Chimpanzees to the East of the Mahale Mountains.
A long-term research project on chimpanzees has been conducted in Mahale, Tanzania by Nishida and his colleagues since 1965 (for details see 1). This study has been conducted on the western side of the Mahale Mountains, where there is relatively more rainfall as a result of the special landscape between Lake Tanganyika and the mountains. The base camp of the Japanese research team and the National Park office are both located on this side of the lake. This is mainly because access to the Mahale region is nearly limited to a route via Lake Tanganyika, since it is difficult to get there by car. Thus, the lakeside is the front of the National Park. Little research has been done on the back side, i.e. the eastern side, of the Mahale Mountains since 1967, when Kano (2) surveyed extensively including this area.
We conducted a survey on this area from September 9 to 17, 1996. This is a brief report of our safari.

September 9
We departed Bilenge for Konkwa by boat, from where we started to walk southeastward. Until Ntondo, we walked on former human trails along the low ridge. With nothing to shadow the strong sunshine, we suffered from thirst. There were some fire-resistant trees, but most of them had dropped their leaves in the depth of the dry season. We found no sign of chimpanzees, instead only feces of roan antelope.

September 10
We continued to walk along the ridge. At 10:56, we found very dry chimpanzee feces on the ridge. Inside the feces, we found seeds of Vitex doniana. At 11:41, we heard many calls of chimpanzees from the riverine forest up the Mganbo River (Fig. 1). To get there we had to cross a deep valley, so we gave up trying to catch up with them. At 12:10, we found leftovers of the fruit of Strychnos cocculoides. The size of the fruit is about that of a baseball with hard and thick 3-5 mm pericarp and absorbingly juicy pulp inside. It tasted like a sweetened plum, the sweet and sour taste that chimpanzees like most. Those pericarps were singed, surely because of wildfire, but the pulp and seeds remained intact. There were Erythrina abyssinica trees around bearing red flowers, which are also chimpanzee food. At 14:19, we found Strychnos fruits again, this time much fresher ones. They were probably eaten by chimpanzees that morning. We also found leftovers of Piliostigma thonningii and Canthium crassum fruits.

September 11
We saw two leftovers of Piliostigma fruits in the morning. At 15:07, we found two chimpanzee beds that looked a little old at Sihe hill.

September 12
At 9:21, we found a bed in the middle of the hill. At 11:34, we crossed the Lufubu River, which formed the boundary of the national park. Everywhere outside of the national park was burnt just like inside it. At 13:00, we found the skull of a guenon. Judging from the singe on it, it was perhaps killed by wildfire.

September 13
At 10:01, we crossed the Lugonesi River. It had more water than any other river we had crossed so far. At 12:39, at the Kabwe Gaje Hill, we found one piece of feces and two wadges of bamboo shoot chewed by chimpanzees. From the ridge of the hill to the south, we could see the village of Bugalaba. To the north, an affluent of the Lugonezi River flowed. Surrounding it was a riverine forest where chimpanzees were likely to live. At 15:57, we again found leftovers of Strychnos. At 16:26, we reached a human trail that came from Bugalaba and led to Mwese.

September 14
As soon as we set off, it began to rain heavily. It rained even in the dry season perhaps because we were close to the Mwese Mountains. We arrived at Mwese about noon, when the rain had become light.

September 15-17
On the way back from Mwese, we took a road that goes directly to Lukoma in the north of the national park. We arrived at Lukoma on the 16th. We stayed that night and on the 17th we hired a boat back for Bilenge.
We walked behind the Mahale Mountains in the middle of the dry season and the area appeared so dreary because everything was burnt. The remnant trees were also burnt on the surface, but these fire-resistant trees had sprouts on the burnt stocks. The undergrowth was also all burnt but also had spouts and new flowers. These marks of fire were perhaps caused by the fire used by shifting cultivators or poachers coming into the national park. This kind of wildfire may not be unusual but instead it is typical, given the fact that fire is sometimes faintly seen on the ridge even from the front of the national park. Thanks to the wet condition of the forest, such fire does not yet come into the front area.
Unfortunately, we could not observe chimpanzees directly, but we did find clear evidence that they live in this dreary area. They seem to mainly use riverine forests that contain enough water and remain even after a wildfire. In the dry season they use fire-resistant plants such as Strychnos or Piliostigma. This area is also the habitat of many elephants, buffaloes and antelopes. There are also larger carnivores such as lions and hyenas. Considering the abundance of predators and the scarcity of trees for hiding, it must be a severe place for chimpanzees to live in. How do they manage to live in such conditions? Some studies on chimpanzees were done in such very dry areas (3, 4, 5), but it is still a big question in chimpanzee studies.

References

1. Nishida, T. (ed), 1990. The Chimpanzees of the Mahale Mountains, University of Tokyo Press, Tokyo.
   
2. Kano, T., 1972. Distribution and adaptation of the chimpanzee on the eastern shore of Lake Tanganyika. Kyoto University African Studies, 7: 37-129.
   
3. Izawa, K., 1970. Unit groups of chimpanzees and their nomadism in the savanna woodland. Primates, 8: 355-381.
   
4. Kano, T., 1971. The chimanzees of Filabanga, western Tanzania. Primates, 12: 229-246.
   
5. Suzuki, A., 1969. An ecological study of chimpanzees in a savanna woodland. Primates, 10: 103-148.
   

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