Rough and ready
A third instance of animal self-medication is the use of mechanical scours to get rid of gut parasites. In 1972 Richard Wrangham, a researcher at the Gombe Stream Reserve in Tanzania, noticed that chimpanzees were eating the leaves of a tree called Aspilia. The chimps chose the leaves carefully by testing them in their mouths. Having chosen a leaf, a chimp would fold it into a fan and swallow it. Some of the chimps were noticed wrinkling their noses as they swallowed these leaves, suggesting the experience was unpleasant. Later, undigested leaves were found on the forest floor.
Dr Wrangham rightly guessed that the leaves had a medicinal purpose—this was, indeed, one of the earliest interpretations of a behaviour pattern as self-medication. However, he guessed wrong about what the mechanism was. His (and everybody else's) assumption was that Aspilia contained a drug, and this sparked more than two decades of phytochemical research to try to find out what chemical the chimps were after. But by the 1990s, chimps across Africa had been seen swallowing the leaves of 19 different species that seemed to have few suitable chemicals in common. The drug hypothesis was looking more and more dubious.
It was Dr Huffman who got to the bottom of the problem. He did so by watching what came out of the chimps, rather than concentrating on what went in. He found that the egested leaves were full of intestinal worms. The factor common to all 19 species of leaves swallowed by the chimps was that they were covered with microscopic hooks. These caught the worms and dragged them from their lodgings.
It turns out that other animals exploit rough plants, too. In the months leading up to hibernation, Alaskan brown bears leave large dung masses full of tapeworms and fibrous sharp-edged sedges. Canadian snow geese, just before they migrate, deposit huge quantities of undigested grass and tapeworms. Wolves eat grass to scour their guts of roundworms. And, according to Indian folklore, tigers, too, occasionally eat grass, probably for similar reasons.
On top of all this, just as infested animals try to cure themselves, healthy ones take precautions. According to Dr Engel, these precautions include sheep refusing to graze on freshly manured grass (the manure contains pathogens and parasites) and cattle refusing to nibble on fresh bones that might contain flesh (they will gnaw on weathered ones to obtain minerals). Of course, the cattle fed on bone meal in Britain in the early 1980s, which led to an epidemic of bovine spongiform encephalopathy (“mad-cow” disease), did not have the option of refusing.
Following that observation, Dr Engel is now particularly excited about how knowledge of the way that animals look after themselves could be used to improve the health of livestock. People might also be able to learn a thing or two—and may, indeed, already have done so. Geophagy, for example, is a common behaviour in many parts of the world. The medical stalls in African markets frequently sell tablets made of different sorts of clays, appropriate to different medical conditions.
Africans brought to the Americas as slaves continued this tradition, which gave their owners one more excuse to affect to despise them. Yet, as Dr Engel points out, Rwandan mountain gorillas eat a type of clay rather similar to kaolinite—the main ingredient of many patent medicines sold over the counter in the West for digestive complaints. Dirt can sometimes be good for you, and to be “as sick as a parrot” may, after all, be a state to be desired.
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