Behavioral ecologist Helen Morrogh-Bernard of the Borneo Nature Foundation spent 20,000 hours studying 10 Orangutans of in the Sabangau Peat Swamp Forest in Central Kalimantan, Indonesia. Her results are published here.
She first observed a female orangutan in 2005 chew a particular plant that isn’t part of their diet. That individual and others chew the plant and spend up to 45 min rubbing it on their arms and legs. They interpret that they do that to treat pain.
The plant, Dracaena cantleyi, is also used by locals in a similar way to treat pain. The plant is known to reduce cytokines in inflammation. Chimpanzees and gorillas ingest rough leaves or chewed plant pith to help flush out intestinal parasites. A few monkey species and one species of lemur are known to rub concoctions, to repel insects or parasites. But wild great apes have never before been seen rubbing ointments onto their fur which implicates the orangutans use the plant to reduce inflammation and treat pain.