Chimpanzees of the Budongo Forest : Ecology, Behaviour, and Conservation

smaller (N27). Boesch and Boesch-Achermann (2000: 21–22) also found no evi-
dence for a birth season based on a sample of 75 births, and no birth season was found
for the chimpanzees of Mahale (Nishida et al. 1990).
However, Goodall did find evidence of a non-random distribution of oestrous
swellings (see her Fig. 5.6) and this is the case at Sonso too (Wallis 2002a). This
interesting phenomenon is dealt with more fully in Chapter 6.
The birth rate at Sonso can be calculated by dividing the number of infants born each
year by the number of females in the community. This is done in Table 2.2, which shows
the Sonso birth rate from 1994 to 2002, together with the mean birth rate (0.16) and the
standard deviation (0.09) for the whole period.
The inter-birth interval (IBI) is a measure, in months, of the time between one infant
and the next. This is longer when the first infant survives than when it dies. For Sonso
the mean IBI for 10 intervals was 67 months (range 58–84 months).^10 For mothers of
more than one infant where the preceding infant died we have three cases from Sonso:

1. Kigere gave birth to a premature stillborn infant in early September 1997. She gave
   birth to Keti, an infant female, in September 1998 (IBI one year).


2. Zimba’s infant Zip disappeared soon after birth in August 1996. She gave birth to
   Zig, a male infant, on 7 July 1997 (IBI 11 months).


3. Melissa’s son Mark disappeared in August 2002. She gave birth to her next infant,
   Monika, a female in July 2003 (IBI 11 months).

A rule of thumb for Sonso therefore is: IBI around 5 years when the baby survives, but
around 1 year when it dies.
At Mahale, the IBI when the first infant survived was 69.1 months for 33 births, and
when it died was 14.4 months (Nishida et al. 2003); the IBI at Sonso was shorter in both

Births 33

Table 2.2: Sonso birth rate 1994–2002 (data compiled by C. O’Hara and L. Bates).

Year No. of births No. of females Birth rate

1994 1 12 0.08

1995 2 15 0.13

1996 3 17 0.18

1997 6 18 0.33

1998 3 18 0.17

1999 1 17 0.06

2000 1 17 0.06

2001 4 18 0.22

2002 4 17 0.24

Average 2.78 16.56 0.16

St. dev. 1.72 1.94 0.09

(^10) Thanks to Melissa Emery Thompson for these calculations.