Factors underlying food preferences: sugars and tannins 63While trees (and other fruit-producing plants) have been evolving mechanisms to
ensure that animals would eat them at the appropriate time and not before, animals have
evolved taste preferences for sweet-tasting foods because the sugars they contain are a
rapidly available source of energy. Thus natural selection has favoured animals evolving
a liking for sugars. A second evolutionary development has been an aversion to tannins.
Tannins are sometimes called antifeedants and have anti-digestive properties which they
achieve by binding to proteins in the diet of the animal and making them inedible. Too
much tannin in the gut leaves an animal unable to digest its food properly and in the case
of some species can have serious consequences up to and including death. Thus natural
selection has favoured animals that avoid tannin-rich foods.
However, the ripe fruits of F. surand other fig species remain astringent even when
ripe and this poses a problem. Why should they remain astringent when ripe? Do they
contain high levels of tannins? There were clues in the literature that they did: a study
by Wrangham et al. (1993) of the chimpanzees of Kibale Forest had found high levels
of tannins in figs, particularly in the seeds.
I approached Jeffrey Harborne^20 of Reading University with this problem, and over
the next several years, together with Jenny Greenham, we investigated it. It was during
this investigation that I realized just how complicated the relationship between plants
and animals is. I make no claim that we eventually solved the question we set ourselves,
but at least we made some progress.
We decided the critical test would be to take ‘fruits seen eaten’ and compare them
phytochemically with ‘fruits not eaten’, ‘fruits rejected’ and ‘fruits left on the tree’. Here,
immediately, we hit the first set of problems: at the point of collection. If a fruit is eaten,
how can it then be collected for analysis? And if a fruit is not eaten but left on the
tree, how can you then collect it? Fortunately for us, chimpanzees are wasteful feeders,
especially when it comes to figs. When they reach out and break off or pull in a branch of
figs they often drop one or more pedicles or single fruits to the ground. There are enough
such accidents to make collection of ‘fruits seen eaten’ (i.e. fruits that would have been
eaten if they had not been accidentally dropped) a fairly simple process. On the second
question, pedicles of F. surfruits contain both riper and less ripe fruits. If, having broken
off a pedicle and eaten the fruits that were wanted, the chimpanzee then dropped the pedi-
cle apparently on purpose and the ‘rejected’ fruits fell to the ground, we collected them.
We repeated similar procedures, adjusted to the particular tree and species, for all the
other fig species, and for as many other species as we could, over a three-year period,
always distinguishing between fruits eaten and fruits not eaten. Every fruit was taken to
our drying area at camp and carefully dried, marked with the species and date, and when
enough samples were dry they were packaged up and sent to the Reading laboratory for
analysis.
We undertook two main analyses: for sugar content (measuring fructose, sucrose,
glucose and galactose) and for proanthocyaninins (condensed tannins). The results were
as shown in Table 4.4 and Fig. 4.3.
(^20) Sadly, Prof. Jeffrey Harborne died on 21 July 2002, during the writing of this book.