Ranges and ranging behaviour
The area over which the Sonso chimpanzees move has been studied by Newton-Fisher
(1997, 2003) who focused on the males, and Fawcett (2000) who focused on the
females. According to Newton-Fisher the community’s core area amounts to 6.89 km^2
of forest, which is small and indicates a food-rich area of forest, but this does not include
the wider area they use towards the outer borders of the Sonso territory. Fawcett reports
that the range utilized by the females falls entirely within the range of the males, female
range being about 80% of male range. This is very similar to the situation in Mahale
(Hasegawa 1990) and is in keeping with the idea that females are less inclined than
males to venture out towards the margins of the community’s territory, and that males do
so and include patrolling of community borders in their ranging activities (see also
Chapman and Wrangham 1993).
Fawcett observed patrolling behaviour by parties composed mainly of males and
oestrous females. ‘During a patrol the chimpanzees would silently leave a feeding tree
and move purposefully towards a boundary area, stopping occasionally as if to listen
and/or wait for other chimpanzees’ (Fawcett 2000: 173). In August 1999 at 07.30,
Dempsey (pers. comm.) observed the four senior Sonso males, Duane, Jambo, Maani
and Nkojo, to the north of camp. While observing them calls and drumming were heard
coming from the Waibira community’s range, some distance away to the north. At this
the males moved silently in a southerly direction, away from the calls. It may therefore
be that they had been patrolling to the north but on hearing calls from the Waibira direc-
tion they became afraid (at that time the Waibira community males had been making
sorties into the Sonso community’s range) and moved away. Once they were well inside
the Sonso range they paused and sat quietly for 13 min. Then, at 08.36, more calls and
drumming were heard from the north and this time they responded loudly with ‘aggres-
sive sounding calls and drumming’. They called again 2 min later after which they
moved on and joined a feeding party on some Broussonetiatrees.
Individual females were found to have extensively overlapping ranges, which were
approximately half the size of male ranges. Range overlap was greater than at Gombe
and more like the situation at Mahale (Hasegawa 1990: 111). Male ranges overlapped
with each other even more than those of females, and they also overlapped with female
ranges, so that the community forms an integrated bisexual whole. Females in oestrus
increased the size of their ranges to the same size as males. Females not in oestrus were
inclined to remain in their core areas to a greater extent than other females, especially
mothers with infants, and males. Also, female core areas overlapped less than male core
areas, in line with the ideas of Wrangham (1979); such core areas were no more than
15% of the community range. Why range overlaps should be greater at Sonso than in
some other areas (but not Taï, where male and female ranges are congruent) may be due
to the extensive food supply of the Sonso chimpanzees which reduces feeding competi-
tion; Fawcett (2000) draws attention to the importance of the exotic Broussonetiain
the food supply at Sonso, and Hasegawa (1990) writes that food supply may underlie
the overlap of ranges at Mahale.
Ranges and ranging behaviour 103