Animal Self-medication—an untapped resource?
[This article was first published in The Ecologist]
To avoid disease, plants produce a wealth of antibacterial, antifungal, and antiviral compounds. To avoid being eaten, they produce physical and chemical defences such as barbs, spines, bitter-tasting alkaloids, astringent tannins, or mouth-numbing narcotics. Plant eaters though have found numerous ways to avoid and disarm these defences resulting in a continuous co-evolution of plant defences and animal counter strategies.
Herbalists have long known that many of these same plant defences make excellent medicines. Compounds designed to be toxic or repellent to microbes for example can be used analogously in medicine. Alternatively dose related effects mean that a toxin is not always a toxin; just as two Paracetamol can relieve pain while 200 can kill. Finally, what is toxic to one species can be beneficial to another.
However, the idea that animals might self-medicate has been dismissed by scientists as romantically anthropomorphic. Currently though scientists are discovering that animals have found ways to use plant defences to their own advantage, not just by using the defensive chemical against their own predators but by using them against infection, and—most controversially of all—as health modulating ‘medicines’.
Nature’s potential pharmacy is vast, incorporating not just chemical ‘drugs’ but physical substrates with beneficial attributes too. Plants, fungi, insects—even earth provide a seemingly ubiquitous supply of potential medicines.
Survival has an aspect of quality about it, in that, an animal has to survive in as healthy a condition as possible in order to successfully compete with others and avoid predators. The result is a world full of animals actively managing their health as each individual attempts to avoid, prevent and treat ill health—an image contrasting strongly with the commonly held view of animals passively enduring the ravages of pathogens, poisons, and injuries that come their way.
Behavioural strategies have evolved for avoiding, preventing and curing injury, disease, parasite infection, poisoning, stress and pain; not due to any supernatural ‘animal wisdom’ rather to selective pressure. Animals need no conscious understanding of their pharmacy or self-medication (although the degree of awareness no doubt increases with increasing neurological complexity). They merely need to link action with consequence; to feel the effects of their actions and make an association—something even the simplest organism can do.
Insects, for example, make good use of a plant’s own defences against fungal and bacterial attack. Gall wasps choose to lay their eggs near those oak leaves which have the highest tannin concentrations, gaining protection from infection for their own emerging larvae[i]. Similarly, when egg-laying, the spotted cucumber beetle leaves its host corn plant and seeks out squash, gourd, or cucumber plants to harvest their bitter defensive compounds, cucurbitacins, which it passes to its own eggs. The eggs and hatchling larvae are thereby protected from both predators and soil fungi[ii]. At least one fungus-growing ant (Cyphomyrax minutes) farms a fungus that secretes antibiotics preventing infection of their shared food[iii].
Even attempts at biological pest control in agriculture are thwarted by an insect’s ability to self-medicate. Attempts to kill the tobacco hornworm, an insect that feeds specifically on Solanum plants such as tobacco and tomato, by spraying them with a lethal bacterium (Bacillus thuringiensis), are undermined by the hornworm’s consumption of large amounts of the plant’s antibacterial alkaloid nicotine. This suggests that to deter herbivorous insect pests by breeding plants containing greater concentrations of toxic compounds may be fundamentally flawed and even counterproductive, as the toxins can actually be used to the pest’s advantage[iv].
The woolly bear caterpillar of the tiger moth is parastized by tachinid flies which inject their eggs into unlucky caterpillars. The fly larvae develop inside the caterpillar’s abdomen, feeding off its fat reserves and eventually taking up the whole body cavity. Finally, the larvae emerge by making a hole in the cuticle wall. When studied under laboratory conditions most caterpillars, quite understandably, die from this experience, but when Rick Karban and his colleagues at UC Davis, started rearing their caterpillars in outdoor field cages, they noticed that the survival rate of parasitized caterpillars was much higher.
Given a choice of plants, healthy caterpillars preferred to feed on lupine (Lupinus arboreus) while parasitized caterpillars preferred to feed on hemlock (Conium maculatum). Having parasites affected diet choice, and the change in diet improved their chances of survival. Although hemlock (known to contain at least eight alkaloids) does not kill the parasite, it somehow helps the caterpillar survive the infection. This experiment clearly demonstrates the importance of studying animals under natural conditions where they are able to make health-enhancing dietary choices[v].
Some birds use herbal volatile oils to enhance the health of their chicks. During nesting time, male starlings collect a selection of aromatic herbs to bring back to the nest. In North America, they carefully select wild carrot (Daucus carota), yarrow (Achillea millefolia), agrimony (Agrimonia paraflora), elm-leaved and rough golden rod (Soldiago sp), and fleabane (Erigeron sp). They prefer these even when they are not the most common plants nearby. The preferred herbs contain more volatile oils, and in greater concentrations, than aromatic plants nearby that are not selected. In other words, they are the most complex aromas around.
Back at the nest, the fresh herbs are woven into the nest matrix and topped up all the while the chicks are hatching. The benefits of these herbs to the chicks are evident. Chicks in herb nests have a significantly greater chance of surviving into the next season than chicks in nests from which the herbs have been removed[vi].
The birds don’t eat or actively rub against these medicinal herbs. It is thought that the leaves’ volatile oils may fumigate the nest. When herbs are removed from nests, chicks become infested with more blood-sucking mites suggesting that the volatile oils are keeping numbers of ectoparasites in check. More specifically chicks in nests containing wild carrot have higher haemoglobin levels than those without, again, suggesting they are losing less blood to ectoparasites (external parasites).
The preferred plants contain monoterpenes and sesquiterpenes (such as myrcene, pinene, and limonene) that are harmful to bacteria, mites, and lice in the laboratory[vii]. Although they retard the hatching of louse eggs, and the emergence of mite larvae, the preferred plants do not kill either adult lice or adult northern fowl mites. The preferred plants are also particularly effective against the harmful bacteria Streptococcus aurealis, Staphylococcus epidermis and Psuedomonas aeruginosa.
Back in 1972 primatologist Richard Wrangham noticed chimpanzees were doing strange things with certain leaves. Early each morning the wild chimpanzees he was studying in Gombe, Tanzania, were carefully selecting hairy leaves and folding them concertina fashion before swallowing them whole. He found these folded up leaves entire on the forest floor apparently having passed through the gut undigested. When he tried some himself, he found them disgusting—akin to swallowing folded up sand-paper! Why would chimpanzees do this to themselves?
Many years of research revealed that these rough hairy leaves act as a scour, scraping through the gut any loose nodule or tapeworms. An individual may swallow anywhere from one to 56 leaves in one bout. Although it is an extremely rare behaviour it is more common after the beginning of the rainy season when worm infestation is greatest and, in nearly all instances, clear indications of worm infestation has been established. The relief of pain from parasite infection appears to the main stimulus for self-medication in apes. To date, chimpanzees, bonobos, and gorillas throughout Africa have been seen to ‘leaf swallow’ at least 34 different species of bristly hairy leaves in this way[viii].
Physical scours are safer than chemical anthelmintics (which by necessity must be toxic enough to harm parasites) and do not contribute to the rapid drug-resistance seen in parasites around the world. Such strategies may not be marketable (and research on them will therefore be difficult to fund) but they are, nonetheless, an important element of health maintenance.
Nearly all vertebrates that feed on fruits, seeds and leaves also eat clay. Elephants clear and mine huge areas of forest to feed on the clay subsoils they need. Chimpanzees, rhinoceroses, giraffes, and buffalo feed on the clay-rich soil of termite mounds in Africa. In Peru, up to 900 parrots from 21 species, and 100 large macaws, gather each day to feed on the clay of eroding riverbanks, biting off and swallowing thumb-size chunks of orange clay[ix].
Clays can bind mycotoxins (fungal toxins), endotoxins (internal toxins from pathogens), man-made toxic chemicals, bacteria and viruses; they also protect the gut lining, act as an antacid, and adsorb excess fluids thereby curbing diarrhoea. In short, clay is potentially a multipurpose medicine.
In 1999, the hypothesis that animals eat clay in order to inactivate plant alkaloids was tested experimentally with macaws by James Gilardi and a team of scientists at Davis, California. First, they established that seeds eaten by macaws contain 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 ate the quinidine with clay, had 60% less alkaloid in their blood than the control group, demonstrating that clay can indeed prevent the movement of plant alkaloids into the blood. What surprised the scientists, though, was that the clay remained in the macaws’ gut for over 12 hours, meaning that a single bout of geophagy could protect the birds for quite some time. They suspect that clay not only prevents plant toxins getting into the blood, but also lines the gut and protects it from the caustic chemical erosion of seed toxins[x]. As macaws don’t have a diarrhoea response to toxins, the consumption of clay may be an essential part of their diet, allowing them to successfully utilise foods that other animals are unable to tolerate.
Interestingly, animals also self-medicate their psychological ills (although this is incredibly difficult to establish in the wild). In one experiment, mice were exposed to two different types of stress. One group received electric shocks to the feet (described as “acute physical stress” but known to most people as “pain”) the other group was forced to witness another mouse getting a foot shock (described as “acute emotional stress”). Both groups of mice had free access to mind-numbing and pain-numbing morphine, but only the mice exposed to emotional stress self-administered the morphine[xi]. The researchers suggest that whereas the mice can associate the physical pain with an obvious source and therefore cope with the stress, the emotionally stressed mice have no way of knowing what is going on and are less able to cope with the situation. A similar effect is seen in emotionally stressed rats and cocaine self-administration[xii].
Laboratory rats are also able to use bio-feedback to calm themselves. Scientists in the Ukraine found that stressed rats learned to self-administer strobe lighting at certain frequencies that changed electrical activity in the brain and thereby calmed heart rhythm and lowered blood pressure. The rats thereby ingeniously calmed themselves down, reducing the likelihood of heart attack[xiii]. A feeling of anxiety is clearly unpleasant, and it is surely the animal’s desire to feel better that drives this kind of self-modulation.
The welfare of animals in intensive farming is a contentious issue, and any objective measure of their suffering is useful in the debate. A team of veterinary scientists at Bristol University have used chickens’ ability to self-medicate as proof that they suffer 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 broken leg bones, yet they receive no analgesia. Lame birds go off their food and remain still, unwilling to walk—even to the water trough. However, one month old birds can rapidly learn to select feed containing the pain-killing analgesic carprofen, and the amount of pain-killer the birds eat increases with the severity of lameness. Carprofen tastes slightly peppery (to a human) and can cause gastrointestinal upset. Sound birds tend to avoid the drugged feed, suggesting that they find it unpleasant—an additional indication that the lame birds prefer the distasteful food for its analgesic properties[xiv].
Animals aren’t only using the chemical and physical properties of substances in their environment to keep themselves well; they also use each other. Blue tangs, herbivorous group-living Caribbean coral fish, frequently sustain small cuts and abrasions yet don’t seem to get any infections. When seriously wounded, these fish, like many mammals, leave the group and stop feeding. But they increase their visits to ‘cleaner stations’ where they allow wrasse fish to feed off dead and infected tissue. After the injury is completely covered by a scab, the coral fish resume normal feeding and reduce their visits to the cleaners. The help they get is beneficial as even after deep wounding of subcutaneous tissue, there are no visible signs of scarring[xv]. Elephants frequently cover their own wounds, or pack the wounds of others with mud, and pull out spears with their trunks[xvi].
Primates even practise what we might consider more ‘advanced’ medicine. One captive capuchin monkey called Alice, for example, was wounded by other monkeys so badly that she required stitches. She groomed the area intensively for days, which was not unusual in itself, but then she took a stick, chewed one end to make a brush, and used it to apply syrup (supplied as food) to the wound area. She didn’t use tools to groom any other part of her body, nor did she apply any substance other than syrup. Syrup, a strong sugar solution, is an excellent ointment for wounds—soothing and antibacterial [strong sugar solutions literally explode bacterial cells]. A natural correlate, honey, is commonly used in traditional medicine for wounds and is recommended by western medics as a first aid treatment for wounds. Alice has never applied syrup to her body at any other time—only when wounded. A few years later, Alice’s infant received a lethal wound to the head from other monkeys. Alice not only licked and groomed the wound but she made a similar tool and applied syrup to the wound, as she had done to her own wound long before.[xvii] Chimpanzees even practise rudimentary dentistry. In captivity, one chimpanzee carefully inspected the mouth of another chimpanzee and then wrenched out a rotten tooth with a simple wooden lever she had made herself[xviii].
It is evident that animals (from insects to apes) use chemical, physical and social elements in their environment to enhance both their own health and that of their offspring. Far from being a romantic interpretation of nature, animal self-medication is an integral part of ecological dynamics. A greater understanding of the processes involved could provide us with a novel yet sustainable approach to health management.
Cindy Engel PhD is author of “WILD HEALTH: How Animals Keep Themselves Well and What We Can Learn from Them”
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27 Tallamy D W (1998) “Sequestered cucurbitacins and pathogenicity of Metarhizium anisopliae (Moniliales: Moniliaceae) on spotted cucumber beetle eggs and larvae (Coleoptera: Chrysomelidae)”, Environmental Entomology, 27 (2) pp 366-372.
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[v] Karban R and English-Loeb G (1997) “Tachinid parasitoids affect host plant choice by caterpillars to increase caterpillar survival,” Ecology 78 (2) pp 603-611.
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[vii] Clark L and Mason, J R (1985) “Use of nest material as insecticidal and anti-pathogenic agents by the European starling,” Oecologia (Berlin) 67 pp169-176.
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[ix] Mayer W (1999) “Feat of Clay,” Wildlife Conservation Magazine June 1999.
[x] Gilardi J D, Duffey S S, Munn C A and Tell L A (1999) “Biochemical functions of geophagy in parrots: detoxification of dietary toxins and cytoprotective effects,” Journal of Chemical Ecology 25(4) pp 897-919.
[xi] Kuzmin A, Semenova S, Zvartau E E and Van Ree J M (1996) “Enhancement of morphine self-administration in drug naïve, inbred strains of mice by acute emotional stress,” European Neuropsyhcopharmocology, 6 pp 63-68.
[xii] Ramsey N F and Van Ree J M (1993) “Emotional but not physical stress enhances intravenous cocaine self-administration in drug-naïve rats,” Brain Research 608 pp 216-622.
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[xiv] Danbury T C, Weeks C A, Chambers J P, Waterman-Pearson A E and Kestin S C (2000) “Self-selection of the analgesic drug carprofen by lame broiler chickens,” Veterinary Record M arch 11 2000.
[xv] Foster S A (1985) “Wound Healing: A Possible Role of Cleaning Stations,” Copeia, 4, pp875-880.
[xvi] Elephants and wounds
[xvii] Ritchie B G and Fragaszy M (1988) “Capuchin monkey (Cebus apella) grooms her infant’s wound with tools,” American Journal of Primatology 16 pp 345-348; Westergaard G and Fragaszy D (1987) “Self-treatment of wounds by a capuchin monkey (Cebus apella),” Human Evolution, 1 (6) pp 557-562.
[xviii] McCrew W C and Tutin C E G (1973) “Chimpanzee tool use in dental grooming,” Nature 241 pp 477-478.
