet and seasonal fruit production.
Long-term studies have described mountain gorillas as primarily folivorous
and, for some time, this was assumed true for western gorillas as well.
However, studies of the feeding habits of western lowland gorillas suggested
that they might be considered frugivorous, and the relative importance
of these dietary aspects formed the main body of Nishihara's work. The
feeding ecology of chimpanzees was also examined using the same methods,
enabling us to make comparisons between the two species and to describe
ecological features of gorillas. Gorilla and chimp densities were estimated
using nest counts (for gorillas) and direct counts of individually identified
chimps. The important results were as follows:
- There is one main fruiting season (April-September) and a period of relative scarcity of fruit (October- March).
- Gorilla and chimpanzee population densities are remarkably high at the study site (size approximately 20 km²). Densities were 1.92-2.56 animals/km² for gorillas and 2.65 animals/km² for chimpanzees.
- The gorilla's basic diet consists of the fibrous parts of plants, though they also consume a large quantity of fruits corresponding to their seasonal availability. Chimpanzees mainly depend on fruits all year.
- Gorillas and chimpanzees share a large number of fruit food species, but overt competitive interactions over food have not been observed. On the contrary, we observed gorillas and chimpanzees peacefully co-feeding in fig trees during the period of relative fruit scarcity.
- Gorillas feed on mineral and protein rich swamp vegetation year round. Aquatic herbs in marsh grassland are particularly favoured.
- Niche separation between these apes is found over mineral food and protein food; chimpanzees acquire minerals and protein preying on birds, mammals, and insects whereas gorillas take them from herbs, leaves, seeds, and different termites species from those eaten by chimpanzees.
Poor visibility as a result of dense vegetation has made direct observation
of gorillas difficult. In May and June 1994, an extensive survey was conducted
to find a study site where observation conditions would be better than
those in the "Njinji" area. A good study site at an area of
marsh grassland (about 1,500 x 100 m) called "Guga Bai" was
selected, where gorillas could be observed frequently with good visibility,
and where several chimpanzees could be habituated. In late 1994, Nishihara
established a study camp at this study site and started an intensive survey
and habituation project. At Guga, gorillas can be located with ease, either
as they pass through the open Gilbertiodendron forest that surrounds the
bai, repeatedly using the same routes, or as the gorillas are already
in the bai through the sound of food washing, moving through marsh or
feeding grunts emitted by them. In order to increase observation frequency,
platforms were set up in the trees at the points where gorillas habitually
enter and exit the bai. Observation time could be greatly increased using
these platforms. For example, between January and February 1995, we had
gorillas in view for 427 out of 2,728 minutes (15.7%) we spent at the
platform. At least 3 groups and 2 solitary males that utilize the bai
have been identified; each group was cohesive and contained one silverback.
The maximum group size was 16 individuals. We found that gorillas in a
unit group moved in procession when they entered and exited the bai. Nine
cases of such processional movement were observed. Data from these entries
and exits suggest that silverbacks do not always lead group movement.
Gorillas frequently scratch soil on the floor in the Gilbertiodendron
forest near the bai. The purpose of this is not clear but it probably
serves to search for insects or other small animals, possibly including
earthworms. It was difficult to find such animals during our own attempts
at soil scratching, and therefore this behaviour did not seem to be an
efficient form of nutritional intake. In addition, no fragments of insects
or small animals were found in gorilla dung.
In 1994, Nishihara began a training program for Congolese research assistants
at Guga. Students graduating from Brazzaville University were instructed
in field research methods related to chimp and gorilla ecology. Results
were presented by the students at seminars held in Brazzaville and at
a primate conference in the United Kingdom. Unfortunately, the study and
training program at Guga was interrupted by the civil war in 1997 and
currently remains suspended.
Tomoaki Nishihara
Prof. Tomoaki Nishihara has studied gorilla ecology in Nouabalé-Ndoki forest since 1089. From 1995, her realized a training program for primate research. Since 1997, he was the park base manager of the WCS project.
