Chausiku, an adult female chimpanzee, wasn't well. She moved painfully slowly, lagging behind the rest of her group, and later lay listlessly on her bed of branches.
Her appetite seemed to have vanished, except when it came to one particular plant: Vernonia amygdalina, or bitter leaf. Plucking the youngest stems, she removed the leaves and bark before chewing on the inner pith. By the next afternoon she was up and running, apparently having made a complete recovery.
The primatologist on Chausiku's tail, Mike Huffman, was puzzled by her behaviour, which he later saw repeated by other ailing chimps in the Mahale Mountains National Park, Tanzania.
When his research assistant, Mohemadi Seifu Kalunde, mentioned that bitter leaf was used by local Tongwe people to treat malarial fever, stomach cramps and intestinal parasites, his puzzlement turned to excitement. Was Chausiku dosing herself for some intestinal malaise?
The local people, he discovered, used the leaves, bark and roots of the plant, whereas Chausiku used the pith. Did she know something the Tongwe didn't?
Huffman made his observations of the sick Chausiku in 1987, triggering a new wave of research into the medicinal properties of bitter leaf - a plant that was already well-known to chemists.
"This plant has been intensively studied for more than half a century," says Huffman, who is based at the Primate Research Institute of Kyoto University, Japan.
"But when we looked at the plant part that the chimps were using we discovered 13 new chemical compounds."
Many of those new compounds turned out to have therapeutic effects, and that work created a flurry of excitement and helped to establish the new field of animal self-medication.
That field might now be about to bear fruit, with researchers discovering new compounds almost monthly. In a paper soon to be published, Sabrina Krief and her colleagues from the French National Museum of Natural History in Paris report on three novel plant compounds called saponins, isolated from plants eaten by chimps in the Kibale Forest of western Uganda, which suppress tumour cells in the lab.
Last August, they described two novel anti-malarial compounds from the leaves of a shrub called Trichilia rubescens, which they had spotted chimps eating in a way that suggested it wasn't for food.
Huffman is convinced this line of investigation has the potential to provide humanity not only with a rich new source of drugs, but also with new methods for using them. However, no company has yet officially invested in the approach.
One reason for their wariness may be that the research is difficult, requiring as it does multidisciplinary expertise in animal behaviour, botany, ethnomedicine and chemical analysis. Another possible explanation is that attempts to identify new drugs by watching chimps has already thrown up a few red herrings.
Take the strange case of leaf swallowing. Primatologists have long known that chimps have a penchant for the hairy leaves of certain plants, such as those of the genus Aspilia.
In 1985, Eloy Rodriguez of the University of California, Irvine claimed to have isolated a new compound from a species of Aspilia, called thiarubine A, which is a potent antifungal and antibiotic agent.
Primatologists, including Jane Goodall and Richard Wrangham of Harvard University - one of the pioneers of the study of chimp self-medication - suggested that the chimps were exploiting its chemical effects.
But when, in 1997, Jon Page and Neil Towers of the University of British Columbia, Canada, tried to replicate Rodriguez' findings with Aspilia specifically chosen by Mahale chimps, they found no trace of any thiarubine.
The alternative explanation Huffman came up with was that the animals were using the leaves as a mechanical scour, to clear worms and other parasites from their digestive tracts by what he dubbed the "velcro effect". And there is now evidence that leaf swallowing serves this purpose not only in all of the African great apes, but also in species as diverse as civets, dogs, bears and geese.
Thiarubine A is still under investigation as a treatment for HIV, but if it turns out to be useful in humans, it will have nothing to do with chimp self-medication, says Huffman.
To avoid misinterpreting the animals' behaviour, he says, it is important to pay as much attention to how they use a plant as to the plant itself. He has spent the last decade collecting information about primate diets, identifying those plants that are likely to be used for medicinal reasons and then isolating their active ingredients.
Krief, who works with Wrangham and the Kibale chimps that he has long studied, says that all the chimps she followed carried some parasites. Like humans, wild chimps are prone to a wide range of diseases but their most common ailments are wounds and flu-like symptoms. "It's amazing that they survive even with the wire of a trap cutting into their flesh, something that often leads to natural amputation," says Krief. "The fact they do suggests they are self-medicating."
Cindy Engel, a biology lecturer at the Open University and author of a 2002 book on animal self-medication, Wild Health, says that all animals have to self-medicate to survive. She gives examples of caterpillars that, when infected with parasites, start consuming plants that are toxic to those parasites.
There is no suggestion that animals are knowingly selecting their drugs to match their ailments - simply that they are constantly monitoring their own body state, and eating more of what makes them feel better. But in some primates, there is at least the possibility of learning, of adaptation and passing on of practices that work.
"We've got primates using particular remedies, and other groups using different remedies that have the same effect," says Engel.
She doesn't share Huffman's confidence that animal self-medication will provide ingredients for Western medicine, because she says animals use a combination of avoidance, prevention and cure.
"Animals don't use drugs the way we do," she says. "They are constantly monitoring how they feel and adapting their behaviour, so they're only going to resort to strong, potentially toxic drugs when all else has failed."
But Huffman argues that Western medicine could absorb some of the ingredients that animals use as alternative or combination treatments, just as it has absorbed elements of Chinese medicine. So far, he and his colleagues have studied the diets of gorillas, chimps, bonobos, Japanese monkeys and lemurs in Madagascar.
In every case, he says, it looks as if 15 to 20 per cent of the animal's diet is made up of plants that have some kind of health-related biological activity. And as his studies progress, they are revealing degrees of subtlety.
For example, chimps will use both leaf-scouring and bitter leaf to treat the same parasite, but not at the same time. Bitter leaf is strong and can be taken only around once a week, whereas hairy Aspilia can be consumed more often. In combining the two, says Huffman, "They are avoiding the resistance that could develop in the parasite if it is only challenged by a single chemical treatment."
And there is plenty of proof that humans continue to learn from animals just as they have done historically, he says. In Krief's Kibale study she interviewed local practitioners of traditional medicine living near to the chimp study site.
Of 163 plants eaten by the chimps, she found that humans used 35. In her videotaped interviews with one local healer, he says he prescribes the leaves of the plant Phytolacca dodecandra to treat stomach ache. He goes on to explain that local chimps eat the same leaves, and are able to do so because they know of other plants in the forest that counter those toxins.
"I have no doubt that a lot of what humans traditionally know about medicine is based on what they see in nature," says Huffman. "That's the only place it could come from."