It thus appears likely that we have a high population density of chimpanzees at Sonso.
However, there is an important proviso to any estimate of density from range size.
Often, and this is the case at Sonso, the full extent of the range is not known. It is very
hard in the forest to determine where the patrolled borders of the Sonso range are,
because the chimpanzees move silently on patrols. A. Plumptre (pers. comm.) stresses
the need to map several contiguous communities before calculating density, in order to
determine the full extent of their ranges. Taking this into consideration, it may be that
lower density figures are more accurate than higher ones, though at present this remains
uncertain.
Community size
The size of any community changes over time according to the number of births and
deaths, emigration and immigration. What is the size of the Sonso community?
Table 2.1 shows its size, by age–sex groups, over an eight-year period, beginning at
1996 when all individuals were known. The table shows a growing community with,
until recent years when some of our younger males have matured, a declining number
of adult males.
A detailed analysis of birth, death and migration, and how these affect the size of the
whole community over time, shows two things. First, the different age–sex classes did
not gain (through birth) and lose (through death) evenly. For seven years, until 1997, our
adult males seemed immortal; we had a good number of sturdy adult males, the core of
the community. Females came and went but the males remained. From 1997 we started
losing adult males: Chris last seen August 1997, Kikunku last seen July 1998, Zesta
killed (see Chapter 8) November 1998, Vernon last seen June 1999, Magosi found dead
July 1999, Muga last seen March 2000, Andy found dead July 2000. Seven adult males
lost over a three-year period. This was a heavy blow because adult males provide the
defence of the community. In four cases, Chris, Kikunku, Vernon and Muga, we do not
know what happened to them.^8 They may have died in snares or been killed by adult
males from other communities. None of them was sick at the time they disappeared. For
a while, during these three years, the Waibira community to the north was making raids
30 The Sonso community
(^8) It is possible that Vernon was speared while crop-raiding. This emerged in 2003, when our fine adult male
Jambo was speared while raiding a sugar cane field at Nyakafunjo. See Chapter 9 for details.
Table 2.1: Size of the Sonso community, broken down into sex–age classes, 1996–2003.
1996 1997 1998 1999 2000 2001 2002 2003
AM 15 15 14 13 11 10 13 16
AF 17 18 18 17 16 17 21 22
Juv 61212 6 7111211
Inf 13 8 9 13 14 11 9 10
Total 51 53 53 49 48 49 55 59
Key:AMadult males, AFadult females, Juvall juveniles, Infall infants.