cases. At Taï the IBI when the first infant survived was 69.5 months, when it died the IBI
was 12.9 months (Boesch and Boesch-Achermann 2000). Again the IBI was shorter
at Sonso. Data for other sites on the IBI when the first offspring survives are: Bossou
61.2 months, Gombe 66 months and Kibale 86.4 months. All these intervals are longer
than the equivalent ones at Budongo.
Emery Thompson et al. (in press) in a study of reproductive parameters (including
endocrinological ones) of wild chimpanzee females, compared the IBIs of Sonso
females with those of Kibale’s Kanyawara community. They found that whereas there
was considerable variation in IBI at Kanyawara, at Sonso the IBIs were more clustered
around the mean. This indicates more regular birth spacing at Sonso and may perhaps
reflect the more reliable or more productive food supply. Comparing peripheral females
(those seen infrequently, defined as seen in less than 15% of all scans) with central ones,
they made the surprising discovery that, in a reversal of the Kanyawara pattern, periph-
eral females at Sonso have slightly shorter IBIs than central ones. The reasons for this
are not yet clear but may relate to the fact that food is not in short supply for peripheral
females, or that they are less stressed socially, or both. Unlike Gombe and Kanyawara
(Kibale) chimpanzees, ‘central’ and ‘peripheral’ Sonso females do not show any obvi-
ous distinctions in core area. Thus they are socially peripheral, but not necessarily spa-
tially so. Distinct core areas would be surprising given what appears to be a relatively
small community range and the high density of Broussonetiaand large fig trees in the
central, logged area.
The birth of Katia
It is highly unusual to observe a chimpanzee giving birth, perhaps because females tend
to hide themselves away at this vulnerable time. However, our field assistant Zephyr
Kiwede did observe a complete birth which, even more unusually, took place high in
a tree. The events are described in full in Kiwede (2000) from which these extracts
are taken:
At 07:30 on 30 December 1998, chimpanzees of the Sonso community were heard calling. Many
chimpanzees were found feeding on ripe figs (Ficus mucuso). A nulliparous female named Kewaya
(KY) was sitting on a branch of the tree at a height of 30 metres above the ground just below the
crown. At 08:07 KY was not feeding but appeared to be in pain. At 08:38 she pushed her left hand
into her vagina for 40 seconds. The right hand of KY was deformed and lacking functionality due
to a snare injury. When she removed the hand it was wet. She licked her fingers. At 08:42 she
pushed the hand again into her vagina. When she removed it, a quantity of fluid came out from the
vagina. It was assumed that the amniotic sac had broken. She moved half a metre on the same
branch and sat, then stood up quadrupedally, then moved back, sat again and again moved back.
She pushed her hand into her vagina several times. At 08:50 while removing the hand from the
vagina, fluid emerged again. She remained ‘unsettled’ and pushed her left hand into her vagina. At
08:55 when she removed the hand, some more fluid (a smaller amount than before) came out of the
vagina. At this time she stopped pushing her hand into her vagina, but she placed/pressed her hand34 The Sonso community