80 Diet and culture at Sonso
rivers flow, are found strips of riverine forest. These used to join the main forest block
but now no longer do so because of tree-cutting by the local population. In recent years
we have started to study the chimpanzees living along the Kasokwa River. At the present
time, 13 chimpanzees live permanently in Kasokwa Forest.
They live a very different lifestyle from the Sonso chimpanzees and others in the
main forest block. They are much closer to human habitations, more so in recent years
since the population to the south of Budongo has increased greatly, and they practise
crop-raiding, particularly at times when few forest fruits are available in their narrow
forest strips. Their predicament is that they cannot find enough food in the riverine forest,
they cannot move easily to the main forest block, and nor can they move easily into the
other forest fragments. As a result they move out into the farmers’ fields and gardens
to feed on sugar, paw-paws and mangoes. This has led to a deterioration in human–
chimpanzee relationships in that area, and the situation seems to be worsening at the
present time (Reynolds et al. 2003). We shall discuss this problem more fully in Chapter 11,
where we look in more detail at the complex interactions of chimpanzees and people in the
Kasokwa area.
Sonso culture
Among the various chimpanzee cultures described, the culture of the Sonso chimpanzees
is marked by a shortage of technological sophistication compared with chimpanzees
from some other sites. The termite fishing of Gombe and the nut-cracking of Taï and
Bossou are missing. Are our Sonso chimpanzees somehow deficient, lacking in skill?
I believe they are not; they have a variety of cultural characteristics including the use of
tools, but their technology has remained very simple (see below, and also see Chapter 11
on the Kasokwa chimpanzees). One of the reasons is not far to seek: their habitat, as we
saw in Chapter 1 and Chapter 4, is rich in their preferred foods, ripe fruits, and there
seems to be no time of year when they run seriously short of forest foods.^23 But I suspect
that is only part of the explanation. There is also the fact that East African chimpanzees
have not developed the sophisticated stone tools used by West African chimpanzees to
crack nuts, and of the East African chimpanzees it seems to be those to the south that
have developed the greatest skills in termite fishing, while as we move towards the more
northerly forests (and Budongo is furthest north of the major forests along the Rift
Valley escarpment, even though there are two small forest populations in Rabongo and
Otzi Forests which are further north than Budongo) the use of tools seems to decline.
This may be because, as in the case of nut-cracking, the idea of using sticks to fish for
insects has never reached Budongo.
(^23) This implies that functional considerations are important in determining the development of cultural
activities in chimpanzees. This is a hotly debated point in respect of both human and primate societies.
Function is no doubt related to the evolution of culture itself; cultural variations are perhaps less determined
by function. However, as for example Humle and Matsuzawa (2002) show, the relative aggressiveness of cer-
tain ant species determines the tool-length and techniques employed by Bossou chimpanzees when engaging
in ant-dipping. Probably we have to conclude that while some cultural variants can be explained in functional
terms, others cannot.