Animal instinct Why do chimps chew on leaves that they clearly find revolting? And why do elephants risk death to extract rocks from a mountain cave in Kenya? Perhaps they know something we don't about staying healthy, says Jerome Burne

Jerome Burne

Guardian

The shrub was not part of chimpanzees' usual diet, unsurprisingly since its leaves are rough, sharp and extremely nasty to eat. Yet Hugo had not only sought it out but he'd also eaten the leaves in a particular way, carefully folding them up concertina-style and holding them briefly in his mouth before swallowing. From the way he was grimacing, it looked just as if he was taking an old- fashioned medicine.

That observation in 1972 was to lead to a whole new way of thinking about animals and their health. At the time the notion that animals might be deliberately treating themselves with natural medicines was beyond the scientific pale. It's true there was no shortage of anecdotes about animals using herbs to cure themselves, cultures as far apart as China and ancient Rome have them and all pet owners know about cats and dog eating grass when they're sick. But until very recently, scientists dismissed such reports as romantic anthropomorphism.

But, gradually, the researchers in the Gombe began to gather evidence to show that something very deliberate was going on. They found that Aspilia leaves were used by local herbalists for stomach upsets and that they contained chemicals which were both antibacterial and attacked gut parasites. What's more, other chimps were seen occasionally eating from 19 other plants that also had rough leaves, in the same way. The leaves were excreted whole and, when examined closely, tiny nodular worms that infect the gut could be seen wriggling on the barbs on the leaf surface.

This is just one of dozens of examples of animals actively taking care of their health featured in a fascinating new book, Wild Health: How Animals Keep Themselves Well and What We Can Learn From Them. Far from self-medication being romantic nonsense, author Cindy Engel, of the Open University, shows that most animals routinely use a variety of techniques to deal with injury, infection, parasites and biting insects. They use plants, earth and even insects in ways that aren't just about getting energy or nutrients but are specifically aimed at keeping themselves and their offspring healthy. The implications are huge. Not just for how we should look after domestic and farm animals but for what we need to stay healthy ourselves.

Compared with their domesticated counterparts - sheep, for example need seven de-worming treatments a year - surveys have found wild animals are remarkably healthy, muscled and lean, with few parasites. Significantly, they have often been infected with diseases that devastate domesticated animals without showing any ill effects - wild boar with swine fever or wild deer with tuberculosis. The farmers' understandable response is to shoot the wild "carriers", a more far-sighted view, suggests Engel, would be to discover just how wild animals generally stay so disease-resistant.

Just as we don't usually take aspirins unless we have a headache, so animals tend to avoid medicinal plants unless they need them. At the Awash falls in Ethiopia, for example, there are two baboon populations, one above and one below. The tree Balanites aegyptiaca, the fruit of which is used by the locals as a de-worming treatment, grows in both areas. But it is only the lower baboons, which are exposed to a parasite spread by water snails, eat it.

But plants aren't the only source of medicinal substances. There's a cave on the side of Mount Elgon, an extinct volcano in western Kenya, which has been mined by generations of elephants. It's estimated they have taken 5m litres of rock in the last 2m years. Access to it is tricky, but the animals are willing to risk death to get there. The bones of those who didn't make it line the trail. Once inside they dig out the soft rock with their tusks, grind it with their teeth and then swallow it.

The rocks contain 100 times more sodium than they can get from the plants they normally eat, as well as being rich in potassium and calcium. Sodium is vital for all metabolic processes, especially for handling the toxins which are an inevitable part of a plant diet - an estimated 40% of plants contain some sort of defensive chemicals.

This sort of preventative medicine can take a rather shocking turn. In the Shetlands and on the Isle of Rhum in the Hebrides the soils are poor and lack these nutrients, so the sheep and deer have discovered a novel source. They bite the legs off the living chicks of the local nesting sea birds to get at the minerals in their bones.

Many animals also eat clay, which is not only an effective way of binding and excreting various toxins but, by lining the gut, it can treat gastrointestinal problems. Native people often mix clay with tannin-rich foods such as acorns before cooking them (tannins are bitter chemicals produced by plants as a defence but they are also active against bacteria and fungi).

A type of clay regularly mined by mountain gorillas in Rwanda is very similar to the kaolin sold in chemists for relief of upset stomachs. Another good source of clay are termite mounds, and chimpanzees are often seen breaking off chunks of soil from them. In one close study of five chimps seen eating termite soil, all were found to be suffering from gastrointestinal problems.

Most animals are plagued by small biting insects such as fleas, lice, mites, ticks and various parasites which can drain blood and inhibit growth, so they have developed a variety of ways to deal with them. Monkeys, apart from constant grooming, also rub themselves with soothing plants and even insects. Capuchins in Costa Rica, for instance, use the Piper plant, from the chilli family, which contains compounds that deaden pain and kill off insects. Catnip probably does something similar for cats.

Capuchins also rub their fur with millipedes, which make toxic chemicals known as benzoquinones that keep other insects away, as well as killing bacteria. Birds do something similar with a technique called "anting". They lie out on an anthill and encourage the ants to crawl into their feathers because they secrete formic which can kill lice, mites and bacteria.

But the use of nature's pharmacy isn't all serious Engel has come up with some controversial evidence that many animals and birds simply like getting stoned. Certainly elephants can detect the fermenting fruit of the marula tree from 10kms and will coming running for it. The Bohemian waxwing has a taste for rowan berries that have begun to ferment. The birds are often found in heaps, dead on the ground, having fallen off their perch. Postmortem examinations show they were drunk when they died and that they had acute alcoholic liver disease.

So why do they do it? There is evidence to suggest that elephants drink to relieve stress - just like the double Martini executive. Elephants given access to alcohol drank twice as much when their stress levels were raised. A more prosaic explanation is that alcohol is a good source of calories and if times get hard, then it makes sense to stock up on fuel.

Such an explanation doesn't work for the attraction that hallucinogenic plants seem to have for a variety of species. Jaguars, for instance, have been seen gnawing at the bitter ayahuasca vine used by Amazonian shaman, while bighorn sheep in the Canadian Rockies take great risks to get at a narcotic lichen. Engel's highly speculative suggestion is that animals' brains and, by implication, ours may benefit from an occasional boost from these potent neurochemicals.

Whether or not we all need the odd bite of peyote is far from clear. What is certain is that our hominid ancestors ate a far more varied diet than we do. It was a diet that would seem very bitter and astringent to us but was filled with a huge range of potent chemicals, many of which would have been effective against parasites and pathogens. While chimpanzees are known to eat 123 different plant varieties in a year, even the most health-conscious westerner rarely consumes more than 20 or 30. Seventy five percent of our global food supply comes from just 12 crops.

But while we would undoubtedly benefit from adopting a much wider diet and could learn lessons from animals about avoiding antibiotic resistance, there are some self-medicating techniques which will never catch on. Anyone for licking wounds or a daily anointing with urine?

Wild Health: How Animals Keep Themselves Well and What We Can Learn From Them is published by Weidenfeld and Nicolson at £20. Jerome Burne is the editor of the monthly newsletter Medicine Today.