In recent decades, it has become well known that animals use plants to self-medicate.
The History of Traditional Medicines
For thousands of years, various traditional medicine systems and folk medicines have centered on the use of plants and other natural substances. The quest to find new drugs from natural substances has also increased in recent decades. In conventional Western medicine, about half of pharmaceutical products either contain or are derived from natural substances; indeed, approximately 47% of 155 anticancer drugs that were clinically approved between 1981 and 2006 for use in North America, Europe, and Japan were derived from natural products. Up to one-quarter of all prescription drugs also contain natural biologically active compounds. More than 35,000 plant species are now used medicinally around the world, their medicinal value coming from the presence of such bioactive compounds that have drug-like properties.
What Do Animals Do When They Become Sick?
In recent decades, it has become increasingly apparent that many animals also use plants to self-medicate. Indeed, a wide range of animal species—from insects to primates—is now known to use nature’s pharmacy to find a medicinal plant when they need a remedy. The science of animal self-medication is known as zoopharmacognosy, and it was created to describe the study of secondary plant materials and other non-nutritive substances used by animals as self-medication strategies to reduce disease or improve health. Zoopharmacognosy may provide important leads to future sources of medicines.
What’s the Evidence for Self-Medication in Animals?
Many animals seem to have developed the ability to exploit the therapeutic compounds in plants. And, although the evidence for this is circumstantial, many examples exist.
Anyone who owns a dog has, at some point, probably watched their pet eat grass. This behavior is believed to be an attempt to self-medicate, either to stimulate retching or to help to eliminate worms. Other animals, including red and green macaws, eat clay to help digestion and kill bacteria; when some lizards are bitten by a venomous snake, the lizards are considered to eat a particular root to neutralize the venom; and, in Kenya, pregnant elephants eat certain tree leaves to stimulate birth.
However, most of the evidence about self-medication in animals comes from the great apes. For many decades, several researchers have described seeing chimpanzees eating certain leaves—often swallowing them whole. Due to the fact that the chimps gained no nutritional value from the leaves by swallowing them without chewing, one researcher suggested the chimps were self-medicating. Another researcher, Michael Huffman, described a chimpanzee in Tanzania that was constipated and had a high parasite burden—he saw the animal chew on the leaves of a poisonous plant that the animal would normally avoid; the next day, the chimpanzee was completely recovered, Huffman said.
According to Huffman, the plants had a rough texture with bristly leaves. He thought the chimps were eating these plants to use the roughness to help eliminate intestinal parasites. He then described criteria for judging when an animal is using a plant to self-medicate: the plant eaten is not a regular part of its diet; it is not being eaten for nutritional value; it is being eaten during times of the year when parasites are most likely to cause infestations, such as during the rainy season, and other animals in the group don’t eat the plant.
The swallowing of entire leaves—without chewing—by apes across Africa has been reported for more than 40 plant species. One recent study presented evidence for self-medication in bonobos, using the Manniophyton fulvum plant. Researchers saw the animals place pieces of the leaves flat on their tongues, produce saliva, fold back the leaves to form a ball, and swallow them without chewing. This plant is not part of the bonobos’’ typical diet—they only ate it in small amounts and at times of the year when parasite infections were more prevalent. In addition, only a small proportion of animals in the group ate it.
In addition to using the leaves for their scouring effect to eliminate parasites from the intestinal tract, researchers hypothesize that the leaves may help to heal wounds caused by parasites and may also have anti-inflammatory effects. By rolling the leaves into a ball, the bonobos may help to keep them intact longer, so the leaves can have medicinal effects lower down in the intestinal tract.
How Do Animals Know How to Self-Medicate?
Experts consider that animals use both learned and innate responses to select plants to self-medicate. Some scientists believe that evolution has provided animals with an innate ability to select plants to self-medicate, because, with respect to natural selection, animals who could find medicinal plants in the wild had a greater chance of survival. Some parasitized caterpillars, for example, eat certain plant toxins that help them become resistant to lethal tachinid flies. Although these toxins are harmful to healthy caterpillars, they help the fly-infected caterpillars to survive. Even fruit flies and butterflies can choose food for their offspring that reduces the effects of disease in the next generation.
However, other studies have shown that this behavior can be learned, in particular among animals such as primates that have high cognitive abilities. Apes watch each other carefully and communicate both vocally and through gestures, so their offspring watch as they self-medicate. In this way, young apes actively learn from their parents about how to self-medicate.
So, it seems that animals have been studying medicine longer than people have been studying it. Animals have successfully used plants as medication for various conditions and some medicines have even been developed from plants. Considering the mounting problem of antibiotic resistance, experts in zoopharmacognosy therefore emphasize that humans can learn from animals, particularly when it comes to finding new medications. In a news feature in Proceedings of the National Academy of Sciences of the United States of America, Jacobus de Roode, PhD, an associate professor of biology at Emory University concludes, “[i]f we can learn from animals that have used medicinal plants for millions of years, and then look at what they use it for, then we could learn interesting things.”
Dr. Parry graduated from the University of Liverpool, England in 1997 and is a board-certified veterinary pathologist. After 13 years working in academia, she founded Midwest Veterinary Pathology, LLC where she now works as a private consultant. She is passionate about veterinary education and serves on the Indiana Veterinary Medical Association’s Continuing Education Committee. She regularly writes continuing education articles for veterinary organizations and journals, and has also served on the American College of Veterinary Pathologists’ Examination Committee and Education Committee.