Chausiki, a wild chimpanzee in the Mahale Mountains of Tanzania, was sick. She dozed lethargically while those around her fed. Her urine was dark, her stools were loose, her back was visibly stiff, and she ignored her whimpering young son. She then sought out a little shrub, Vernonia amygdalina, the ‘bitter-leaf’ or ‘goat-killer’. Chausiki stripped away the highly toxic outer layers of its shoots. For twenty minutes she chewed and sucked the more mildly poisonous inner pith. The son tried to copy his mother, but he spat out the bitter plant in obvious disgust. A day later, Chausiki was cured. She travelled swiftly ahead of her group, and fed as usual.
Two scientists were watching her: Mohamedi Seifu Kalunde, a Tanzanian traditional herbalist, and Michael Huffman, an American biologist. The herbalist informed his companion that the bitter-leaf plant is a strong, broad-spectrum medicine used against malaria, schistosomiasis, amoebic dysentery and intestinal worms. Later chemical analysis showed that bitter-leaf pith contains at least ten different compounds active against parasites.
Huffman and Seifu’s account of Chausiki’s cure was one of the first scientific reports of animal self-medication. Cindy Engel tells the story well in Wild Health, a wonderful collection of tales about the ways in which animals prevent and cure ill-health. A general in Han Dynasty China noticed that sick horses gained vigour from eating Plantago asiatica, and fed it with good effect to his sick soldiers. Modern laboratory studies have found that poisoned rats turn to eating clay, and rats in pain will choose food laced with painkillers. Engel develops the argument that wild animals maximise their chances of good health when they live in rich natural environments where they are free to direct their own behaviour.
Engel is not a sentimentalist. She notes the widespread belief that wild creatures – and house cats – go off to die in stoic solitude, but points out that if an animal is sick and weak, the safest thing it can do is to hide from predators and pushy rivals of its own species. It goes off alone not to die, but to give itself the best chance of living. Similarly, she doesn’t believe that animals have a mystic ability to identify specific cures for specifically diagnosed diseases. They choose foods that cure the symptoms of disease. Our folk feeling that medicine has to be bitter to do any good may derive from the fact that natural cures often taste of tannins or alkaloids. A chimp with a stomach ache may seek out a broad-spectrum antidote like the bitter-leaf. Or it may swallow whole leaves from several different kinds of hairy plants: leaves with ‘Velcro’ hooks help clear out intestinal worms. Bears before hibernation and snow geese before migration also turn to Velcro plants. Domestic dogs and cats chew grass, which has the same scouring effect. Many wild animals, and some people, develop ‘pica’ when ill, a craving to eat earth – particularly clay, which assuages diarrhoea and binds to many plant poisons. Among the most famous clay-eaters are the parrots of the Amazon. Scarlet macaws, blue and gold macaws, and hosts of smaller birds perch together in their hundreds to excavate the best clay layer along a riverbank. Parrots’ regular diet is tree seeds, which the trees defend with toxic chemicals, and clay is an essential buffer to the toxins.
Engel’s book has lessons for the new field of ‘conservation medicine’ – veterinary science applied to wild populations. One reason for taking up conservation medicine is to save wild creatures threatened by shrinking habitat and contact with humans – the Baltic seals and Serengeti wild dogs felled by canine distemper, or wild gorillas that catch pneumonia from tourists. Another is to save ourselves and our livestock from cross-species plagues such as rinderpest, Aids and Ebola. A third is to understand disease from the point of view of the evolved and active parasite, so that we can estimate how epidemics are born and transmitted, especially in those cases where a parasite can jump between wild and domestic species. Practical policy – what we did with the last foot and mouth epidemic, what we will do with the next one – depends on understanding cross-species disease transmission.
But the book itself is more concerned with what animals themselves eat, seek and avoid. Engel turns common medical attitudes inside out, suggesting that, in many ways, the animal (and this goes for people, too) that responds to its own feelings of health or sickness has the right end of the stick. Infection by pathogenic parasites may be a symptom of ill health, rather than the simple or single cause. We are all surrounded by zillions of bacteria, fungi, worms, insects and predators out looking for an easy meal. Each host species carries many parasites specialised in attacking that species alone, and many more that can be shared with others. There are more species of parasite than there are host species. Parasitism is the norm for life on this planet, not the exception. Nonetheless, a healthy animal host usually resists severe infection. It is the already debilitated animal, or the one living in an exceptionally infectious environment, that succumbs.
In a remote field site in the Central African Republic, human and non-human primates carry many kinds of intestinal worms as well as dysentery-causing amoebae. Parasites infest not only the faeces of wild gorillas and chimpanzees and mangabey monkeys, but those of BaAka pygmies and Western researchers. According to a recent study by the parasitologist Alecia Lilly, especially high concentrations appear in mangabeys, which pick through elephant and gorilla faeces for undigested seeds, and in the BaAka people, who live in crowded villages. Ill health, in other words, appeared to be the result of unhealthy food or poor sanitation. Yet the highest load of all was in a lone silverback gorilla male, roaming on his own and suffering the stress of social exclusion. It doesn’t seem to matter, then, whether the social or the physical environment is more at issue: when defences are insufficient to the task, the parasite population becomes uncontrollable, and the plant or animal gets sick and may die. In death, the rot really sets in, as other species take advantage of the now undefended nutrients.
Plants and animals have evolved myriad strategies for defending themselves against the continuous onslaught. Those that cannot do so efficiently become extinct. Animals have coherent, self-repairing skin, closing, mucus-lined orifices, complex immune systems, and behavioural dodges to avoid being eaten alive. Plants have bark and hairs and, above all, toxins. Fungal toxins range from antibiotics that repel and kill predatory bacteria, to poisons strong enough to kill off mushroom-collecting gourmets. No surprise, then, that small doses of some of these bio-active substances are effective as medicines. If plants use them against their own micro and macro-parasites, animals can borrow the effects to medicate themselves.
Proving that animal self-medication takes place in the wild is, of course, a tall order. To provide rigorous evidence of self-medication a zoologist would have to be able to establish what constitutes a normal diet, spot when particular animals were suffering from particular infections, identify which new plants were being consumed, show that the animal got better as a result, and ideally, confirm the efficacy of the plant against the disease agent in question. Multiple observations, showing significant correlations between consumption of plants with active constituents and actual healing, would clinch the argument – a near impossibility for a field zoologist.
Though the zoological and epidemiological evidence is very thin, Engel’s book convincingly demonstrates that animal self-medication is plausible, and the more we look, the more examples we are likely to find. In much the same way that water-seeking is driven by the discomfort of thirst, so the discomfort of sickness probably leads to the search for a remedy. But how does an animal know which substances to use? If we assume a lack of medical insight, animals might learn by trial and error, by watching what others do, or by being born with pre-programmed behavioural routines. The evidence that might enable us to distinguish these mechanisms is thin, but a proper understanding of such behaviours might shed light on the tangled issue of human behavioural motivation.
Aside from the practical difficulties of studying the epidemiology of disease and healing in wild animal populations, there is the problem of what constitutes medication. The line dividing medicine from food is blurred. Missing a key nutrient, an animal may fall sick, then appear to be cured when the missing compound is administered. Endless epidemiological studies show associations between the human consumption of carrots, green vegetables, fruit, cereals and wine, and reduced rates of cancer, circulatory problems and infectious disease. In the Stone Age, human beings ate more than a hundred species of plants; a modern urban diet typically consists of fewer than twenty. Skeletal remains suggest that in shifting from a hunter-gatherer to an agrarian lifestyle, people became stunted and undernourished. It may be that the modern urban diet leaves out some of the dietary constituents on which we evolved, and that increased consumption of fruit and vegetables helps mimic the Stone Age diet to which we are adapted. It is even possible that the urge to consume bitter and astringent compounds such as tea, coffee and chilli is an ancient drive to eat foods that help combat our many parasites.
Engel’s suggestion is that we would have much to gain from studying animals’ health-related behaviour. Animals occasionally indulge in recreational drugs – they get drunk, sedate themselves into a stupor, and eagerly consume stimulants. Nature’s pharmacy provides many intoxicants. They work by mimicking the action of neurological chemicals, and plants produce them because they defend against herbivorous predators. Getting high ought to reduce an animal’s adaptive fitness; falling out of trees and stumbling away from predators is not the best way to get your genes into the next generation. One proposed explanation is that animal alcoholics are after the high calorie nourishment of ethanol. A better theory is that drugs are a short circuit to the pleasure centres. Pleasure is usually a reward for behaviour patterns that are good for survival and reproduction. By taking a psychoactive drug, animals (and people) skip the hard work of getting food, getting resources or getting laid, and get the pleasure pay-off directly. Because psychoactive chemicals are rare in the wild and come in small doses, the casualty rate in spaced-out animals hasn’t been high enough for it to be a factor in selection. Human technology has given us artificially purified drugs, often attractively packaged and marketed for anyone who wants them. ‘The social brake of legislation seems powerless against the unhealthy combination of natural desire and technological know-how,’ Engel remarks, ruefully.
Animals don’t always do the healthy thing. There are wild bears in the Yosemite National Park that scavenge waste dumps and become obese, often twice their normal weight. Baboons, too, jump (or rather don’t) at the opportunity to become couch potatoes. In the Serengeti reserve they can lie about all morning till the waste trucks from the hotels arrive, binge on high-sugar, high-fat, high-protein leftovers, and relax all afternoon. Over the years the baboons have got fatter, reached puberty earlier and acquired higher levels of cholesterol and insulin. The lesson is, of course, that when an animal is removed from the environment in which it evolved, drives and desires that were once adaptive can become highly maladaptive – a lesson it has taken people a long time to understand.