Sick chimpanzees seek out range of plants with medicinal properties

Several plants eaten by chimpanzees when they are ill or wounded have been found to have medicinal effects, providing some of the strongest evidence yet that our close relatives practise self-medication.

Reports of chimpanzees self-medicating with plants have been around for decades, but it is difficult to establish when wild animals are ill and what effect their diet has.

Elodie Freymann at the University of Oxford and her colleagues followed wild chimps through Budongo Forest in Uganda, recording when they were ill and what they ate. The researchers identified chimps with obvious wounds or with gut infections by analysing their faeces for signs of intestinal worms, as well as checking urine samples for raised levels of immune cells.

Analysis of 53 extracts of plants consumed by the ill or injured chimps showed that 88 per cent were active against bacteria that are pathogenic in humans, including antibiotic-resistant strains like MRSA. Extracts from every sampled species had anti-inflammatory effects.

Such a systematic approach allowed the researchers to identify and characterise the species that the chimps use beyond what had been done before, says Kirsty Graham at the University of St Andrews, UK, who wasn’t involved in the study. “It’s a very impressive project.”

Sick chimps often left the safety of their group to eat specific plants, and picked out species only rarely eaten at the site. The infrequency of these events is what makes self-medication behaviour so hard to observe, but also provides one of the strongest pieces of evidence that it is a targeted response to illness.

Chimpanzees are usually reluctant to try unfamiliar foods that might be dangerous, says Freymann. Choosing to eat unusual plants therefore suggests they have particular reasons for doing so. “If you’re sick, you’re not going around stuffing things that could make you sicker in your mouth,” she says.

This might not capture everything that is going on, though, says Graham, as chimps’ diets are still very diverse. If the animals learn which plants to eat from others, over generations the community might overcome this reticence towards new foods. Directly comparing the diets of sick and healthy chimps at the same time might clarify whether these are active choices, she suggests.

Team member Fabien Schultz at the Brandenburg University of Applied Sciences in Germany hopes that identifying the active compounds in the plant extracts might lead to promising candidates for human medicines. “What if human lives can be saved by following the ways of our animal relatives?” he says.

An orangutan, another member of the great ape family, was seen applying plant leaves directly onto a wound in an apparent act of self-medication reported this year.