Orangutans have been observed using medicinal plants to sooth joint and muscle inflammation "for the first time ever".
The Borneo based apes chew leaves of the Dracaena cantleyi plant to create a white lather, which they then rub onto to their bodies.
This is the first report of self-medication in an Asian ape, and also the first evidence for the use of anti-inflammatory medication in animals.
Indigenous people on the Indonesian island are however, known to use this plant for the same purpose.
"For the first time ever, self-medication activities of orangutans have been confirmed through this research," said Dr Ivona Foitova who co-authored the study published in the Scientific Reports journal.
As well local knowledge about the plant's healing properties, chemical analysis of the leaves suggested to the researchers that its anti-inflammatory properties may be beneficial to the apes.
Those tests demonstrated that extracts from the leaf inhibit the production of substances called “inflammatory cytokines”, which can aggravate joints and muscles.
Dr Foitova of Masaryk University in the Czech Republic and her colleagues have been observing wild orangutans in the Sabangau Forest for 15 years.
During this period they have collected more than 20,000 hours of observational data.
However, self-medication behaviour has been observed only seven times.
On one of these occasions the team was able to capture it on camera.
The footage showed a female orangutan called Indy applying the plant-based lather to her upper left arm. The researchers noted that the leaves she chewed to produce the lather were not swallowed.
"In the Sabangau Forest, it has been primarily adult female orangutans observed performing this behaviour,” said Dr Helen Morrogh-Bernard, co-director of Borneo Nature Foundation and lead author of the study.
“We believe that females may be using this plant to soothe sore muscles and joints from the extra weight of carrying their infants while climbing through the forest canopy."