Chimpanzees heal their wounds with plants used in traditional medicine.

Like humans, chimpanzees heal their wounds. They lick them, pack them with their hands, apply plants with healing properties, and even chew leaves and stems before using them as a poultice. A study compiled over 30 years of observations and published in the scientific journal Frontiers in Ecology and Evolution shows that they also practice hygiene after intercourse or defecation. Previous studies had already demonstrated similar practices in other great ape species, such as orangutans and gorillas, but chimpanzees also heal others, something very close to human altruism.

In the Budongo rainforest (Uganda), chimpanzees have been studied for more than 30 years. With patience, primatologists have managed to accustom members of two different communities to their presence, allowing them to approach within a few meters. The Sonso community, studied since 1990, is home to 68 identified individuals. Meanwhile, in the Waibira community, where scientists have been present since 2011, the population exceeds one hundred. The result of this work was the discovery last year that these apes were eating tree bark with antibiotic properties .

“The chimpanzee communities in this article are the same as those in the previous one,” says Oxford University researcher and first author of the paper, Elodie Freymann. “This time, instead of analyzing the plants these chimpanzees consume as potential medicine when they are sick or have parasites, I focus on forms of external care (wound and injury care, trap removal, hygiene behaviors, etc.),” she adds. Furthermore, “in the other study, we focused solely on self-care, self-medication. Now, we also report prosocial behaviors, directed toward others,” she adds. In addition to her own field observations during eight months of fieldwork, Freymann combines the study and analysis of the diaries of her colleagues who have been in Budongo since 1990.

Since 1993, nearly 50 cases of wounding have been recorded. Most of them, 34 of them, involved the injured chimpanzee treating itself. However, there are several in which other members of the group were treated. It is very likely that the total number was much higher, but that at the beginning of the research, these behaviors were not systematically recorded. In fact, during the four months Freymann was in Sonso, in the summer of 2021, he was able to observe 12 cases of wounding. All the injuries were due to violence within the group; two of them occurred during infanticides, in which the aggressor was injured in one case and the mother in the other.

In Waybira, Freymann recorded four injuries in another four months of his stay. The most serious was caused by a trap. “I observed Pavela , a young female, caught in a wire snare. It was wrapped around her foot and looked quite recent. Unfortunately, she didn't survive and was never seen again after that day,” he recounts. Made of nylon or wire, humans set them to hunt antelope, but they also catch chimpanzees. In fact, 40% of the members of the Sonso community bear scars from one of these traps.

“When chimpanzees are caught in traps, their mobility is drastically affected. They often lose a limb, other times they die,” Feymann laments. “When a chimpanzee is trapped, they often disappear and stray from the group for a while, especially if they have limited mobility. This can have serious social consequences for them, in addition to posing a risk to their health,” she adds. “I returned to Budongo a few weeks ago, and Sonso's alpha male had gotten caught in one of them. This made it difficult for him to maintain control of the group,” the researcher emphasizes. During her stay, she didn't manage to see them, but Sonso's journal records several instances in which scientists observed one chimpanzee helping another free itself from a trap.

The study describes how wounds heal. The first and most common behavior is licking the wound. In addition to cleaning it to prevent infection, saliva may have antimicrobial properties, as studies with other animals have shown, although not in chimpanzees. Another behavior involves putting one's fingers in one's mouth and then placing them on the wound. The two most elaborate behaviors are directly applying leaves or chewing them and then applying them to the laceration. Interestingly, they don't use just any plant. The study includes four different species, all with recognized or supposed properties. This is the case with the leaves of Pseudospondias microcarpa , used in several Central African countries for the treatment of various pathologies. The leaves of Argomuellera macrophylla are used by chimpanzees to treat their wounds, but in the Ivory Coast, humans ingest its sap as a purgative and to treat ascites.

Although this was not the focus of this study, it also records several instances of other behaviors more closely related to hygiene. Several animals have been recorded cleaning their genital area with leaves after two events: intercourse or defecation.

In the last year, several studies have emerged showing how three of the four great apes use plants to treat their wounds or illnesses. The image of the orangutan Rakus applying a plaster to a wound on his face, which had disappeared in less than three weeks, is exceptional, but because it is difficult to observe these animals in the wild. Other orangutans have been seen eating ginger leaves, used as medicine in Southeast Asia.

Regarding gorillas, Fabien Schultz, ethnopharmacologist at the Bernhard Nocht Institute for Tropical Medicine (Germany), is finalizing a study they've been conducting since 2019 in Bwindi Impenetrable National Park, home to half of the remaining mountain gorillas . These species have co-evolved with their pathogens, so they must have developed defensive behaviors. But there's one thing that hasn't been observed in either gorillas or orangutans, but has been observed in chimpanzees: healing others. Regarding bonobos, although there are no direct observations, given their greater empathy than chimpanzees, there's a good chance they also exhibit these prosocial behaviors.

Susana Carvalho, Associate Director of Paleoanthropology and Primatology at Gorongosa National Park (Mozambique), acknowledges that there has been much debate among primatologists about altruism, reciprocity, and cooperation in chimpanzees. “Many researchers argue that chimpanzees lack the behavior of helping others unless they receive a reward. However, longitudinal studies on the topic are lacking, and some studies in captivity have shown that chimpanzees spontaneously help others, even without rewards and despite the high cost,” she recalls. Hence the importance of this work, in which she appears as a senior author.

For Carvalho, caring for others' wounds among chimpanzees should lead to a rethinking of the study of the roots of prosociality. "We probably share a longer evolutionary history of altruism with chimpanzees than we had previously considered," she writes in an email. In her opinion, it could have originated in analogous contexts, where caring for others became fundamental to the survival and health of the group, justifying the care of unrelated individuals. "Therefore, its origins could be more related to caring for others than to sharing objects," she concludes.