Effects of snare injuries on feeding and social life 175Effects of snare injuries on feeding and social life
A number of studies have been made on the effects of snare and trap injuries on the life
of the Sonso chimpanzees.
First, feeding. Injuries reduce the ability of individuals to feed near the trunk of the
tree in certain species such as Ficus sur, because it is harder for them to hold on to the
trunk and large branches while feeding than it is for fully able-bodied chimpanzees
(Burch 1994; Smith 1995). This is important in the case of F. sur, because not only is
this a top-ranking food for chimpanzees but the majority of its fruits hang in pedicles
from the trunk and large branches of the tree. High ranking, able-bodied chimpanzees
tend to feed on the fruits hanging from the trunk and lower branches of F. surtrees, but
injured chimpanzees cannot feed there. Smith (1995) recorded the feeding height of 15
uninjured and 10 injured Sonso chimpanzees at three heights in F. surtrees: low (on
main trunk), intermediate (on large branches) or high (on small branches), over an
8-month period from February to October 1994. The mean height of each individual was
plotted relative to the mean for all individuals in the tree (the group mean). This showed
that uninjured chimpanzees fed significantly lower than injured ones (Fig. 9.6).
In some other species, such as Celtis gomphophylla, the injuries are less of a problem
because the fruits occur only on the outer and smaller branches of the tree, where injured
chimpanzees can support themselves more easily.
Smith (1995) found that the feeding rate (based on the number of fruits of F. sureaten
during 30-min sampling intervals) was the same for injured as for uninjured chim-
panzees. Nor was there a difference in the amount of time spent in visits to feeding trees
between injured and uninjured chimpanzees. As he points out, however, despite the
disadvantage of not being able to reach certain parts of some tree species, the injured
animals do not show a loss of condition such as poor coat quality or other signs of ill-
health. He suggests that this may be because the fission–fusion system of chimpanzees
allows an injured individual to choose to feed on less popular foods where competition
is less intense.
A study of the relative disadvantage of injury on chimpanzees’ feeding techniques
and the adaptations made by individual chimpanzees to their injuries is that of Emma
Stokes (Stokes 1999) who worked closely with Richard Byrne (Stokes and Byrne 2001).
From a 13-month study (August 1997 to September 1998) based on 22 able-bodied and
8 injured individuals, these authors showed that chimpanzees with injuries modify the
feeding techniques shown by able-bodied chimpanzees in their feeding behaviour, and
this enables them to work around their impairments.
Stokes and Byrne (2001) focused on the techniques used by the Sonso chimpanzees
when feeding on young leaves of Broussonetia papyrifera, a much preferred food. By
breaking down the feeding process into its component parts, they were able to distin-
guish 14 different feeding techniques, two of which were preferred by all individuals. In
this food species, leaf blades are eaten but the petioles are discarded. The two major dif-
ferences in technique they noted were (a) stripping leaves towards and into the mouth,
then discarding the petioles, and (b) detaching the petioles first, then stripping leaves