skewed to the left, i.e. smaller parties are more frequent than larger ones. Maximum
party size in Taï was 41 (53.9% of the community size of 76); in Sonso for 2001 it was
31 (65% of the community size of 48 at that time). Boesch (1996) argues that mean
party size should be expressed as a percentage of the total community size because total
community size limits the size parties can reach. Expressed in this way, he found that
mean chimpanzee party size over the study period was between 9% and 21% of total
community size. In the Sonso community in 2001 mean party size was 9.12 or 19% of
the total community size which falls within the range found at Taï. The distribution
of party sizes for the year 2001 is shown in Fig. 5.1.
For the same reasons that mean party size should be expressed in relation to the size
of the community, when calculating the mean number of males and the mean number of
females in parties, these should be expressed as a proportion of the total number of
males and of females in the community. This is done, for example, by Emery Thompson
and Wrangham (in press) and I have used this method in this book (see Table 5.1).
Party duration
We carried out an analysis of party duration: the length of time party size remained con-
stant, i.e. how long, on average, it was before one or other of the party members left or
new individuals joined the party. In a subset of 162 parties, the mean time parties
remained constant was 19 min. Mean duration of parties at Taï was 24 min (Boesch and
Boesch-Achermann 2000: 91).
Summarizing, parties at Budongo, over this four-year period, consisted on average
of 5–7 individuals and ranged from 1 to 29 individuals, with a change in composition,
on average, every 19 min.
Party composition
Parties can be made up of almost any age–sex composition. The smallest party consists
of a lone individual. An analysis was done of the composition of parties observed during
the period January–December 2001. Over this period, the composition of 482 feeding
parties was recorded. During this period the number of individuals in each age class^30
who were present during the whole year was as follows:^31
Males(N18)
9 adult, 2 subadult, 4 juvenile, 3 infant
Females(N30)
14 adult, 2 subadult, 3 juvenile, 11 infant
Total 48
88 Social organization
(^30) Definitions of age classes can be found in Chapter 2, Table 2.3.
(^31) Two adult females and two juvenile males were excluded from the analysis because they were not present
for much of the year.