Taï (Boesch 1996). However, the importance of food as a determinant of party size has
also been questioned. Isabirye-Basuta (1988) found that this relationship did not hold
when multiple sources of important foods were available. Boesch (1996) found
that despite apparently similar levels of fruit production in each of two wet seasons,
chimpanzee party size was large in one but small in the other. Stanford et al. (1994)
found that parties of Gombe chimpanzees were largest during the dry season when food
supply was restricted.
Newton-Fisher et al. (2000) explored this topic in detail. Over several years we made
three independent studies at Budongo. In the first of these I took data from our long-term
study of party size and composition, determined the mean size of feeding parties in 1-ha
blocks of forest over a four-year period, and then compared this with the abundance of
chimpanzee food using data from a study of the size and species of chimpanzee food
trees in those blocks. There was no simple relationship. In the second study, Newton-
Fisher compared party sizes obtained by scan-sampling over a 15-month period with the
abundance of food on which they were observed feeding; again there was no simple
correlation. In the third study, Plumptre observed chimpanzees over a 14-month period
(following the period observed by Newton-Fisher) and obtained scan data on party size
and also on food abundance; as before no simple correlation was found. The situation is
very complex, but there was evidence from each of these studies that what matters in
determining party size is not only the abundance of food, but its dispersion (how
‘clumped’ or otherwise it is), species and food type (whether leaves, ripe fruits, unripe
fruits, etc.). We concluded that ‘factors other than food supply were primarily responsi-
ble for the size of chimpanzee parties at Budongo’ (Newton-Fisher et al. 2000: 623).
Why should this be? We posited a number of reasons. One is that it seems that the
food supply at Budongo is particularly rich for chimpanzees (see Chapter 1) because of
the logging history of the forest (Plumptre and Reynolds 1994). We concluded that ‘with
increasing food abundance the importance of feeding competition on party size decreas-
es, eventually reaching a point where the relationship becomes negligible and other
factors control the size of chimpanzee parties’ (Newton-Fisher et al. 2000: 625). And in
particular we posited that the presence of oestrous females might be an important factor
determining the movement of males, and to some extent females as well, so that as food
availability increases, after a certain point party size reflects the number of oestrous
females present rather than of food.
Fawcett (2000) in a 16-month-long study focusing on female relationships and food
supply at Budongo did find a positive correlation between food supply and party size.
She wrote ‘A frequent predictor of party size was the combined measure of abundance
of fruit, flowers, buds and leaves, highlighting the importance of leaves, in addition to
fruit, of the diet of the chimpanzees at Budongo’ (p. 217). Fawcett also found that the
number of females in the Sonso community exhibiting sexual swellings increased at
times of high food availability. What she called ‘sexual parties’, i.e. parties with one
or more females in oestrus in them, were larger when there were more oestrous females
in the community. This moves us forward on the question of how food availability
and presence of oestrous females affect party size: it seems the effects may be both
Influence of food supply on party size 95