Chimpanzee pharmacy

Found across tropical Africa, Aneilema aequinoctiale is a perennial flowering shrub that can grow to 2m. Its leaves and their leaf-sheaths are covered in fine, hooped trichomes that make them feel sticky when touched — hence the English vernacular name, clinging aneilema.

The shrub’s roots have been used in a variety of disorders, including skin conditions, eye diseases, constipation, leprosy, kwashiorkor, amenorrhoea and osteomalacia. But perhaps the plant’s most interesting use is as an anthelminthic.What makes it so interesting is chimpanzees rather than humans use it for this purpose. It is not a normal part of the apes’ diet, and research has confirmed that chimpanzees self-medicate by swallowing the leaves of aneilema and similar plants.

This activity has been observed mainly during the rainy seasons, when chimpanzees are most likely to be affected by parasitic worms such as Oesophagostomum stephanostomum, a nematode.

Examination of chimpanzee faeces has shown that the swallowed leaves remain intact but are egested along with expelled worms. One faecal sample contained 50 undigested leaves and 20 worms.

The precise mechanism by which the unchewed leaves have their effect is not yet established, but it is thought that the roughness imparted by their trichomes has a mechanical action in helping to expel worms from the intestines.

Chimpanzees are by no means the only animals to self-medicate. Various other mammal and bird species have been observed selecting and using plant materials, insects and soils to treat and prevent disease. The study of self-medication by non-human animals has been given the name zoopharmacognosy.