HER SYMPTOMS read like a classic hospital chart: lethargy, loss of appetite, darkened urine, difficulty and irregularity in excreting wastes. No doubt about it, this was one sick chimp.

However, a few hours after her illness began, the patient known as CH began chomping on Vernonia amygdalina, a shrub native to her home in Tanzania's Mahale Mountains National Park, but rarely eaten by chimpanzees there. She sucked and swallowed the bitter juice from the plant's macerated pith, spitting out the fibrous remains.

By the next afternoon, CH had perked up.

Was it mere coincidence that she had munched on V. amygdalina?

Perhaps. But the fact that chimps rarely ingest this plant -- known in English as "bitter leaf" -- strongly suggests she sought out the shrub for reasons other than sustenance, assert Michael A. Huffman of Kyoto University in Japan and Mohamedi Seifu of the Mahale Mountains Wildlife Research Center. Moreover, they say, studies have documented that African tribes use extracts from bitter leaf bark, stems, roots, seeds and leaves to treat a variety of human ailments -- including intestinal upset and appetite loss -- resembling CH's undiagnosed affliction.

The two primatologists suggest that ailing Mahale chimps turn to the local pharmacy -- the plants that thrive in their habitat -- for treatment.

Though their case study, detailed in the January 1989 Primates, is one of the first to document a sick animal recovering after eating a plant with known medicinal properties, Huffman and Seifu note that other researchers have recounted many incidents in which animals appeared to use plants for health and healing. Indeed, recent reports point to a common theme: Some animals appear to doctor themselves, perhaps even practicing preventive medicine.

Studies of animal self-medication may help confirm the wisdom of traditional cultures that have used these same compounds for centuries, says Eloy Rodriguez, a plant chemist at the University of California, Irvine. Taking a medicinal cue from animals may be one way to identify potential drugs for humans, Rodriguez and others suggest.

Rodriguez has helped analyze the chemical properties of other medicinal plants eaten by chimpanzees in Mahale and in Gombe National Park, about 100 miles apart.

He undertook the analysis with Harvard anthropologist Richard Wrangham, who had noted in 1983 that Gombe chimps occasionally dine on two species of shrubs belonging to the genus Aspilia. These plants, members of the sunflower family, have a long history as folk medicines in Tanzania. Wrangham observed that chimps consume A. mossambicensis and A. pluriseta in an uncharacteristic way: They pluck a single leaf at a time, swallowing it whole without chewing.

Six years ago, Rodriguez and others discovered a red, sulfur-containing oil in the leaves of these species. Now known as thiarurbrine-A, the oil kills disease-causing bacteria, fungi and parasitic worms. At a recent symposium in Oxford, Miss., Rodriguez reported that thiarurbrine-A shows as much anticancer activity in laboratory cell cultures as agents vincristine and vinblastine, standard chemotherapy agents derived from the shrub periwinkle (genus vinca).

Because chimps swallow Aspilia leaves whole, the "chewing," or surface rupture, occurs during digestion, presumably enabling the body to extract just the right amount of thiarurbrine-A for use as an antibiotic or antiparasitic agent, Rodriguez and Wrangham say. Whole leaves removed intact from chimpanzee dung show significantly lower levels of this compound than uneaten leaves, Rodriguez finds. But both researchers caution that the notion of chimpanzees using the plants as medicine will remain speculative until scientists measure thiarurbrine-A concentrations in the blood of chimps that have eaten the leaves, and until studies demonstrate that this compound or some other Aspilia ingredient can improve animal health. In a more recent field study, Wrangham observed that chimps at Uganda's Kibala Forest swallow whole leaves of another plant, Rubia cordifolia, which Ugandans reportedly use to treat upset stomachs. Kenneth E. Glander, a biologist at Duke University, says he has circumstantial evidence suggesting that some howling monkeys -- a tree-dwelling species native to Costa Rica, Panama and Mexico -- may use plants to dictate the gender of their offspring. He observed that some female howlers eat an as-yet-unidentified collection of plants before or after copulating but not at other times in the reproductive cycle. This, Glander says, may explain another finding that emerged during his field studies: A significant number of female howlers bore exclusively male offspring (or, more rarely, only female offspring) over a period of 20 years -- a gender bias unlikely to occur purely by chance.

Glander says he suspects some fertile females eat plants rich in estrogen-like compounds chemicals that change the pH, the acidity or alkalinity levels, of the vagina -- methods known to help shift the gender odds in humans. Glander observed that most female howlers at the top or bottom of the colony's pecking order produced only male offspring. This may give the mothers a reproductive advantage, Glander says, because most offspring at the extremes of the pecking order get killed during infancy by other members of the colony, who view the low-ranking ones as future threats. When the few offspring that survive are male, the mother's genes have a greater chance of widespread perpetuation, Glander speculates.

Glander and his colleagues are now measuring vaginal acidity and estrogen levels among Costa Rican howling monkeys and plan to investigate whether ingestion of particular plants can alter the animals' vaginal pH.

The bizarre behavior of an elephant led another researcher to consider whether some pregnant animals seek out specific plants for medicinal use. Ecologist Holly T. Dublin spent nearly a year tracking the daily routine of a pregnant elephant at Tsavo Park in Kenya. (Elephant gestation lasts 20 to 22 months.) The 60-year-old expectant mom almost never varied her routine, walking about 5 kilometers a day in search of a standard mix of palate-pleasing bush plants.

But one day, the elephant marched 28 kilometers to a riverbank, and stopped in front of a small tree of the family Boraginaceae. This particular species had never been listed among the elephant's meal choices. The elephant devoured the entire tree, leaving nothing but a stump. Four days later, back at the old stomping grounds, the elephant gave birth to a healthy baby.

Though this remains a single, unpublished and inconclusive case report, Dublin said she suspects that some compound in the tree helped induce labor. Working with the World Wildlife Fund in Nairobi, she and her colleagues are analyzing chemicals from the tree. Dublin has documented a provocative connection: Pregnant women in Kenya commonly brew a tea from the bark and leaves of the same tree to induce labor or abortion.

Among the 141 plants eaten by a colony of rhesus monkeys on the Caribbean island of Cayo Santiago, about half have known medicinal uses in humans, says Bernadette M. Marriott, a behavioral ecologist with Johns Hopkins University in Baltimore and the Institute of Medicine in Washington.

Beginning in 1979, she spotted unusual behavior among wild rhesus monkeys in Nepal and in research colonies at Cayo Santiago, near Puerto Rico, and Morgan Island, off the coast of South Carolina. These monkeys all ate dirt.

"It was really odd at first. I couldn't believe it," Marriott said. "I thought they're getting insects, or they're eating roots in the soil. But in fact, it's the soil itself they're eating."

Along riverbeds, near excavation sites and in fields, she saw monkeys digging and eating. Some dug little caves used by successive generations of monkeys; a few of the caves were big enough for a monkey to sit inside while dining on dirt. "They dig with their tiny little fingers, just scraping away," says Marriott, who reported her observations last June at a meeting of the American Society of Primatologists.

Virtually all the digging takes place in areas where the soil contains a high concentration of nutrients, Marriott notes. But she adds that evidence from an X-ray crystallography study and documentation of human soil eating -- a practice known as geophagy -- indicate that the primate penchant for soil may have a medicinal function, too.

Certain clay soils contain a high concentration of kaolin, the active ingredient in the antidiarrheal medication Kaopectate, according to a 1985 report by Donald E. Vermeer of George Washington University. Marriott and Vermeer say there are anecdotal and scientific reports from around the world indicating that some people who, like the monkeys, eat certain forms of Type B soil (the layer beneath topsoil, containing few organic compounds) regard it as a stomach settler.

In western Bolivia, for instance, people traditionally slather a slurry of soil on the skin of certain bitter-tasting potatoes before eating them, according to a report in the March 1986 Journal of Chemical Ecology by chemical ecologist Timothy Johns, of McGill University, Quebec. Johns says the slurry apparently offers protection from glycoalkaloids, secondary compounds in the potato skin that could otherwise cause illness.

He adds that the Navajo, Hopi, Zuni and Keres tribes in northern Mexico and the U.S. Southwest also use clays to protect against toxins in the skin of wild potatoes. In a report for an ecology publication, Johns and Martin Duquette of McGill will describe their laboratory simulation of an unusual culinary tradition of the Pomo tribe in California, showing that the practice of adding clay to acorns during cooking causes the soils to bind and break down bitter-tasting tannins from the seeds, thus preventing a possible bellyache.

Marriott says such reports suggest that the dirt eaten by rhesus monkeys, while serving primarily a way of obtaining adequate nutrition, may also function as a detoxifier.

She and her colleagues plan to analyze the feces of dirt-eating rhesus monkeys to determine what materials in the soil might benefit the animals. Marriott stresses that the soil's role in the body appears highly complex.

Ethnobotanist Shawn V. Sigstedt adds a bear story into the tales of animal self-medication. For centuries, humans have used preparations of the plant genus Ligusticum as medicines, says Sigstedt, a graduate student at Harvard University. In China, India, Mexico and the U.S. Southwest, traditional uses for various species of this plant have included antibacterial agents, dewormers, insecticides and treatments for upset stomach and rheumatism. More recently, researchers have derived from the plants a host of anticoagulant compounds known as coumarins, widely prescribed for patients suffering heart disease and stroke.

In the late 1970s, after seven years spent living with a Navajo family and learning about traditional tribal medicines, Sigstedt began to investigate the American Indian folklore behind Ligusticum. A legend holds that the bear gave Native Americans these plants; their Navajo name means "bear medicine," Sigstedt notes.

Sigstedt says his observations of captive bears, together with anecdotal reports of wild bear behavior, suggest a biological basis for the legend.

Sigstedt focused on Ligusticum porteri, a vanilla-celery-scented herb that grows below the canopy of oak and Douglas fir in the Rocky Mountains, among other habitats. He gave samples of the herb's root to groups of caged polar and grizzly bears in Colorado. Rather than eating the root, the bears chewed it and then spit out the mixture of saliva and macerated plant, methodically rubbing it on their paws and fur, he says.

Sigstedt says the bears' behavior, while inconclusive, is consistent with the plant's recorded use as an insecticide and antiparasitic agent. Sigstedt reported on his work in Tempe, Ariz., last March at the annual meeting of the Society of Ethnobiology.

For now, the evidence linking animals to medicinal compounds in nature remains tentative. "All of this is hypothesis," says Glander. "We really don't know the outcome of some of these apparent associations." Much more research is scheduled.

As Janzen puts it, "If you can 'talk' to animals, understand their 'language' by observing what they eat and what they don't," you can begin to catalog the wide variety of pharmacological compounds in nature.