a subset of the data to see if there was any difference between party size or composition
according to season. The results are shown in Table 5.3. The difference in party size was
highly significant: party size was larger in the dry season.^35
Why should this be? One of the main food sources in the dry season is the seeds of
Cynometra alexandri. When this tree is fruiting, chimpanzees gather in large parties to
feed on its seeds which are lipid-rich and provide one of the most nutritious vegetable
foods the forest has to offer. Unlike some other forest tree species,Cynometrais highly
seasonal. Only once in the 12 years of study has it not fruited between December and
February and that was in January 1995, probably a side-effect of El Niño.
A second factor associated with increased party size is the presence of oestrous
females (see above). Could the increase in party size in the dry season be a result of
an increase in the number of oestrous females at that time? An analysis was done of
party size comparing number of oestrous females in groups in the dry season with
the wet season. The results are shown in Table 5.4. This result was, as before, highly
significant.^36
Thus the increase in party size seen in the dry season is likely to be associated with
both the presence of Cynometraseeds and also with the larger number of adult females
in oestrus at that time. Whether these two factors are causally related to each other is not
known.
Influence of food supply on party size
The question of the effect of food supply on party size remains a central focus because
we believe that food abundance and dispersion are important factors determining not
only party size but also the evolution of the chimpanzee’s fission–fusion social system.
Evidence for a positive correlation has come from Gombe (Wrangham 1977), Kibale
(Ghiglieri 1984; Isabirye-Basuta 1988; Wrangham et al. 1992; Wrangham 2000a) and
94 Social organization
Table 5.3: Party size according to season.
N Mean s.d.
Dry 173 8.50 5.95
Wet 1927 6.87 4.87Table 5.4: Number of oestrous females
in parties according to season.
N Mean s.d.
Dry 173 1.07 1.21
Wet 1926 0.58 0.99(^35) The seasons were compared using a t-test (two-tailed). For purposes of the test, equal variances were
assumed (Levene’s test, 36 p0.000). The result was highly significant (t4.13, df2098,p0.000).
Levene’s test indicated equal variances and the resulting t-test gave t6.08,p0.000.