Chimpanzees spend most of their waking hours searching for sustenance, and their diet consists of a varied mix of leaves, fruit, insects, and even meat. As a rule, food is chewed carefully and not swallowed intact, but a different eating strategy was described more than twenty years ago by Richard Wrangham, a Harvard anthropologist working in the Gombe National Park of Tanzania. He observed chimpanzees selecting the leaves of Aspilia species, folding and rubbing them against the inside of their mouths for a few seconds, and then gulping them intact. Often the leaves were gathered and swallowed early in the morning, possibly by animals afflicted with illness or parasites. This behavior is quite different from the usual pattern of leaf-eating, in which a handful of leaves is swallowed after being thoroughly chewed. The unchewed Aspilia leaves are often excreted intact in the chimpanzees' feces. Aspilia mossambicensis (Plate 10) is a shrub in the daisy family (Compositae) with large leaves covered in dense hairs; it was assumed that secondary compounds produced by Aspilia had medicinal effects in the chimpanzees. Indeed, early reports from the lab of Eloy Rodriguez of Cornell University suggested that Aspilia leaves produce a red oil known as thiarubrine-A, which kills viruses, fungi, and parasitic worms, but this work has not been confirmed by other investigators.

Chemistry aside, an alternative explanation for the swallowing of the intact Aspilia leaves is that their densely hairy surface may physically dislodge parasitic worms from the chimpanzees' intestines as ingested material moves through during digestion. The chimpanzees at Gombe swallow more Aspilia leaves during the rainy season, when there are more parasite larvae in their habitat and the likelihood of infection increases; they may consume between fifteen and thirty-five leaves at a sitting. African people in the same area use Aspilia to treat infections, malaria, and scurvy, as well as conditions such as sciatica and lumbago. The feeding behavior of a chimpanzee is learned as the young animal associates with its mother and other elders. A young chimpanzee closely mimics the selection and preparation of foods, such as the removal of leaves and peeling away of bark to reveal the edible pith of a twig. Adult chimpanzees do not teach each other or urge peers to consume medicinal plants, but behavior such as leaf-swallowing and pith-eating could be acquired in youth. Possibly a few mature individuals experiment with plants that are not normally part of the chimpanzee diet, benefit from their medicinal effect, and pass this behavior along to their offspring and other younger members of their social group.