62 Diet and culture at Sonso
Factors underlying food preferences: sugars and tannins
Chimpanzees eat some species of fruits and leaves frequently, some occasionally and
some never. In the early 1990s we decided to investigate what factors were common to
the most eaten species and not to the less eaten or avoided ones. We also wanted
to investigate this with respect to fruits on a single tree: normally the ripe ones were
preferred but sometimes unripe fruits were eaten. What were the factors involved?
We had one clue to go on from the start: many fruit-eating species (ourselves included)
prefer ripe fruits because we find them sweeter. So one thing to measure in the eaten and
not-eaten fruits was sugar content (fructose, sucrose, glucose and galactose). But there
was an interesting problem when it came to figs. Ripe figs as well as unripe ones of the
species eaten by chimpanzees often taste horrible to humans — full of latex, they are
astringent to the point where we have to spit them out. This astringency is found in many
fruits when they are unripe, and declines as the fruits ripen, while the sugar content, at
first low, rises so that the ripe fruits lose their astringent taste and become sweet tasting.
The reason is that many species (including figs) depend on animals eating them to dis-
perse their seeds far from the parent tree where they have a better chance of survival. As
a result the fruits that contain seeds become fleshy and attractive to eat, but not immedi-
ately: the edibility quotient of the fleshy outer part is closely geared to the development
of the seeds inside. Natural selection has favoured fruits that evolved mechanisms to
ensure they did not get eaten when unripe with underdeveloped seeds that would fail to
germinate when excreted, and it has favoured mechanisms that attract animals to eat
them when the seeds are ripe and ready to germinate. Seeds of some species are crushed
when eaten but rapid eaters like chimpanzees swallow many seeds intact. In some tree
species (e.g. Maesopsis) the seeds are too hard to be crushed and come through in the
faeces intact. Such seeds have the advantage of being deposited away from the parent
tree and away from all but a few competitors of their own species.
Do seeds that have passed through the gut of chimpanzees germinate faster or survive
better than those that have not? This and other aspects of seed dispersal by primates was
studied by Chapman (1995) who concluded ‘evidence suggests that the seeds primates
disperse that are not found by rodents or secondarily dispersed by dung beetles probably
are capable of germination if the conditions are right. When researchers take seeds from
primate dung and attempt to germinate them in controlled settings, evidence typically
suggests that the passage through the frugivore gut has improved the rate of germination
and reduced latency to germination’ (p. 77). A study at Sonso of the germination of tree
seeds taken from chimpanzee dung vs. seeds picked up on the forest floor found better
germination in the former (Plumptre et al. 1997, pp. 76–7). A study of Uvariopsisseeds
at Kaniyo Pabidi in the northeast of Budongo Forest had the same result: 160 plots were
studied, each plot being planted with 10 seeds. Half of the seeds in each plot were taken
from chimpanzee dung in the forest, the other half from seeds found under the parent
trees. There was higher germination of seeds that had passed through the chimpanzee
gut (Muhumuza, pers. comm., based on Plumptre, unpublished data).