In 1987, in the beginning of rainy season, Hufmann and Mohamedi Seifu Kalunde—game officer, Mahale Mountain National Park observed a sick non-feeding female chimpanzee self-medicating. It selected a small shrub— Vernonia amygdalina(called goat killer for its extremely poisonous bitter leaf) bent down several shoots, carefully stripped off the outer layers and began sucking and chewing on the exposed inner pith. The plant is used by local people to treat stomachaches, parasitic diseases like schistosomiasis, ameobic dysentery, and so on. This was one of the bitterest plants and healthy chimpanzees seemed to avoid it and took to pith eating only in sickness. Further to this self-medication, the sick animal had an unusually long rest and appeared better the next day, as it had even regained appetite and was feeding as usual.

Phytochemical analysis of the pith revealed the presence of a series of steroidal glycosides (vernosides) and sesquiterpene lactones—vernodalin, vernolide, hydroxyl vernolide and vernodalol. In vivo testing of compounds found in the pith revealed powerful anthelmintic action of the steroidal glycosides and the sequiterpene lactones were found to be anti-amoebic, anti-tumor and anti-microbial. The outer bark and leaves, which were carefully discarded by the chimps, were found to contain high levels of venonioside B1, which is extremely toxic to them. Consumption of the powerful anti-microbial, anthelmintic pith had impacted the nodule worm infestation of the animals. Thus bitter-pith chewing like leaf swallowing was seen to be more common at the start of rainy season, when nodular worm infestation is on the rise. Further it was noticed chimpanzees with higher worm loads were more ill and tended to chew more bitter pith than those with lower infestation levels.

This study was the first to document sickness at the time of ingestion of a medicinal plant by a wild animal. Also the animal was followed up to apparent recovery as a result of this self-medication. This has further triggered extensive research on other plants of chimpanzee diet and sesquiterpene lactones in general for possible identification of lead molecules effective against schistosomiasis, leishmaniasis, dysentery and drug-resistant malaria.