Humans are not the only ones to use medicine to treat diseases. There’s even a science dedicated to animal self-medication: zoopharmacognosy, from the roots zoo (“animal”), pharma (“drug”), and gnosis (“knowing”). Who knew?

Humans have been medicating using plants for thousands of years — but we’re not the only ones.

Our understanding of animals using medication has shifted over time. One of the earliest researchers in zoopharmacognosy established criteria for defining when an animal is using medicine.

The plant should not be a regular part of the animal’s diet, have no nutritional value, be consumed in time of the year when parasites are more active, and should only be used by a single animal at a time. These criteria are the most relevant with primates, which actively choose medication when they are sick (presumably, a similar practice was employed by early humans).

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In addition to active learning, some animals learn innately (such as through natural selection), such as insects and other invertebrates with tiny brains. Other types of medicine use in animals are recognized, such as giving medicine to family members and using the same substance as usual but in higher quantities.

Here are some interesting cases of animals that use medicine when they are sick:

Primates

As they are our closest relatives, it is probably isn’t surprising that primates use medicine. At around the same time that Jane Goodall was observing chimpanzees in Tanzania in the 1960s, so was a Japanese anthropologist called Toshisada Nishida. He saw chimpanzees eating aspella leaves, which he found a bit strange as they don’t have any nutritional value for the chimps.

Likewise, at Jane’s Gombe reserve and other locations, chimpanzees were seen swallowing whole leaves. The researchers hypothesized that they were using the leaves as medication. Additional support came many years later in 1996, when the biologist Michael Huffman saw a sick chimp ridden with parasites that chewed on leaves of a noxious plant and recovered by the next day. Other researchers have observed bonobos take leaves that cause itchy skin and layer many of them on their tongues, carefully avoiding touching their skin. They use saliva to stick a whole ball of leaves together that they then swallow whole. The reason that they layer the leaves is so that it becomes a sort of time-release medication that acts over a longer period of time.

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A bonobo eating bamboo leaves. It turns to special, rough leaves when parasite-ridden. Image credits: Tambako The Jaguar.

In general, the primates seem to swallow these rough leaves to scrape the parasites out of their intestines and speed up elimination. They wouldn’t normally eat these leaves because they are prickly, noxious, and without nutritional value. A long time ago, a primate ancestor probably happened to grab a leaf when s/he was sick, and felt better afterwards. Then others picked up the behavior from the first discoverer.

Monarch butterflies

Not all animals actively learn to take medicine like primates. In most cases, the animals learn innately and a behavior is promoted through natural selection. The iconic monarch butterfly depends on the milkweed plant as its caterpillars exclusively eat it.

There’s a special type of anti-parasitic milkweed and only infected butterflies lay their eggs on it; healthy butterflies don’t look twice at it. However, by laying their eggs on it, infected butterflies ensure that their offspring are protected from infection. It is hard to say how consciously they are making this decision, but it does seem to be an innate behavior. Perhaps the parasite changes the physiology of the monarch butterfly and how she perceives the vegetation around her changes; she could genetically prefer beneficial plants in her parasite-ridden state. If the medication works then her offspring survive and can pass the behavior on to the next generation. In other words, the use of medicine could undergo natural selection. It doesn’t require a conscious choice of a medicinal compound like a human or primate would do.

Monarch on milkweed. Image credits: USFWS Midwest Region.

Other insects also take medication against parasites like the woolly bear caterpillar, which ingests plants that are toxic to parasites, and fruit flies, which lay their eggs in the alcohol from fermented fruit to keep parasitic wasps away from their offspring.

Birds

Some birds have started stuffing an unlikely material into their nests: cigarette butts. No, they’re not ne’er-do-well parents. On the contrary, they could actually be using the chemicals in the butts as medicine against parasitic mites, protecting their little chicks. It’s not as crazy as it seems as tobacco leaves contain chemicals that repel pests; tobacco juice and nicotine sprays can be used as garden pest control.

A bird with a cigarette. Image credits: Tony Wills.

Researchers studied two bird species that are common in North America, house sparrows and house finches, and measured the amount of cellulose acetate, a synthetic fiber found in cigarette butts, present in their nests. They found that nests with higher levels of this fiber contained fewer parasitic mites. Additionally, the researchers placed unsmoked and smoked cigarette butts in bird nests and found that there were half as many parasites in the nests with smoked cigarettes than non-smoked cigarettes. Smoked cigarettes contain much more nicotine because the smoke has passed through them.

Therefore, the researchers have a hunch that nicotine could be what the birds are using as medicine against a mite infestation in their nests.

Bees

Honey bees often collect resins produced by plants and stick them onto their hive. In particular, they use resins as medication after a fungal infection. After being affected by harmful fungus, such as chalkbrood, the bees collect more resin than they do normally. As a test to see if increased amounts of resins can protect against fungal infections, researchers added resins to experimental bee colonies. The result: colonies with more resin had fewer fungal infections. Therefore, the resin is an effective type of medication.

A fungal disease affecting a bee colony. Although it is hard to tell as the image is from a beekeeping book from 1900, dead larva of different ages can be seen in the cells. Image credits: Internet Archive Book Images.

This case of animal self-medication is particularly interesting because the medicine doesn’t act on a single individual but rather on the whole colony. The whole colony assesses the need for more resin after an infection and allocates workers for the task. The resin is also not individually ingested but has positive effects for everyone. This finding has implications for bee-keeping as beekeepers usually choose tidier bees and not those that cover their hives in annoying, sticky resin. However, by selecting for “cleaner” hives, the overall health of the bees may be reduced as they do not self-medicate after fungal infections.

There are many, many other cases of animals self-medicating. A few other colorful cases are lizards that eat a particular root after being bitten by a venomous snake, baboons with flatworms that cause schistosomiasis eat the leaves from a particular plant to get rid of those nasty parasites, and pregnant elephants in Kenya that eat tree leaves to induce delivery. As you can see, there is a wide spectrum of animals using medicine, and surely, there are more cases that we do not know about yet!

Understanding how animals use medicine can influence how we manage animals like honeybees and model diseases in wildlife populations. Some animals have been known to use plants that combat a lot of human diseases, and some medicine has been created from them that are in use for humans. Learning from animals has influenced traditional medicine and may still help us to find compounds that are powerful against certain diseases.