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

sessions recorded at Sonso over a five-year period (1995–2000) shows the number of
occasions on which males and females groomed each other (see Table 6.1; the analysis
is based on adults only so as to exclude mother–offspring pairs). In each year, in the
community, there were more adult females than adult males (see Table 2.1).
As can be seen, males groomed one another more than six times as often as females
groomed each other, and they groomed females more than females did. Females
groomed males five times less often than males did. Females, in fact, groomed less than
a quarter as much as males. However, they should not be thought of as rarely interact-
ing, as seems to be the case at some sites (Gombe: Goodall 1986; Kibale: Wrangham
et al.1992; Mahale: Nishida 1989); as we saw in the previous chapter, females do often
form all-female groups consisting of mothers, infants and childless females (Bates
2001; Munn 2003).
Arnold and Whiten (2003) also studied grooming among the Sonso chimpanzees.
They found that adult males had an average of 13 grooming partners of either sex of
which 7.5 were other adult males. Adolescent males typically had 7 grooming partners,
but adult females averaged only 2 grooming partners, of which only 0.17 were other
adult females. Both sexes groomed males most frequently. After correcting for the num-
ber of available partners in each age–sex class the same pattern emerged. Grooming
tended to be directed up the hierarchy, with individuals ranking close to each other
grooming more than those less close. Higher ranking males had more grooming partners
than lower ranking ones.
Looking at the above from the viewpoint of cognitive psychology (Chadwick-Jones
1998), it looks as if males are more interested in, or stand to gain more from, grooming
exchanges than females do. Why should this be so? In some chimpanzee populations
where hunting is common, grooming can be seen as a mechanism for male bonding that
then expresses itself in co-operative hunting. But at Sonso hunting, while it occurs, is
infrequent. A more important reason for co-operation among males is territorial
defence, for the Sonso range is juxtaposed on three sides by other communities,
the males of which appear to be hostile to the Sonso males. The bonds established by
grooming may be important in achieving good co-ordination between Sonso males in
the event of territorial disputes. Females do not have any clear-cut co-operative role to
match that of males.
However, as we have seen, males do not just groom any other males, they have
specific grooming partners. Here the psychological explanation is likely to be linked

Grooming and other affiliative behaviours 113

Table 6.1:Grooming sessions between Sonso adults,

both sexes, 1995–2000.

Groomer AM Groomer AF Total

Groomee AM 1927 362 2289

Groomee AF 534 285 819

Total 2461 647 3108

Key:AMadult male; AFadult female.