Chimpanzees of the Budongo Forest : Ecology, Behaviour, and Conservation

66 Diet and culture at Sonso

than is normally the case. Thus fluctuations occur from year to year in what is available.

However, there is no month in which food is scarce, and even in the drier times of year

some ripe fruit is available.

With regard to food abundance, because you cannot find all the trees of a given

species in the forest (this can be done in a very small patch of forest but not for a region

of 15 km^2 , the range of the Sonso chimpanzees) some kind of sampling technique is

necessary. Having estimated the number of fruit trees you then need to have a method

for estimating the quantity or biomass of the fruit on each tree. This can be done by placing

plastic squares or buckets under the tree and calculating the weight or mass of fruit

falling on them, then extrapolating up to the estimated area or volume of the whole

crown of the tree. Or it can be done by eyeballing the crown, counting the number of

fruits in a small section and then extrapolating up. Alternatively, a more indirect method

can be used. Chapman et al. (1994) showed that the diameter of a tree at breast height

(DBH) is a good predictor of its fruit abundance for trees in Kibale Forest and we have

also used DBH to estimate fruit abundance at Budongo (Newton-Fisher et al. 2000).

Our work has led us to conclude that the Sonso chimpanzees live in an area of forest

with high food, and especially fruit, abundance (Plumptre et al. 1997; Newton-Fisher

1999 a). This contrasts with the situation at Gombe where there is a period each year

of food scarcity (Goodall 1968) and at Kibale (Wrangham et al. 1993) where

0

D J FMAM J J ASOND

1994 –1996

JFMAMJJASOND

20

40

60

Percentage of diet

80

100

Ripe fruit

Seeds

Unripe fruit

Leaves

Other Flowers

Bark/wood

Fig. 4.4: Sonso diet over a two-year period, 1994–1996 (courtesy of A. Plumptre, WCS, from

Plumptre et al. 1997).