independent and interactive. A rich food supply (including the presence of nutritious
leaves) increases party size; and it increases the number of females in oestrus; the number
of females in oestrus also increases party size. Thus, the near doubling in size of parties
with one or more oestrous females (see earlier in this chapter) can be explained as an out-
come of a two related causes, with food pushing sex and both pushing party size.
Yet another study of food abundance and patch size in relation to party size at
Budongo was undertaken by Harris et al. (in progress) designed to test some of the
above ideas. To overcome some of the difficulties presented by multiple species and
food types we used the idea of ‘food quality’, with ripe fleshy fruits being of highest
quality, unripe and hard fruits of medium quality, and leaves of lowest quality. In the
analysis we related food quality, party size (recorded by scan-sampling with a 20-min
interval), and presence of oestrous females, paying particular attention to changes in
party size, i.e. whether individuals were joining the party or leaving it. The results
showed that higher food quality was positively correlated with increases in party size
over time and vice versa. Thus when food quality is lower, chimpanzees stay on the food
for a shorter time and leave sooner than when food quality is high. Party size was greater
when one or more oestrous females was present, but presence/absence of oestrous
females did not have any effect on party duration or dynamics (movements to and from
the group).
Nesting and nesting parties
Brownlow et al. (2001) collected data on the nesting behaviour of the Sonso community
between October 1995 and December 1996. Chimpanzees were followed from dawn to
dusk over periods of 24 and 48 h over this 15-month period; 104 day nests and 601 night
nests were recorded.
A significant sex difference was found in night nest height, with adult males nesting
lower than adult females (Table 5.5). Males nested lower than females, perhaps because
they are better equipped to deal with nocturnal predators. Alternatively, the greater
weight of males may lead them to nest on sturdier branches lower down trees than
females, or their greater strength may lead them to choose somewhat sturdier branches,
especially as these produce a better sprung bed than do the smaller branches further up
96 Social organization
Table 5.5: Comparison of adult male and adult female nests.
Males Females Significance
(Mann–WhitneyU)
Night nest height
Mean 10.91 m (N334) 13.55 m (N267) p0.0001
Range 2.5–32.5 m 2.5–32.5 m
s.d. 5.83 5.77
Day nest height
Mean 14.75 m (N40) 15.78 m (N64) n.s. (p0.59)
Range 0–27.5 m 7.5–27.5 m
s.d. 5.96 5.51