Food availability 65ability to secrete protein-rich mucins from the mucosal surfaces of the mouth, which go
into their saliva and serve to neutralize tannins. For example, children who regularly drink
red wine (even diluted) develop salivary mucins and are more readily able to tolerate red
wine in later life. In the UK we don’t normally give wine to children but we drink it in
adult life, developing salivary mucins as a consequence. We may find wine a bit astringent
at first, especially dry wine with low sugar content, but in the end we come to neutralize
the tannins, and to tolerate and like such wine. Another example is tea and coffee, both
are rich in tannins. If we don’t like the tannins we neutralize them with milk, which is
rich in proteins, and we can go one step further and add sugar to make them sweet.^21
So to some extent chimpanzees may develop a tolerance to the tannin-rich figs they eat
almost daily in Budongo. It seems likely that they produce salivary protein-rich mucins, as
we do. But the high levels of tannins they eat may also reflect a genetic factor. Assuming
they have been eating figs and other tannin-rich foods over many thousands, even
millions of years, this is to be expected. However, while they can tolerate the high level
of tannins found in figs, they do reduce the number of tannin-rich seeds they swallow.
To do this they make ‘wadges’ — a wadge is a bolus of compacted fig seeds that is kept
in the mouth while the flesh of the figs is masticated and the juice is swallowed, after
which the wadge is spat out. Thus we see that chimpanzees can tolerate tannins in the
mouth but make an effort to avoid swallowing this antifeedant secondary compound.
Returning to Broussonetia, we found that the much favoured young leaves, flowers
and fruits of this species were characterized by high sugar levels and a complete absence
of condensed tannins. This absence helps to explain why this species is so readily eaten.
Food availability
Availability of food is the combination of the amount — abundance — and the spatial
arrangement — dispersion — of food within a given location at a given time. Over time
food availability varies according to what the various food species are doing — whether
shedding their leaves or growing new young ones, or flowering or fruiting. Some tree
species are highly seasonal, others not at all. Examples of important seasonally fruiting
food trees in Budongo are Cynometra alexandri,Celtisspp.,Pseudospondias micro-
carpaandMaesopsis eminii. Non-seasonal species include the figs. The list of foods of
chimpanzees in Appendix B shows the months in which these species have been seen
providing food.
Plumptreet al.(1997) recorded the diet of the Sonso chimpanzees between
December 1994 and December 1996. As shown in Fig. 4.4, this study showed that ripe
fruit formed over 50% of the diet for much of the time except during the December–
February dry season. In most dry seasons the chimpanzees feed heavily on the seeds of
Cynometra alexandri, as they did in 1995–96. However, in the corresponding period for
1994–95C. alexandrifailed to fruit and more unripe fruit was consumed in this period
(^21) I am indebted to the late Jeffrey Harborne for this information. Little is known about chimpanzee mucins
and in particular no work has been done on their possible role in neutralizing tannins so this must remain
speculative. I am grateful to Tony Corfield (pers. comm.) for help with this topic.