Two decades ago, made a discovery that changed his scientific career. While researching the ecology and evolution of parasites and their hosts, he came across something truly surprising: the monarch butterflies he was studying seemed to be exploiting the medicinal properties of plants to treat themselves and their offspring.
Back then, the notion that an insect might be capable of self-medicating seemed far-fetched. Now, de Roode is a world expert in the burgeoning field of animal medication, with a lab of his own at Emory University in Atlanta, Georgia. He spoke to New Scientist about his work, his new book, , and his belief that animals possess medicinal knowledge that we can use to improve our own health.
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Graham Lawton: How did this unlikely area of research get going?
Jaap de Roode: It started during work in Tanzania in the 1980s with a chance observation. Michael Huffman of Kyoto University was working with Mohamedi Seifu Kalunde, a national parks ranger, to look at the role of elderly chimpanzees in society. While tracking one called Chausiku, they noticed that she was withdrawn, she was taking naps during the day and she had diarrhoea. They saw her go to a plant called Vernonia, also known as bitter leaf. She stripped off the bark and started sucking the pith. This is not normally part of their diet. Seifu, who was also a traditional healer, told Huffman that he uses it as a medicinal plant. The next day, they saw Chausiku had recovered. That was the moment Huffman thought, well, maybe she is using medication.
Was she?
A lot of studies followed to figure that out. They looked at every step. Was a chimp sick? Does the plant have medicinal properties? If chimpanzees are sick and use this plant, does it help them? They found there are chemicals in this plant that kill intestinal worms. So, yeah, , and chimpanzees use it when they are sick.
That was the first proven example. Any others?
People including Jane Goodall had already observed that . They fold them over and swallow them – they don’t chew or digest them – and they poop them out whole. The idea was that those leaves also have chemicals that are bad for worms. But it turned out that they are really rough, like Velcro. Worms get stuck on them. The leaves also irritate the gut, so it purges the gut. .
How did you make your own discovery?
I was studying monarch butterflies, trying to understand parasite evolution. There is a parasite quite similar to malaria, Ophryocystis elektroscirrha, that infects monarchs. One feature of monarch caterpillars is that they eat only milkweeds, which contain chemicals called cardenolides. They use these plant toxins to make themselves poisonous to predators. I knew about the chimpanzee studies, so I thought, let’s see if milkweeds also have an effect on the parasite. I reared monarchs on different species of milkweeds and found that when the plants were more toxic, the monarchs became less infected and suffered lower parasite burdens and less disease. My thought was: “Wow, maybe these butterflies can actually use this as medication.”
Animals have evolved all these fascinating ways to deal with infections
How did that idea go down?
I think it was easier to accept that chimpanzees can medicate, because we give them credit for intelligence that we don’t give to other animals. When we wrote that showed the milkweeds had a medicinal effect [in 2007], a peer reviewer said there was no way monarchs can do this. So we had to remove it. But that didn’t stop us. We did experiments. We had infected and uninfected caterpillars, and we gave them a choice between medicinal and non-medicinal milkweed. They had no preference. Then we looked at female butterflies. One of the ways the parasite spreads is from mothers to offspring: when the female is infected, she is covered in parasite spores and these get scattered onto the milkweed and the caterpillars eat them. What we found is that infected female butterflies have a strong preference to lay their eggs on the medicinal milkweed. And that helps their offspring. It was a very clear medicinal effect.
Did those results convince the sceptics?
From the late 2000s, a lot of papers started appearing. When woolly bear caterpillars are infected with parasitoid flies, they , a type of chemical that kills these flies. Wood ants and it helps against the growth of microorganisms. For the woolly bear caterpillars, it is self-medication. For the wood ants, it is prophylaxis. So, it was becoming a trend at the time, which helped in the acceptance of our findings.
Many more examples have since been discovered. Which are your favourites?
When sheep are infected with worms, they prefer to eat plants that have more tannins in them: sheep normally hate tannins, but tannins kill worms. .
There is also fumigation. A lot of animals do this, but I love the birds and the butts story. Researchers at the in Mexico City discovered this when they were studying how urbanisation is affecting nest building in house finches. They saw all this white fluff in the nests and they didn’t know what it was. But then, when a nest was wet, one of them realised, “Oh, it smells of smoke. These are cigarette butts.” They also found that when there were more cigarette butts in the nest, there were fewer lice, mites and ticks.
To demonstrate that these birds do this on purpose, they took nests, removed the lining and put in an artificial one. Then they added live ticks, dead ticks or no ticks. What they found is that the birds that had live ticks in their nest started . It’s not an accidental thing. They are purposefully changing their behaviour in response to a threat. In natural conditions, finches collect all sorts of aromatic plants and put them in their nests. They especially use nicotiana plants, which have the same chemicals as the butts. I think they can smell the chemicals.
What other methods do animals use to stay healthy?
One of the most intriguing examples is anointing. Cats love catnip. When they are exposed to it, they start rolling around in it. They go into this frenzy and you can tell they’re having a good time. But the question has always been, why do they like it? Research in Japan found that when cats roll on catnip, they coat themselves in a .
How do animals know when and how to medicate?
To some extent, they don’t need to. Cats don’t have to know that they are coating themselves in chemicals. All they need to know is that what they are doing gives them a lot of pleasure.
The question is, how does it evolve? For a lot of systems, we don’t know. But with the woolly bear caterpillars, the researchers really . The caterpillars have four different taste receptors and one of them tastes the alkaloids that kill the parasitoids. When they have an infection, that taste receptor starts firing more rapidly, so they get more nerve impulses going to the brain. Alkaloids suddenly taste really great and they want more of them.
So, there are some innate mechanisms where physiology tells the animal how to respond. There is also individual learning. A lot of animals taste different things by trial and error, and they can make associations. For example, there is work showing that or chemicals. And then there is social learning. Chimpanzees learn from each other how to swallow leaves. They can fold them in many different ways before they swallow them and .
There was a documenting a wild orangutan treating a facial injury with medicinal leaves. Why don’t you give any examples of wound healing in your book?
I felt I had to focus on where the evidence is clear. With that orangutan, I would like to see more observations. I would like to see that it actually helps, ideally in experiments, but you cannot do that with orangutans.
How can people who live with animals allow them to take advantage of their ability to medicate?
We have dictated everything for our pets without letting them be their own doctors. I spend a lot of time in parks and people’s response to seeing their dogs’ natural behaviours is always interesting: “Don’t eat the dirt!” “Don’t eat the grass!” When my dogs decide to eat grass, I let them, because their body is telling them to do it. Sometimes they poop it out. Sometimes they throw up. With cats, if you have a yard, or even a balcony, make sure to plant some catnip.
And what about farm animals?
Right now, we don’t give them opportunities for natural medication, so we have to give them tons of antibiotics and that kill worms. By allowing them to eat more varied diets that they can use for medication, we don’t need those. It makes our animals healthier. It also prevents drug resistance from evolving, which is a direct medical benefit to us.
The examples in your book were all discovered by accident, so surely there must be many other instances of animal doctors out there?
I think we are going to find others. For now, it is really only vertebrates and insects. We have ignored worms, sponges, snails, all sorts of things. And octopi: they are really smart. One thing I hope to achieve with this book is to say: “Hey, now that we know that animals do it, let’s do a study to find out what animals use medication – and what we can learn from it.”
What can we learn?
People have looked at animals for inspiration for a long time. Healers in Seifu’s community learned from porcupines how to treat diarrhoea. They learned from the elephants that make treatments for upset stomachs. Many Native Americans have medicines that are based on looking at what bears do. Oshá root, which treats infections, is a good example.
Animals have evolved all these fascinating ways to deal with their infections and their health using all sorts of medicines that they find in nature. We can see what they use their medicines for and we can use that as a way of narrowing down what things we should look at to make our own medicines. There is just so much knowledge out there in the animal kingdom.
The great monarch butterfly migration: Mexico
Witness one of the world's most astounding wildlife events, the monarch butterfly migration, which occurs each year in the forested Central Highlands of Mexico.
