Wound-treating behavior in a Sumatran orangutan

Humans often use various medicines to treat injuries and illnesses. In the natural world, many animals also use substances with medicinal properties to alleviate discomfort. However, most of these cases involve ingesting plants with therapeutic effects, whereas deliberate treatment of external wounds with medicinal substances is rarely documented. Among simian primates (Simiiformes), only three relevant cases have so far been recorded.

The first case involved a captive female tufted capuchin (Cebus apella). After being injured in fights with conspecifics, she was observed using a tool coated with syrup to groom her wounds. This behavior was considered to be associated with previous care by keepers, who routinely applied medicinal ointment to her wounds. The capuchin had likely learned to associate wound treatment with the application of a substance and therefore used syrup as a substitute ointment to apply to her body.

The second case comes from chimpanzees. In the Loango National Park of Gabon in West Africa, a chimpanzee community was observed applying insects to wounds on themselves or on their companions. Individuals first captured a dark, winged insect about 5 mm in length. They then immobilized the insect by squeezing it between their lips, after which they applied it to a wound and moved it back and forth using their fingers or mouth. The insect was then placed again between the lips and subsequently reapplied to the wound. These chimpanzees were observed repeating this process several times, apparently performing wound care.

The third case involves a male Sumatran orangutan (Pongo abelii) named Rakus. He was observed using the leaves of yellow root (Fibraurea tinctoria) to treat a wound. This represents the first documented instance of a non-human animal applying a plant as a topical medicine to treat an external injury. The following account focuses mainly on this observation of Rakus.

Rakus was first observed in March 2009. At that time he was already close to adulthood but had not yet developed secondary sexual characteristics. Based on estimates, he was likely born in the late 1980s. His secondary sexual characteristics were not fully developed until August 2021.

On June 22, 2022, Rakus was observed with a wound on the right side of his face. Earlier that day, sounds of fighting between Sumatran orangutans had been heard, leading researchers to suspect that Rakus had sustained the injury during a confrontation with another male.

The condition of Rakus's wound from June 22 to August 5（Image source：Laumer, I. B et al. (2024). ， CC BY 4.0 ）

On June 25, Rakus was seen feeding on leaves of yellow root (Fibraurea tinctoria). After about thirteen minutes, he began chewing the leaves without swallowing them and used his fingers to apply the plant juice from his mouth onto the wound on his face. This behavior lasted for about seven minutes and was repeated several times.

Shortly afterward, unidentified flies began to appear around the wound. Rakus then applied the chewed leaf pulp directly onto the wound, covering it completely with the plant material. After treating the wound, he continued feeding on yellow root leaves for about thirty-four minutes. On the following day he consumed the leaves again for approximately two minutes, and in the days afterward he was not observed eating the plant again.

By June 30 the wound had already begun to heal and showed no signs of infection. Observations also indicated that Rakus rested significantly more than usual during the healing period, a factor that may contribute to tissue recovery and wound repair.

Yellow root (Fibraurea tinctoria) is widely distributed across Southeast Asia and parts of China. It is commonly used in traditional medicine to treat conditions such as malaria, dysentery, and diabetes. The plant is known to possess analgesic, antipyretic, detoxifying, and diuretic properties. Chemical analyses show that it contains furanoditerpenoids, compounds known to exhibit antibacterial, antifungal, anti-inflammatory, antioxidant, and even anticancer activities.

The plant also contains high concentrations of protoberberine alkaloids, which are associated with anti-inflammatory, analgesic, anticonvulsant, anti-amnesic, anesthetic, anti-arrhythmic, anti-hemorrhagic, hypotensive, antioxidant, antitumor, antidiuretic, anti-ulcer, and muscle-relaxant effects.

In addition, yellow root contains various other bioactive compounds with medicinal properties. These findings indicate that the plant has clear therapeutic potential. Rakus's behavior of applying the plant juice and covering the wound with plant material, possibly to prevent flies from approaching, therefore appears likely to be intentional.

Appearance of Fibraurea tinctoria leaves and a photograph of Rakus feeding on the leaves of Fibraurea tinctoria（Image source：Laumer, I. B et al. (2024). ， CC BY 4.0 ）

Nevertheless, during 21 years of observations at the study site, researchers never recorded any other Sumatran orangutans using yellow root to treat wounds. One possible explanation is that the social structure of the local orangutan population is relatively stable, resulting in few injuries and therefore few opportunities for wound treatment behavior.

Another possibility relates to orangutan dispersal behavior. Male orangutans typically leave their natal area during or after puberty and migrate to new regions. As a result, adult males in a particular area are usually not born there. Rakus may therefore have acquired this behavior in his natal population, although his origin remains unknown and this hypothesis cannot yet be confirmed.

Alternatively, the behavior could have arisen through individual discovery. While feeding on yellow root, Rakus may have accidentally applied the plant juice to his wound and experienced its analgesic effects. This could have encouraged him to repeat the behavior and eventually use the plant deliberately to treat the injury.

Finally, the wound-treating behaviors observed in chimpanzees, Sumatran orangutans, and humans suggest that the common ancestor of humans and other great apes may already have possessed similar behavioral tendencies. Such observations provide valuable insights into the evolutionary origins of medical behavior in humans.

Author: Bai Leng

Reference:

Laumer, I. B., Rahman, A., Rahmatei, T., Azhari, U., Hermansyah, Atmoko, S. S. U., Schuppli, C. (2024). Active self-treatment of a facial wound with a biologically active plant by a male Sumatran orangutan. Scientific Reports.