answer may have something to do with the fact that chimpanzees are very good at moving
between forest patches, as we know from our own studies at Budongo and from studies
elsewhere. They are not confined to continuous forest by any means, and so would be
quick to explore northwards along forest strips and patches. But we need to remember that
blue monkeys, redtail monkeys and black and white colobus monkeys, all true denizens of
the forest which do not normally cross open country for any great distances, also made it
to Budongo. This leaves the question open as to whether and when the Budongo Forest
was last reduced to a fragment of its present size, or disappeared altogether. If it disap-
peared during the last glaciation 18 000 years ago and did not reappear until around
10 000–12 000 years ago, then we can assume that the three forest monkey species and the
potto made it to Budongo at that time or later. As regards the chimpanzees, perhaps they
never became locally extinct in the way we think the monkeys would have done.
Depending how dry it became, they may just have dispersed into the woodlands or bushy
terrain where the forest once was. We know from studies in West Africa by Nissen in
Guinea and by McGrew in Senegal that chimpanzees are able to survive in very dry con-
ditions, albeit at very low densities. This would mean that their response to environmental
change may be less dramatic than that of the monkeys and prosimians. Genetic data on
East Africa’s chimpanzees indicate a high degree of relatedness between all the chim-
panzees of western Uganda (Goldberg 1997; Goldberg and Ruvolo 1997) but this can-
not tell us whether they dispersed at the time of forest decline or disappeared to be
replaced by new arrivals from the southwest. It is also not clear why chimpanzee distri-
bution is confined to the western forests of Uganda and does not extend farther east,
despite the existence of forest patches and some fairly continuous forest between the
Rift Valley and Lake Victoria, e.g. the ‘Mubende corridor’ (D. Pomeroy, pers. comm.).
The forest in more detail
Today Budongo Forest is a medium altitude, moist, semi-deciduous tropical rain forest. It
is situated between 1 37 N–2 03 N and 31 22 –31 46 E. The average altitude is
1100 m. The whole forest slopes down gently in a southeast–northwest direction, towards
the Albertine Rift. Four rivers run through the forest: from east to west they are the
Waisoke, the Sonso, the Kamirambwa and the Siba. The mass of the forest comprises
some 35 200 ha (352 km^2 ) to which we must add the Siba Forest, which is an extension of
Budongo to the southeast and comprises some 8300 ha (83 km^2 ) making a total of 435 km^2
of continuous forest cover. Outside these two main areas there are numerous lengthy strips
of riverine forest to the south and southwest of Budongo, perhaps some 100 km of riverine
forest in all, forming arms of forest that stretch out into the surrounding areas, grassland in
the past but today mainly cropland (much of it sugar cane) or under human occupation.
Climate
Rainfall data were collected at Busingiro and at Nyabyeya in the past but both these sites
are on the forest edge. The BFP has now been collecting rainfall and temperature data
Climate 9