Mar 8, 2022 7:00 AM

Are These Chimpanzees Using Insects as Medicine?

Researchers observed chimps in Gabon applying insects to wounds—and it’s raising big questions about animal altruism and self-medication.

Central Chimpanzee with a baby chimpanzee on its back

In 2019, Alessandra Mascaro, a research assistant at the Loango Chimpanzee Project, was filming the chimps for fun when she saw a female named Suzee grab a mysterious speck out of the air and hold it to an open wound on her son’s foot.

"It was very difficult to understand what was going on, because it's something very quick and was never observed before,” says Mascaro, who works at the Loango National Park in Gabon, Africa. And even worse, the images weren’t very high resolution; to avoid influencing the wild animals’ behavior, her footage had been taken dozens of feet away, and her view was obstructed by nearby brush.

“We weren’t really sure what we saw at the beginning,” agrees Lara Southern, another researcher at the site. Without a good internet connection, they had to resort to textbooks to try to look the behavior up. Still no luck. It took a few days for a coworker to suggest that the specks captured on camera were insects. But once Mascaro began looking for the behavior, it seemed like it was everywhere. Over the next 15 months, project researchers noticed chimps rubbing insects in either their wounds or another’s a total of 19 times. “We had to figure out all together that it was not in our imagination, but was really happening," Mascaro says.

Why were the chimps doing it? In a study published in February in Current Biology, Mascaro and her colleagues detailed the behavior and posed two nonexclusive possibilities. One is that the animals were attempting to medicate themselves. The other is that it’s an instance of prosocial behavior—otherwise known as altruism.

Male chimpanzees often skirmish, making wounds fairly common, although rarely serious. So perhaps unsurprisingly, in all but one of the instances the researchers observed, the wounded chimp was male, and the insect was being applied either by themselves or another member of the group. Because of their distance from the animals, the research team isn’t sure which insects were used, but in at least three instances, chimpanzees took insects from near or under leaves. Then, using either their fingers or mouth, they pressed the whole insect to the wound, in some cases moving it around. It was unclear whether the insects remained in the wounds after this step, or whether the chimps discarded them.

If there were no benefits to rubbing an insect on a wound, the probability of seeing it repeated among an entire community would be very low, Mascaro says. But it’s hard to say exactly what the benefit would be. Self-medicating behaviors have been observed before in the animal kingdom, including among chimps. Some are likely instinctual, like cats or dogs licking their wounds with naturally antimicrobial saliva. Moths, ants, and fruit flies have also been called “animal pharmacists,” creating, seeking out, and consuming foods and substances with medicinal properties.

Other behaviors, however, are not purely instinctual. Instead, they may have elements of culture and generational knowledge tied up in them. Michael Huffman, an associate professor at Kyoto University's Primate Research Institute, has spent his career researching “zoopharmacognosy,” or animal self-medication. In 2003, he observed a sick chimpanzee in Tanzania’s Mahale Mountains National Park eat a leaf he’d never seen the animals eat before. “What's the name of the plant?” he recalls asking his field assistant, a game scout for Tanzania’s national parks. “Well, it's very strong medicine for us,” the assistant replied. It turned out to be Trema orientalis (L.) Blume, a member of the cannabis family that is used in traditional medicine among people in West Africa, Tanzania, East Africa, and Madagascar.

Photograph: Lara Southen/Ozouga Society

Huffman theorized that the chimpanzees were consuming bunches of these leaves in order to expel parasitic worms. In a number of studies, he analyzed the feces of chimpanzees and great apes to see whether phytotherapy, or leaf-swallowing, correlates with the presence of worms. ("You can ask the animals questions, but the answer doesn't come out of their mouth. Sometimes, it comes out of somewhere else," he says.) In one, he and colleagues observed 27 instances of Tanzanian chimpanzees swallowing leaves whole, then identified the plants in the animals’ excrement and counted the number of adult nematodes in it. In general, when they found both worms and leaves, they also found diarrhea. And generally, when they found worms but no leaves, they didn’t. To Huffman, this suggested that the chimps “knew” to consume these leaves when they were infected with worms, and the plants somehow facilitated the speedy expulsion of the parasites. (Phytotherapy also has been documented in porcupines, elephants, and goats.)

To Mascaro, insect application is a different kind of behavior—for one thing, when animals swallow leaves, it’s not always clear if their purpose is to medicate or eat. Leaf-swallowing is also an exclusively solitary undertaking, while at least three times during the researchers’ observations, the Gabonese chimpanzees applied insects to one another, suggesting a cultural or interpersonal aspect to the behavior. It even seems to transcend normal social hierarchies. Southern, a coauthor of the paper and a graduate student at the University of Osnabrück, notes that the first instance observed—of Suzee applying an insect to her son’s wound—fits within the expected close bond between a mother and child in chimpanzee communities. Later, though, they saw a female engage in the same behavior with an unrelated male, and an instance involving two unrelated males.

If the chimpanzees believe the bugs to be medicinal, this would make their application by others a form of caretaking, and a sign of altruism. Male and female chimpanzees, for instance, “don’t have a very strong social bond outside of a mating context—and this goes way beyond that, obviously,” Southern says.

Mascaro says they also haven’t discarded the theory that the behavior is actually a form of the placebo effect: that the theater of grabbing a bug and applying it to a wound is just a performance. “All around human society, we use so many things that only have a placebo effect—some types of homeopathy, for example,” she says. Is it too big of a stretch to think other species might flock to snake oil for the same reasons we do?

Some say yes. Thibaud Gruber, a psychology professor at the University of Geneva who was not involved in the research, says he considers the study to be evidence of cultural behavior, but further observations would be required to show that it’s altruistic. The study leaves two big questions unanswered, he points out: the identity of the insects and whether they have a curative effect on the chimps’ injuries. Without retrieving remnants of the bugs or using a high-powered camera that can show them in greater detail, there’s no way to know if the insect could impart health benefits, he says. “It's really worth bringing into the scientific world,” he says of the study, “and at the same time, there's so much missing that you really want to know more about.”

Huffman agrees. “We still don't know the insect, and we don't know the compounds that could possibly be helping in relieving the symptoms—or, in the case of a wound, treating a bacterial infection,” he says. “Once they've been able to show that, then it's an amazing new example of self-medication.”

Southern concedes this point, but she says it was nearly impossible to find tiny fragments of bugs, which had been mashed up by wild animals that could only be observed from a distance. “We still have a lot of things to figure out about what's really going on here,” she says, such as a way to locate and identify these fragments, and to track wounded chimpanzees to see if insect application correlates with any health effects, in much the same way Huffman has done with leaf swallowing.

So while the new paper offers an intriguing look at a novel chimpanzee behavior, it still can’t tell us why they’re doing it. Still, Huffman says people shouldn’t be too quick to discard the idea that it’s some form of caretaking—whether that benefit is medicinal or purely social. “Our egos are too big,” he says of humans. “Maybe a study like this can put us in our place and help us to realize that we're not the only ones on this planet that look after each other."