In Mexico City, house sparrows and house finches are picking up cigarette butts and weaving individual fibres into the lining of their nests. When researchers first discovered the butts – while studying what plastics end up in nests – they assumed it was simply a fluffy material being used as insulation. But through a series of ingenious tests, they discovered that the butts were actually medicinal: the birds actively collected them because the toxin nicotine reduces mites and other blood-sucking parasites. The birds are treating themselves – and their offspring.
It’s one of many fascinating examples of animals medicating themselves revealed in Doctors By Nature, a new book by the US-based Dutch academic Jaap de Roode. Apes deliberately swallow leaves to dislodge intestinal worms. Caterpillars switch diets to repel parasitic flies. Bees incorporate sticky resins in their homes to combat disease.
Our medicinal skills were once considered a key difference between humans and other animals. “In western society, we love being unique,” says De Roode, speaking from his home in Atlanta, Georgia, where he is professor of biology at Emory University. “We come up with all sorts of characteristics – it’s tools, it’s walking upright, it’s big brains, it’s language, it’s culture, it’s medicine. And all of those things we debunk, one after another – and discover that we’re just another animal. I’m cool with that. I think what makes us unique is the wish to be unique.”
In fact, as De Roode points out, our knowledge of medicine probably began as mimicry – by observing and copying animal teachers. And today, a greater understanding of how other animals use natural medicines may in turn help us to find new cures for human diseases, as well as keep our livestock healthier and more productive.
The sparrows and finches in Mexico City are De Roode’s favourite example because they have adapted so quickly to a human environment. “Instead of collecting plants like a lot of wild birds, finches and sparrows found a shortcut and went for the cigarette butts littered around their nests, and it keeps these bloodsuckers like lice and ticks away. It’s an example of the birds helping their chicks rather than themselves.”
De Roode began his studies by making a similar discovery in monarch butterflies. These large orange butterflies are famed for their epic migrations between Mexico and northern America, but they are afflicted by a single-celled parasite called ophryocystis elektroscirrha (OE). Millions of black parasitic spores form on their skin, causing weight loss and dehydration. Many afflicted butterflies die on their migrations.
Through a series of experiments, De Roode discovered that monarch caterpillars eat a toxic class of chemicals called cardenolides in the plant milkweed. The variety of tropical milkweed has more cardenolides than its sister plant swamp milkweed, and tests found that 20% fewer of the caterpillars that ate tropical milkweed became infected with OE. Further studies revealed that female butterflies infected with OE laid more eggs on the medicinal tropical milkweed than on the non-medicinal plants. De Roode believes that they are trying to protect their children by doing so.
“When you think about how evolution and natural selection works, there’s so many things that parents do for their offspring – maybe it’s not that strange,” says De Roode. As he writes in the book: “Animals with brains smaller than a pinhead can be just as good at medicating as those with brains like our own.”
All medicine begins with plants and fungi. Most plants can’t move, and so need to produce chemicals that will defend them from attack. Cocaine, for instance, is a substance deployed by coca plants to repel hungry insects by messing up their nervous systems and causing tremors and death. “Plants and fungi provide a vast pharmacy that humans and other animals can use to find drugs to battle their parasites and pathogens,” writes De Roode.
One of the first known human medics was Ötzi the Iceman, whose 5,300-year-old body was discovered in the Alps in 1991. His gut contained the eggs of a parasitic worm, and he carried two cork-shaped fruiting bodies of the birch polypore fungus, well known to have antibiotic and laxative effects, suggesting that the neolithic man was treating his infection. Since then, it’s been discovered that Neanderthals were using medicine 50,000 years ago.
The early humans may have made their medicinal discoveries by following the examples of other animals. The modern scientific field of animal medication began in the 1980s when the primatologist Michael A Huffman and Mohamedi Seifu Kalunde, a Tanzania national parks ranger, observed a chimpanzee, Chausiku, fall ill. Chausiku stopped at a shrub Huffman had not seen the chimps feed on before and chewed on the pith of a branch. Kalunde told Huffman that the local people used the leaves of the plant, bitter leaf, as a traditional medicine treating stomach upsets, diarrhoea and intestinal parasites.
They kept following Chausiku, who built herself a nest and rested. The day after she ate bitter leaf, to their amazement, she got up and was leaping about with her daughter again. Huffman took plant samples back to his university and found they contained a class of chemicals known for their medicinal value against parasitic worms, bacteria and even cancerous tumours.
Huffman’s further studies of other individual chimps proved beyond doubt that they were using this plant – and others – deliberately for medicine. As scientists such as Michael Singer, a professor of environmental studies at Wesleyan University, point out, one of the definitions of medicine is that it comes with a cost: drugs have side-effects, and this bitter leaf was unpleasant to take. Since then, primatologists have found that 25 different wild primate species across 26 countries use plants with well-described medicinal properties.
Domesticated animals are still smart enough to possess what scientists call “nutritional wisdom”: they listen to their gut. Worm-infested livestock seek out tannin-rich shrubs, which, when eaten, can kill worms. In another revealing experiment, two groups of calves were given different foods. One was given a ready-mixed portion of grains and hay. The other was allowed to choose from a buffet of the same foods. The buffet calves actually ate less but gained just as much weight as the calves given a ready-mixed ration. So the cost of producing a kilo of beef fell by 20% when animals were given a choice. In the wild, free-ranging ruminants may eat up to 50 plant species a day. It’s not hard to see the health (and productivity) benefits if livestock are given more naturalistic management – in cows’ cases, a more varied diet and, ideally, outdoor grazing in plant-rich pastures with hedges, rather than grass monocultures. For instance, the organic farmer Rosamund Young has noticed that, if sick, her free-ranging cows seek out willow bark or buds, a source of salicylic acid, a chemical compound altered by chemists to produce aspirin.
De Roode finds similar lessons with honeybees, which seem increasingly afflicted by deadly mites and other pathogens. Bees use propolis – a glue-like substance they collect from plants – to stick things together, smooth surfaces and fill up crevices. Beekeepers have, over time, selected bees that don’t produce as much propolis, as it is so sticky it becomes a nuisance for them. However, this was a bad move.
Humans have used propolis as a medicine for thousands of years (most recently against HIV) and, unsurprisingly, it is deployed by bee colonies to protect themselves from pathogens. Experiments by Marla Spivak at the University of Minnesota and Michael Simone-Finstrom found that bee colonies infected by a fungus dispatched more bees to collect propolis for their hives. When hives were afflicted by chalkbrood, a disease that affects honeybee larvae, the adult bees incorporated more of this medicinal resin in their nests.
Modern hives with smooth wooden interiors have discouraged the production of propolis, but De Roode explains how simple experimental tweaks in hive design in the US are encouraging honeybees to produce more of it again. Using rough wood instead of smooth, and adding internal grooves to the wood, encourages bees to produce more propolis, leading to bigger, healthier colonies.
“Humans have used propolis for so long as a health supplement but didn’t make the link that maybe it helps bees. That stems from the idea that we’re the only ones who know medicine,” says De Roode.
He hopes his book will challenge that mistaken sense of superiority. It’s also a powerful message to halt the extinction crisis. In the Democratic Republic of the Congo, a research group is following chimps and bonobos and extracting chemicals from the plants they use. “We need all sorts of creative ways to find new drugs, especially with all the antibiotic resistance,” says De Roode.
Destroying biodiversity means losing not only plants and fungi with valuable future medicinal uses, but also losing our teachers. “For every lost plant species, we lose another potential drug for infectious disease or cancer. For every lost animal, we lose another potential pharmacist or doctor,” De Roode says. “You can protect nature because you think it’s the right thing to do. Or you can do it because it also helps us, and those things come together.”
• Doctors by Nature – How Ants, Apes and Other Animals Heal Themselves by Jaap de Roode is published by Princeton University Press (£22). To support the Guardian and Observer, order your copy at guardianbookshop.com. Delivery charges may apply.
