Soil-eating 43Parasite control is achieved not only by the physical action of leaves being swallowed
whole, however. As Huffman points out, some of the plants used by chimpanzees in this
way, in particular species of the genus Vernonia, are also used by human beings and for
their livestock as medicinal treatment for such illnesses as malaria, dysentery, intestinal
worms and general stomach upsets. Some people also cook parts of the plant as a tonic
food which enhances appetite and gives strength (Huffman 2001). Likewise, a female
chimpanzee at Mahale observed by Huffman with symptoms of diarrhoea and a heavy
parasite load, ate the pith ofVernonia amygdalina after which the output of
Oesophagostomum eggs in her faeces declined over a 20-h period (Huffman et al.
1993). Steroid glucosides and sesquiterpene lactones have been found in the leaf, stem,
pith and roots of Vernonia amygdalina(Huffman 2001) and have been demonstrated to
be among the compounds that are medicinally active in this species.
One other plant deserves a mention in the present context:Khaya anthotheca, the
Budongo mahogany. From time to time, at irregular intervals, chimpanzees avidly
remove flakes of the bark of this tree and scrape the inner side of the flakes with their
incisor teeth. Reynolds et al. (1998) found the bark to be very high in condensed tannins,
with a bitter or astringent taste. However, a clear medicinal role for this bark, which is
also used by human populations to control disease, has not been demonstrated in the way
it has for Aneilema. Possibly its high antioxidant activity serves to scavenge for free rad-
icals in the foods eaten (Greenham, pers. comm.). Our studies of Khaya-eating have also
led us to study the gum that is present under and around flakes of bark, which we have
found to have a high level of polysaccharide. We are currently examining whether this
may be attractive to chimpanzees. Such gum may be the tree’s response to attack by
insects laying eggs under its bark. This has been found to lead to the production of gum in
Acaciatrees, and this gum provides an important part of the diet of galagos (Bearder and
Martin 1980). According to T. Wagner (pers. comm.) many species of beetles use dam-
aged areas of forest trees for nutritional purposes, feeding on phloem sap, while some,
e.g. Nitidulidae, lay eggs on the ‘wound’ and their larvae feed on the rotting cambium
between bark and wood. This may be of interest to chimpanzees.
Soil-eating
The Sonso chimpanzees eat termites and at other times they eat the soil of Cubitermes
termite mounds. Fifteen cases of geophagy by 12 individuals (4 females, 8 males)
have been observed to date. Chimpanzees used their hands and teeth to break open
the mounds of Cubitermes speciosus, including both active and inactive mounds (see
Fig. 3.4). In 60% of cases, termites were eaten along with the soil. This activity is not
a common one, frequency of observation being 0.79 instances per 100 h of observation.
The peak time for this behaviour to occur was between 0900 and 1000 h.
It is likely, though not as yet proven, that this is done for medicinal reasons (as a preven-
tive or as a cure) but we do not currently know. Parasite studies by Huffman and Pebsworth
(pers. comm.) showed that all individuals involved in termite-soil-eating were infected
by at least two nematodes,Oesophagostomumsp. and Strongyloides fullebornii, and