Chimps use more plant medicines than any other animal

For several decades, evidence has accumulated that animals turn to medicinal plants to relieve their ailments. Chimpanzees (and some other species) swallow leaves to mechanically clear the gut of parasites. Chimps also rely on the ingested pith of an African relative of the daisy, Vernonia amygdalina, to rid themselves of intestinal worms.

Dolphins rub against antibacterial corals and sponges to treat skin infections. And recently, a male Sumatran orangutan was observed chewing the leaves of Fibraurea tinctoria, a South Asian plant with antibacterial and anti-inflammatory properties, and dabbing the juice onto a wound.

These instances of animals playing doctor with therapeutic plants have typically been identified one by one. Today, in PLOS ONE, a multinational team proposes adding 17 samples from 13 plant species to the chimpanzee pharmacopia.

“The paper provides important new findings about self-medication behavior in wild chimpanzees,” a topic that’s still relatively unknown, says Isabelle Laumer, a cognitive biologist at the Max Planck Institute of Animal Behavior and lead author on the orangutan self-medication paper who was not involved in the new chimp research.

Observers with the team behind the paper spent 4 months with each of two chimp communities habituated to human observers in Uganda’s Budongo Forest. The researchers supplemented their own observations with historical data.

From the 170 chimps in the two communities, the observers zeroed in on 51 individuals suffering bacterial infections and inflammation as indicated by abnormal urine composition, diarrhea, traces of parasites, or apparent wounds. For 10 hours a day they followed the sick chimps through the forest, noting which plants they ate and when, and watching in particular to see whether the animals went out of their way to find and consume plants not part of their usual diet.

In one example, researchers observed an individual suffering from diarrhea very briefly venture outside the group’s safe home territory to eat a small amount of dead wood from Alstonia boonei, a tree in the dogbane family. Chimps rarely eat dead wood, which is not nutritious for them, the team says.

A different chimp with a hand wound was seen gobbling leaves of Christella parasitica, a type of fern, while nearby companions ate other plants. Fern eating had been documented only once before in this community.

The team—made up of researchers from the University of Oxford and groups at other institutions in Europe, Japan, and Uganda—tested samples of all the plants eaten by sick chimps for antibiotic and anti-inflammatory properties. They also reviewed the scientific literature for reports of use in local traditional medicines. A. boonei, the dogbane tree, exhibited antibacterial and anti-inflammatory activity in vitro. Traditional medicine practitioners in Africa use the plant to treat bacterial infections, gastro-intestinal issues, snakebites, and asthma. The fern proved to have anti-inflammatory properties as well, which might have benefited the chimp’s hand injury, but humans haven’t been reported to use it for healing.

In all, 11 of the 13 plants the team investigated are used in local traditional remedies, boosting the evidence that chimps use plants to help heal, researchers say.

For all 51 sick chimps, the self-medication appeared to work. “Every individual recovered, and relatively quickly,” says Oxford primatologist Elodie Freymann, the study’s lead author, though she cautions that she can’t be sure their healing was due to the plants.

Questions linger over how deliberately the chimps picked the supposedly medicinal plants, however. Richard Wrangham, a primatologist at Harvard University who did early work on primate self-medication, isn’t completely convinced the chimps are ingesting plants as medicine. He’d like to see whether plants the chimps pass over clearly lack medicinal compounds. But he still thinks the paper “is a nice advance.”

The new plant candidates “need to be looked at in more detail,” acknowledges Michael Huffman, a primatologist at Nagasaki University and a co-author of the study. But the evidence to date suggests the ill chimps select plants that could alleviate their symptoms, he says.

Researchers aren’t sure how animals learn to self-medicate. Huffman says some research suggests illness triggers an instinctual craving for bitter-tasting foods, which often have antiparasitic or antibiotic properties. But John Arnason, a phytochemist and ethnopharmacology expert at the University of Ottawa, thinks “nonhuman primates may have used the same trial and observation methods humans have used to find effective medicines, then passed on the information to their offspring.”