Parties, food and social factors
The pattern of fission and fusion of parties is central to chimpanzee social organization.
Explanations for this uncommon pattern, also found in spider monkeys in South
America (Chapman 1990), have been sought in a number of factors. One is the patchy
distribution of food, which occurs at many different places in the community range;
formation of larger or smaller parties is a result of feeding competition. Individuals
are not averse to forming large parties to exploit rich food patches (e.g. a large fig tree
containing many thousands of ripe fruits), whereas when food is scarce or more
dispersed they can find more food by splitting up into small parties.
A second set of factors affecting party formation can be collectively called ‘social’.
For example, sexually receptive adult females attract and are attracted by adult males.
Non-sexual social factors are also important: alliance partners tend to move together,
dominant males sometimes attract a retinue, friends tend to stay and move together, and
kinship bonds are strong, with mothers and their infants always being together.
Together, food abundance and social factors determine party size.
Party size^27
A variety of methods has been used to collect data on chimpanzee party size (see Boesch
1996 for a review of some of these). We can contrast (a) continuous sampling over the
period of the life of a party (i.e. from when it begins to when it ends), and (b) scan
86 Social organization
No. of individuals in party1 3 5 7 9 11 13 15 17 19 21 23 25 27 31Frequency of party sizes50403020100Fig. 5.1: Distribution of party sizes (data for 2001).
(^27) The parties referred to in this chapter are, except where stated, feeding parties, not parties of other kinds
such as, for example, nesting or travelling parties.