It was 1987 in Tanzania and the chimpanzee Chausiku was doing something very peculiar.
Primatologist Michael Huffman noticed Chausiku was chewing the bark of shrub Vernonia amygdalina, commonly known as bitter leaf—a plant that was not a normal part of her diet.
Since bitter leaf is used across Africa by humans as a remedy against malaria fever, stomachache, and intestinal parasites, Huffman speculated that Chausiku might be self-medicating, as she had been acting lethargic and with little appetite over a period of days.
His hypothesis was met with both excitement and disbelief. Indeed, just 22 hours later, Chausiku was back to her healthy, energetic self. Later research suggested Chausiku might have had a nodular worm infection in her gut. By analyzing the improvement in her feces, urine, and behavior, Huffman made the case for this being the scientifically documented example of self-medication in animals.
Huffman, now a professor at the Wildlife Research Center of Kyoto University, admits that figuring out whether an animal is medicating is somewhat like an educated guess. “We can only describe what we see and then measure the outcomes of those actions,” he says. “But since we cannot talk to animals, we do not know what they are thinking when they self-medicate.”
Nevertheless, he believes there are both innate physiological factors, like cravings for certain tastes, and socially learned ones, like noticing a decrease in pain after a certain action, at play in these behaviors. He and a growing body of scientists posit that medication is not something reserved just for humans or even closely linked species, like primates. Examples of animals using medication are more widespread and varied than ever previously thought.
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Researchers break down five examples of species across the animal kingdom figuring out ways to fight disease and prevent infection.
These caterpillars change their diet to treat their parasitic infections
When, in grasslands of southeastern Arizona, south of Tucson, in August 1993, biologist Michael Singer first observed woolly bear caterpillars—fluffy, black caterpillars that turn into tiger moths—he noticed they were grazing on a variety of plants rather than a narrow selection. “This was very un-caterpillar-like behavior,” says Singer, now a researcher at Wesleyan University.
It turns out the caterpillars were sick. They were, as is common, infected with parasitic flies that lay their eggs inside the caterpillar’s body, develop by eating the caterpillar’s insides, and then burst out, killing their host. The caterpillars’ behavior wasn’t random—they were seeking out toxic plants such as ragwort, fiddleneck, and rattlebox, which contain pyrrolizidine alkaloids.
Singer’s mind immediately thought of self-medication. “When I first talked to people about this, they said, ‘No way, a caterpillar is not going to be able to do that, right?’”
But a series of experiments revealed that although these caterpillars only have four tastebuds, one is specifically tuned to pyrrolizidine alkaloids, and it is specifically activated to make the toxic chemicals taste better when the caterpillar is sick with parasites.
Experiments showed that parasitized caterpillars that eat pyrrolizidine alkaloids will increase their chance of survival—but alkaloids were still poisonous to healthy caterpillars.
This is a trade-off similar to side effects in medicine for humans. Singer says this is what persuaded him these plants have therapeutic, medicinal value, and this was a case of self-medication.
These primates rub themselves in insect-repellant
Some animals don’t want to run the risk of getting infected by disease-carrying and itchy ticks and mosquitoes, so they use their own form of insect repellent. Various species of capuchin monkeys from South America and some species of lemurs from Madagascar have been observed hunting the slinky millipede Orthoporus dorsovittatus, biting and crushing the worm in their hands, and then coating their fur with the oozing goo from its insides.
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They did this more during seasons when mosquitoes and other biting insects were more abundant. And in 2003, scientists discovered that these millipedes secrete toxic chemicals called benzoquinones which actively repel mosquitoes.
“Does this mean they know what they are doing? Not necessarily. They could just be doing do it because it feels good,” says Emory University evolutionary biologist Jacobus De Roode. “But we do know the behavior helps against bites and infections.”
These chimps treat their wounds with bug juices
In a 2022 study on chimpanzees in Gabon, scientists observed a total of 76 different times chimps accidentally wounded themselves. In 19 of those cases, the chimps tended to their wound in an unusual way—with squished insects.
A chimp called Freddy, who had a day-old wound on his left arm, was spotted snatching a leaf and using his mouth to pluck off an unidentified insect, then pressing it softly against the gash. Chimps did this for each other as well: one chimpanzee female caught an insect and handed it over to another unrelated chimpanzee male, who then applied it to the open wound of another unrelated male.
( See rare photos of chimpanzees treating their wounds with insects .)
These behaviors weren’t “just a coincidence,” says Simone Pika, a comparative biocognition researcher at the University of Osnabrück, but they don’t know whether this behavior indeed helps to increase wound healing.
It may just be a social custom the chimpanzees of that community have learned, she says, but given the “unambiguous context” of the wounds, there’s reason to believe this is a medication behavior.
These birds use cigarette butts to keep pests away
In Mexico City Universidad Nacional Anutónoma de México behavioral ecologist Constantino de Jesús Macías García’s students found that the most common man-made item local birds used to make their nests were cigarettes.
Since nicotine, derived from tobacco, is occasionally used as a parasite repellent for garden pests, his team tested whether birds were lining their homes with shreds of cigarettes for this purpose.
In an experiment with over 50 bird nests of local house finches and house sparrows, the team found that nests with the cellulose from smoked cigarette butts attracted significantly fewer mites, lice, and ticks than their non-smoker counterparts. They also found that exposure to those cigarette butts damages the red blood cells of both bird chicks and adults, but that the chicks weigh more—indicating health—the more cigarette butts are used in the construction of their nests.
“Although toxic, the net benefit is positive,” says de Jesús Macías García, suggesting this is a form of prophylaxis (practice to prevent disease) for urban birds. He thinks cigarettes are used in lieu of aromatic plants that these birds would have available in nature.
These butterflies medicate their babies before they’re even born
When monarch butterflies contract the potentially lethal, wing-deforming disease Ophryocystis elektroscirrha, they eat types of milkweed species that contain higher levels of cardenolides which are toxic to the disease’s spores inside their body.
In experiments, when given a choice, female monarchs chose to lay eggs on the more toxic milkweed, and the probability that their eggs would be infected dropped.
“These mothers basically make choices based on the future infection of their offspring,” says De Roode. “Now, does this mean intentionality? Not necessarily, but it does mean that the cause for their behavior is infection. They may not know they are infected, their bodies may simply prefer the more bitter plants.”
But given the growing body of research suggesting a wide variation in the examples of medication in the animal world, De Roode’s take is that medication in the animal world is much more widespread than previously thought.
“It’s been common from the beginning of time, as soon as there were animals, as soon as there were parasites, there would have been ways for animals to take care of them,” says De Roode. “And so the bigger picture is that it’s extremely common, yet very much under-explored.”
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