News Feature: Animals that self-medicate

In an effort to self-medicate, a bonobo female selects stem of a M. fulvum plant for stripping. Image courtesy of LuiKotale Bonobo Project, copyright Max Koelbl.

Many animal species have created their own pharmacies from ingredients that commonly occur in nature.

Birds, bees, lizards, elephants, and chimpanzees all share a survival trait: They self-medicate. These animals eat things that make them feel better, or prevent disease, or kill parasites like flatworms, bacteria, and viruses, or just to aid in digestion. Even creatures with brains the size of pinheads somehow know to ingest certain plants or use them in unusual ways when they need them.

Anyone who has seen a dog eat grass during a walk has witnessed self-medication. The dog probably has an upset stomach or a parasite. The grass helps them vomit up the problem or eliminate it with the feces.

The science of animal self-medication is called zoopharmacognosy, derived from the roots zoo (“animal”), pharma (“drug”), and gnosy (“knowing”). It’s not clear how much knowing or learning is involved, but many animals seem to have evolved an innate ability to detect the therapeutic constituents in plants. Although the evidence is entirely circumstantial, the examples are plentiful. The practice is spreading across the animal kingdom in sometimes surprising ways.

Neighborhood Pharmacy

A wide range of animals self-prescribe the plants around them when they need a remedy.

  • Bears, deer, elk, and various carnivores, as well as great apes, are known to consume medicinal plants apparently to self-medicate.
  • Some lizards are believed to respond to a bite by a venomous snake by eating a certain root to counter the venom.
  • Baboons in Ethiopia eat the leaves of a plant to combat the flatworms that cause schistosomiasis.
  • Fruit flies lay eggs in plants containing high ethanol levels when they detect parasitoid wasps, a way of protecting their offspring.
  • Red and green macaws, along with many animals, eat clay to aid digestion and kill bacteria.
  • Female woolly spider monkeys in Brazil add plants to their diet to increase or decrease their fertility.
  • Pregnant lemurs in Madagascar nibble on tamarind and fig leaves and bark to aid in milk production, kill parasites, and increase the chances of a successful birth.
  • Pregnant elephants in Kenya eat the leaves of some trees to induce delivery.

Most studies of animal self-medication, however, are in the great apes. In the 1960s, the Japanese anthropologist Toshisada Nishida observed chimpanzees in Tanzania eating aspella leaves, which had no nutritional value. Harvard primatologist Richard Wrangham saw the same behavior at Jane Goodall’s Gombe reserve, where chimps were swallowing leaves whole. Other scientists noted the same in other chimp colonies. Without chewing, the animals weren’t getting much nutritional benefit. So why do it?

In 1996, biologist Michael Huffman suggested the chimps were self-medicating. Huffman, an American who has worked for years in Japan at the Primate Research Institute at Kyoto University, first saw a parasite-ridden, constipated chimpanzee in Tanzania chew on the leaves of a noxious plant it would normally avoid. By the next day, the chimpanzee was completely recovered (1).

The plants had bristly leaves, rough to the touch. Huffman theorized the chimps were swallowing the plants to take advantage of that roughness, using the leaves and stems to scour their intestines and rid themselves of parasites. Other researchers observed the same practice among other apes across Africa.

A Learned Behavior?

The obvious question is how do the animals—some of them not noted for intelligence—learn to do this? How did sparrows and finches learn to collect nicotine-heavy cigarette butts to reduce mite infections in their nests? How do honey bees and wood ants know to line their nests with resin to combat bacteria (56)?

Some of the adaptations to self-medication are recent. The widespread collecting of cigarette butts must be less than 100 years old because cigarettes aren’t much older. Other adaptations, however, are part of a long evolutionary process.

A simplistic explanation goes this way: one day a couple of million years ago, an animal, say a gorilla, had a stomach ache. For reasons unknown, he or she grabbed a leaf, chewed on it or swallowed it, and felt better. The animal remembered the action, and went to the same plant whenever the stomach ache returned.

It’s a reasonable scenario, says Mark Hunter, a professor of ecology and environmental biology at the University of Michigan. Learning these practices can be either innate or behavioral, or both. Apes, being intelligent creatures, certainly pass their knowledge on to their progeny. They are always watching each other and can communicate both vocally and through gestures, so their children watch as they treat themselves: active learning. However, there is innate learning as well.

“You should never underestimate the power of natural selection,” Hunter says. “It does not take a smart organism to develop an instinctive behavior.” There could be a genetic variation that leads the gorilla to taste a plant it does not normally eat, and eating the plant makes it healthier. The gorillas with that genetic background live longer and have more progeny and no real thought goes into it. “That level of learning might operate with other kinds of organisms too,” he says.

Take the monarch butterfly that lays its eggs on milkweed, which has antiparasite effects. “All we have to do is look at a healthy monarch butterfly and a sick monarch butterfly,” says Jacobus de Roode, assistant professor of biology at Emory University, who specializes in the beautiful creatures. “Now, a sick monarch butterfly is really affected by these parasites. The parasites bore little holes in the abdomen, and she will lose some of her bodily fluids and doesn’t feel good.” The changes in her physiology can change the way she responds to smells of the vegetation around her and she may have a genetic preference for these that would do her good.

Source: PNAS