Unwell? Ape an ape's remedy

June 21, 2002

We can advance knowledge in many fields by studying the ways animals medicate themselves, says Cindy Engel

Navajo and Blackfoot Indians say that one of their most powerful medicines was discovered by watching bears use a plant as a skin rub. Tales of animals healing themselves with natural remedies persist in folklore around the world. Such stories are often anthropomorphic interpretations of nature that assign animals an almost mystical "wisdom". For this reason, even repeated reports of animal self-medication by amateur naturalists have been disregarded - until now.

In the past decade, the scientific exploration of animal self-medication has come of age as researchers from various disciplines have demonstrated behavioural health-maintenance strategies in mammals, birds and insects. Chimpanzees, gorillas and bonobos swallow the rough hairy leaves of more than 34 species of shrubs, vines and trees most frequently when infestation with internal parasites is at a peak just after the start of the rainy season. Combined field observations and laboratory studies reveal that these folded hairy leaves trap and scrape away tapeworms and nodule worms as they pass through the gut. The cue for such behaviour is, it seems, abdominal discomfort.

Bears, monkeys and coatis use a variety of aromatic plants and insect secretions as skin rubs that are repellent to insects, analgesic and astringent to minor bites. European starlings use aromatic herbs to line their nests at hatching time to reduce the impact of blood-sucking skin pests on new chicks. Plant-eating vertebrates, from elephants to macaws, eat clay - a phenomenon known as geophagy - primarily to detoxify defensive secondary compounds in their diet. Laboratory experiments show that clay effectively binds and thereby deactivates up to 60 per cent of dietary plant alkaloids.

Despite a recent flurry of papers on animal self-medication and consequent public interest, excitement in the scientific community at large has been reserved, no doubt due to the topic's lingering romantic associations. But for me as an ethologist teaching interdisciplinary environmental science and harbouring a personal interest in herbal medicine, it was imperative to get an overview of the topic - to tease fact from fiction. The implications of animal self-medication were too huge to ignore, even at the risk of peer disapproval. What started as a casual interest in 1992 grew steadily over the next decade into a full-time, three-year review.

I found veterinarians using self-selection of analgesics as a way of improving pain relief in domestic species; entomologists exploring how insect pests self-medicate against our attempts at biological pest control (the aim being to sabotage the insects' attempts); phytochemists hoping that wild mammals can lead us to potential drugs for medical research; anthropologists unravelling the evolution of human medicine by studying primate self-medication; medical researchers exploring how humans unconsciously self-medicate a range of ills; psychopharmacologists using self-medication to explore human drug addiction; and even geologists investigating why wild animals focus their geophagy on ancient subsoils.

Assimilating the information from these seemingly disparate disciplines was made difficult by the use of different terminology. Behaviour described by one person as "self-selection" was called by others "self-medication" or "illness-response behaviour". But one thing soon emerged - health-maintenance behaviour is more a self-regulatory process than a "curative" process per se. Young broiler chicks, for example, successfully self-medicate the pain of broken limbs, but their injuries remain unattended. Woolly caterpillars infected with lethal parasitoids increase their chances of survival by selecting a more toxic diet but with no obvious damage to the parasite. In other words, a simple rule of thumb such as "remove unpleasant sensations by whatever means available" could account for most of the curative strategies seen.

Most of the time, the action to remove unpleasant sensations would involve removing the source of the problem, but not always. Self-medication is turning up as if it were a universal - and no wonder. Consider the advantages to individuals who find ways to stay healthy in a competitive market. I find no romantic animal wisdom but rather plenty of adaptive behavioural strategies.

Acknowledging the active role animals take in keeping themselves well is going to change the way we approach animal health research, livestock husbandry, conservation and pest control. Imagine, for example, the improvements to livestock health that might be possible if we understood all inherent health-maintenance strategies that remain functional in domestic species. Might we not then be able to provide at least some of the resources they need to help themselves? Sound simplistic? It certainly sounds cheaper and safer than our current over-reliance on expensive and potentially dangerous pharmaceutical drugs at a time of increasing drug-resistance by pathogens.

Cindy Engel is an associate lecturer with the Open University and author of Wild Health: How Animals Keep Themselves Well and What We Can Learn from Them, published by Weidenfeld & Nicolson, £20.00.

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