Exploring the Self-Medicating Abilities of Animals: From Chimpanzees Eating Bitter Leaves to Caterpillars Seeking Toxic Plants. Do these behaviors hold secrets about their survival?
This is not the only species known to utilize plants for medicinal purposes.
Treating Illness
In 1987 in Tanzania, a chimpanzee named Chausiku did something that astonished the scientific community. One day, researchers found Chausiku chewing on the bark of the bitter leaf tree, also known as Vernonia amygdalina, which is not part of the species’ typical diet. The bitter leaf is a folk remedy in Africa used to treat malaria, diabetes, and digestive issues such as anorexia, dysentery, and gastrointestinal disorders.
Chimpanzee using leaves for medicinal purposes.
Dr. Michael Huffman, a primate researcher at Kyoto University in Japan, speculated that Chausiku was self-medicating due to its lethargy and loss of appetite over several days. After 22 hours of consuming bitter leaves, Chausiku returned to a healthy, energetic state.
Scientists examined and found that Chausiku was infected with intestinal parasites, and its consumption of bitter leaves is documented evidence of animals self-medicating.
According to Huffman, scientists can only hypothesize based on observations and measure the outcomes of such actions. Additionally, they consider innate physiological factors, such as craving certain flavors or social behaviors where animals notice pain relief after specific actions and thus repeat those behaviors. Consequently, more scientists believe that animals, particularly those closely related to humans, have the ability to use medicinal plants.
In 1993, biologist Michael Singer from Wesleyan University in the United States observed that woolly bear caterpillars were feeding on various plant species. This behavior is atypical for caterpillars, which usually feed on specific types of leaves.
It turns out these caterpillars were sick: They were infected with parasitic flies. As is common, parasitic flies lay their eggs inside the caterpillars, developing by feeding on the caterpillar’s insides, eventually bursting out and killing the host.
“The behavior of these caterpillars is not random—they seek out toxic plants like ragwort, fiddleneck, and rattlebox, which contain pyrrolizidine alkaloids.
Singer immediately thought of the possibility of caterpillars self-medicating. “When I first talked to people about this, they said ‘No way, a caterpillar can’t do that, can it?'”
However, a series of experiments revealed that although these caterpillars have only four taste buds, one is specifically tuned to pyrrolizidine alkaloids, which activates to make the toxic chemicals taste better when the caterpillar is infected with parasites.
Experiments indicated that the caterpillars consuming pyrrolizidine alkaloids increased their chances of survival—but these alkaloids are still toxic to healthy caterpillars.
This presents a trade-off similar to side effects in medicine for humans. Singer noted that this convinced him of the therapeutic value of these plants and represented a case of self-medication.
The capuchin monkeys, found in Central and South America, seek to avoid itching and contracting pathogens from ticks and mosquitoes. They catch the giant centipede Orthoporus dorsovittatus, crush it in their hands, and apply the centipede’s internal mucus to their fur.
This behavior is common during seasons when mosquitoes are abundant. In 2003, scientists discovered that the mucus from the centipede contains benzoquinone, which acts as a mosquito repellent.
“This behavior cannot confirm that the monkeys know what they are doing. They may do it simply for comfort, but we know that this behavior can protect against bites and infections,” said evolutionary biologist Jacobus De Roode from Emory University in the United States.
In 2022, scientists documented 19 instances of chimpanzees caring for wounds by crushing insects. For example, a female chimpanzee caught an insect and handed it to a male chimpanzee to apply to another male’s open wound.
This action may not necessarily expedite wound healing but could represent a common social behavior among chimpanzees. However, given the state of being “injured,” it can be considered a form of self-medication in chimpanzees.
Urban birds spreading tobacco fibers in their nests to prevent parasites.
Disease Prevention
Moreover, animal species also know how to prevent diseases. Dr. Constantino de Jesús Macías García, a behavioral ecologist at the Universidad Nacional Autónoma de México, discovered that birds in Mexico often use tobacco to build their nests. This is because nicotine, contained in tobacco fibers, repels harmful garden parasites. The birds collect tobacco scraps and spread them in their nests.
Based on this finding, Constantino’s group conducted experiments with over 50 house sparrow and local sparrow nests. Results showed that nests with smoked tobacco remnants had fewer fleas, lice, and mites compared to nest without tobacco remnants.
Cellulose is a naturally difficult-to-decompose resin known to repel insects.
This is a form of disease prevention for urban bird species. Tobacco is utilized instead of aromatic plants these birds may find in nature.
Similarly, when queen butterflies become infected with the parasitic Ophryocystis elektroscirrha, they consume plants containing cardenolides, a toxin that helps eradicate the parasites within their bodies. Additionally, after they become queen butterflies, many will lay eggs on toxic plants containing cardenolides, reducing the likelihood of their eggs becoming infected.
“We can believe that these mothers are looking for ways to protect their offspring. Or they may be unaware of their infection, and their bodies, when infected, prefer bitter plants,” De Roode noted.
According to this expert, the use of medication in the animal kingdom is more common than scientists initially thought. This demonstrates that animals will always find ways to survive, not only through seeking food and hunting but also in preventing and treating diseases. And plant species will provide opportunities for scientists to research animal care methods in the future.
