Our primate cousins vary their diet depending on their state of health.
TO THE GREEK MIND, barbarians were people who did not change their diet when they were sick. At the Laboratoire d’Écologie Générale of the Museum in Brunoy, Claude Marcel Hladik and his colleagues demonstrated that under certain circumstances monkeys eat earth, or plants containing alkaloids, in order to preserve a balanced diet—even to treat intestinal disorders. In this they are close relatives of the civilized world.
The reaction of primates to sweet solutions is a striking feature of mammalian evolutionary adaptation: The larger the animal, the more efficiently it detects sugars. Large animals, better equipped to recognize sweet foods, are able to acquire more energy for themselves. An exception to this rule, which holds as much for fruit-eating animals as for herbivores, is the loris Nycticebus coucang, a prosimian that is insensitive to sucrose (ordinary sugar). This may be because it must tolerate the bitter taste of insects in addition to various prey that other monkeys do not feed upon.
A Normal Primate
Regarded in terms of their ability to perceive sweet products, humans are normal primates: Our body mass is large, and we are very sensitive to sweet tastes. Nonetheless, this biological basis is modulated by environmental factors. For example, perceptual thresholds for glucose and sucrose differ between the inhabitants of tropical forests and those of grasslands. Thus the Pygmies, who occupy forests where sugar-rich fruits are common, have a less developed sensitivity to sweet tastes than that of peoples who live in savannas, where plants contain less sugar.
Tests of species sensitivity to sweetening agents have brought out surprising differences. For our species, monellin (a protein present in the red berries of the African shrub Dioscoreophyllum cumminsii) is 100,000 times sweeter than sucrose. Nonetheless, although the taste of this protein is identified by African nonhuman primates, it is not perceived by American primates. The same difference is observed in the case of thaumatin, a protein sweetener extracted from the fruit of another African plant, Thaumatoccus danielli. It may also be encountered in the case of brazzein, identified in 1994 in the creeper Pentadiplandra brazzeana.
Differentiation of receptor proteins in the papillary cells of the tongue probably occurred 30 million years ago, after the separation of the New World Platyrrhina and the Old World Catarrhina. In their respective environments these animals found various plants with which they evolved in tandem, eating the fruits of these plants while dispersing their seeds. In the Americas, where no protein sweetener has yet been found, coevolution should have caused new molecules to appear that would not have seemed sweet to Old World monkeys.
It has been known for several decades that vertebrates are able to detect sodium chloride and actively seek it in case of insufficiency. For example, horses lick salt deposits only if they have to, an observation confirmed by the study of salt-deprived rats. In natural environments (particularly forests) salt deficiency is rare, but in 1978 the American biologist John Oates observed that Colobus guereza, a shy monkey that seldom ventures out of its normal tree habitat, comes down to the ground to eat the leaves of the plant Hydrocotyle ranunculoides, which grows in ponds and contains more salt than other available sources.
Natural Medicine
Other primates eat earthy matter even though they do not suffer from salt deficiency. The soil eaten by Colobus satanas (another colobus monkey that lives in the forests of Gabon) contains less salt than the fruits that make up an important part of its normal diet, but this behavior occurs during the two periods of the year when the animal must supplement its diet with mature leaves (feeding the rest of the year on young shoots and leaves in addition to flowers, fruits, and grains). These older leaves contain not only molecules of the polyphenol family (hydroxyl [–OH] groups that attach to benzene rings having six carbon atoms) but also tannins, which inhibit the digestion of proteins by forming complexes with them. Because clay and other soil compounds readily absorb tannins, the geophagy of these monkeys can be explained as a way of compensating for the ingestion of unwanted plant products. “From your food you shall make your medicine,” Hippocrates is credited with saying. Could it be that our primate ancestors whispered this phrase in his ear?
Often an aversion to bitter tastes favors the avoidance not only of dangerous alkaloids but also of astringent compounds such as tannins, terpenes, saponins, and strong acids. Nonetheless, not all toxic compounds are bitter: Dioscin, a lethal alkaloid found in the yam Dioscorea dumetorum, is almost tasteless. The animal kingdom is protected by the phenomenon of neophobia—the fear of eating what is new—and by conditioning from postingestive symptoms that trigger the appearance of an aversion (observed in rats and primates alike). Even so, chimpanzees are known to heal themselves by eating the bitter plant Vernonia amygdalina, generally avoided by healthy animals. This plant contains several steroidal glycosides that are effective in treating gastrointestinal troubles. What shall we call this type of behavior? Natural medicine?