Just after dawn in the dense rainforest of Tanzania, a chimpanzee carefully inspects a few leaves on an Aspilia shrub (a relative of the sunflower family). She picks one leaf, folds it to and fro, concertina fashion, and swallows it whole without chewing. Then she fastidiously chooses another and repeats the folding and swallowing of several leaves. Nearby, there are a few folded leaves on the forest floor that have passed through her gut undigested.

When anthropologist Richard Wrangham of Harvard University first observed this leaf-swallowing behaviour in the early 1970s he was intrigued, for it obviously conferred no nutritional benefit to the chimpanzees. There were rumours among locals that chimpanzees used nature’s pharmacy to self-medicate their ills. Could leaf-swallowing be an amazing new discovery - the first example of chimpanzees gaining some medicinal or intoxicating effect from a plant’s bioactive defensive chemicals? If this were so, chimpanzees could potentially reveal valuable new drugs for human use. It was an exciting possibility.

While laboratory analyses of Aspilia leaves progressed through the 1980s, field scientists established that leaf-swallowing is also a habit gorillas and bonobos have across Africa and that between them the great apes swallow leaves from more than 37 species of plants containing a variety of bioactive chemical compounds. Disappointingly, no one could find a common medicinal function for leaf swallowing. What did emerge, however, was one common denominator: all the leaves had a rough, hairy surface. But why swallow folded hairy leaves?

In 1993 primatologist Michael Huffman of Kyoto University was in Tanzania’s Mahale mountains when he came across a fresh steaming deposit of swallowed leaves and found live nodule worms entrapped in the hairy folds. Further research determined that swallowed rough leaves act as mechanical scours that scrape intestinal nodule worms and tapeworms through the gut and bringing rapid relief from the gastrointestinal discomfort they cause.

Since then biologists have noticed other species using mechanical scours to rid themselves of uncomfortable intestinal parasites. North American brown bears scour out tapeworms before hibernation by feeding on coarse fibrous sedge, and Canadian snow geese clear out gut parasites by gorging on rough grass before their summer migration. Wild hunting dogs and tigers have been observed eating fibrous grasses shortly before shedding gut parasites and it is thought that the habit of grass chewing by domestic dogs and cats could be a remnant of this behaviour.

The benefits of scours are known to traditional herbalists who have long used indigestible material for scraping out intestinal parasites. A traditional Suffolk remedy for intestinal worms in horses was coarse hair - usually horse tail hair - mixed with honey. Scours are safer than chemical de-wormers (which by necessity must be toxic enough to harm parasites) and can therefore be a useful adjunct to veterinary medicine. Not only are they non-toxic but they also avoid contributing to the rapidly evolving drug-resistance seen in parasites around the world.

Wild animals use other strange materials as remedies, too. Nearly all vertebrates that feed on fruits, seeds and leaves, also eat clay - a behaviour known as geophagy (earth-eating). In Africa, elephants clear huge areas of forest to feed on the clay subsoils they need, and chimpanzees, rhinoceroses, giraffes, and buffalo gorge on the clay-rich soil of termite mounds.

In Peru, hundreds of parrots and macaws gather each day to feed on the narrow clay strata in eroding river banks, biting off and swallowing thumb-sized chunks of orange clay.

Up until recently it was assumed that mammals and birds ate earth to find essential minerals and trace elements but today clay consumption is understood to primarily be a form of self-medication. Clays can bind mycotoxins (fungal toxins), endotoxins (internal toxins secreted by pathogens), man-made toxic chemicals, bacteria and viruses; they also act as an antacid and absorb excess fluids, thereby curbing diarrhoea.

In 1999, the hypothesis that macaws eat clay to deactivate dietary poisons was tested by James Gilardi and a team of scientists in Davis, California.

First they established that seeds eaten by macaws contained toxic plant alkaloids. Then they fed one group of macaws a mixture of a harmless plant alkaloid (quinidine) plus clay. A second group of macaws were fed just the quinidine, without any clay. Several hours later the macaws that had eaten the quinidine with clay had 60 per cent less alkaloid in their blood than the control group, demonstrating that clay prevents the movement of plant alkaloids into the blood. What surprised the scientists was that clay remained in the macaws’ gut for more than 12 hours, meaning that a single bout of clay-eating could protect the birds for some time. Clay not only prevents plant toxins from getting into the blood, but also lines the gut and protects it from caustic chemical erosion. Again, humans have traditionally engaged in similar behaviour. Even modern humans use clay as medicine. Kaolin - a form of clay mined in Cornwall - is an important ingredient in the once popular digestive medicine “kaolin and morphine”. In America, Kaopectate is the commercial form of kaolin-based medicine sold in pharmacies for stomach upsets. In West Africa clay biscuits are still sold in markets and Australia’s Aborigines traditionally use clay as medicine for diarrhoea.

Toxins as medicines

Wild animals don’t restrict themselves to safe non-chemical remedies. In extreme circumstances they resort to highly toxic herbal medicines. Michael Huffman has noticed that chimpanzees in the Mahale mountains, suffering from diarrhoea, lethargy, weight loss and nematode infection, suck the bitter pith of Vernonia amygdalina - a shrub so poisonous that is called “goat killer” by local people. The pith contains numerous anthelmintic (combats intestinal worms), antiamoebic, antitumour and antibiotic compounds. Sick chimpanzees recover within 20 to 24 hours of chewing this pith (a similar time to local humans who use this same plant for similar symptoms). In one recorded case, the faecal worm-egg count dropped from 130 to 15 within a day of chewing bitter pith. They are careful to discard all but the inner pith (the outer parts kill browsing domestic goats unfamiliar with the forest flora). Wild chimpanzees have, therefore, not only found a medicine that will help them feel better quickly, but have learned to avoid the more deadly parts of the plant.

Innate wisdom?

Animals do not merely need to survive, they need to survive in as healthy a condition as possible to successfully compete with others and avoid predators. Therefore, the pressure to find successful health-enhancing strategies is strong. Animals need no conscious understanding of their actions (although the degree of awareness will vary according to the neurological complexity of each species). The individual merely needs to link an action with its consequence and make an association - something even an insect can do.

Self-medication is rewarded via hedonic feedback - that is, the removal of uncomfortable sensations. Lab-oratory experiments show that rodents will attempt to reduce feelings of anxiety using whatever is available. In one example, a group of mice received electric shocks to the feet (physical stress) while the other group was forced to witness another mouse getting a foot shock (emotional stress). Both groups of mice had free access to morphine (morphine induces an indifference to pain rather than blocking the sensation), but only the mice exposed to emotional stress self-administered the morphine. A similar effect is seen in emotionally stressed rats and cocaine self-administration.

In another example, stressed rats learned to self-administer strobe lighting at certain frequencies that changed electrical activity in the brain, thereby calming their heart rhythms and lowering blood pressure. The rats had ingeniously calmed themselves down and, coincidentally, reduced the long-term likelihood of heart attacks. A feeling of anxiety is clearly unpleasant and it is surely the animal’s desire to feel better that motivates remedial action.

Similarly, attempts are made to self-medicate pain. Broiler chickens have been artificially selected to grow extremely quickly, turning food into meat at the expense of bone growth. Their legs, therefore, are not strong enough to support their weight and they frequently suffer from broken legs.

Lame birds go off their food and remain immobile, unwilling to even walk to the water trough. However, experiments at Bristol University show that at only four weeks old, lame birds rapidly learn to select appropriate amounts of the pain-killing analgesic carprofen, given the opportunity. Although the bird’s leg bones are not healed by the medicine, their immediate somatic sensation - pain - is remedied.

Helping others

An animal’s use of nature’s pharmacy is not always about self-medication.

Sometimes one individual helps another, often closely related animal.

During hatching, male European starlings bring a selection of aromatic herbs to the nest. In North America, they preferentially select wild carrot, yarrow, agrimony, elm-leaved and rough golden rod and fleabane, even when they are not the most common plants nearby. These herbs contain more volatile oils in greater concentrations than aromatic plants that are not selected.

Woven into the nest matrix, the pungent herbs are kept topped up while there are chicks in the nest. The chicks don’t eat or actively rub against these pungent herbs, yet they have a significantly greater chance of surviving into the next season with the herbs than without them. It seems that the volatile oils within the leaves (such as myrcene, pinene, and limonene) that are harmful to bacteria, mites, and lice in the laboratory, act as fumigants against skin pests. If herbs are removed from nests, chicks become infested with more mites. The presence of wild carrot, in particular, produces chicks with higher haematocrit (the percentage of blood carrying red blood cells), suggesting that they are losing less blood to blood-sucking mites.

We humans consider ourselves unique among mammals at nursing and doctoring others, yet African elephants show an array of helpful actions. They protect their young - and occasionally even those of other species - from predators. They pull out hunters’ spears from the hides of wounded herd members and if an elephant is grounded, herd members surround the injured elephant forming a protective, impenetrable circle. If there is blood showing, they pack the wound with clay to quell blood loss. And when an elephant dies, others cover the body with leaves, earth and twigs.

Cindy Engel is author of Wild Health: how animals keep themselves well and what we can learn from them (Phoenix paperback, pound;8.99).