Animals may not write prescriptions, understand Latin, or have a good bedside manner, but they are experts in medicine.
What’s more, studying animals’ sophisticated self-medication techniques promises to reveal new ways we can improve human health – from fending off mosquito-borne disease to developing new drugs for cancer.
Take Chausiku the chimpanzee. Michael Huffman, an associate professor of primatology at Kyoto University, Japan, was tracking a group of chimpanzees in Tanzania, studying their behaviour, when he noticed the 29-year-old female doing something he hadn’t ever witnessed.
Chausiku, who looked unwell, removed a branch from a particular leafy bush, peeled off the bark and leaves, and – after chewing on it for a few seconds – sucked on the pith (the inner part of the branch) and swallowed the juice, spitting out the fibrous parts.
Known by scientists as Vernonia amygdalina, the plant’s common name is bitter leaf. As the name implies, it is quite inedibly bitter and Professor Huffman had never seen any chimpanzees feed on it.
A new idea struck him: could Chausiku be using the plant as medicine?
Together with Mohamedi Seifu Kalunde, a senior game officer at Tanzania National Parks, Professor Huffman saw Chausiku climb a tree and build a nest – not at a time this would usually be done.
After a long rest, Chausiku left the tree and then stopped to chew on the bitter-leaf plant.
Animals may not write prescriptions, understand Latin, or have a good bedside manner, but they are experts in medicine
‘My colleagues said chimps don’t get ill,’ says Professor Huffman. ‘But all the signs started to make sense to me. Making the bed, spending time in it. She could not walk very long without stopping to rest.’
She also had no appetite and produced dark urine. Chausiku was clearly sick.
The next morning, Chausiku was still moving slowly. But after the group’s noon nap, she suddenly stood up and took off.
It was clear from their observations that Chausiku had fallen ill with a stomach parasite infection, then used a specific plant as a treatment – and recovered within 20 hours.
But Professor Huffman needed evidence to persuade his colleagues that Chausiku was truly using the plant as medicine.
This was back in 1987 – and the idea that animals knowingly self-medicate was viewed with some scepticism.
Some scientists use the word ‘zoopharmacognosy’ – the root words being zoo (animal), pharma (drug), and gnosy (knowing) – Professor Huffman’s observations were a key early breakthrough in this area.
On his return to Japan, the professor handed a bag of dried Vernonia amygdalina leaves to Kyoto University’s experts in plant chemistry.
Extracting chemicals from the leaves revealed a number of sesquiterpene lactones. These are a class of chemicals known for their toxicity and medicinal value against parasitic worms, amoebae and bacteria – as the Kyoto experts reported in the journal Bioscience, Biotechnology and Biochemistry in 1993.
Since then, laboratories in the US and Malaysia have found that vernodalin, extracted from the bitter plant used by Chausiku, also has anti-tumour effects that may make it a potent anti-cancer drug. A patent for its application to treat breast cancer was awarded in 2004, though a clinical drug has yet to be developed.
Last year an international team of scientists , whose research was published in the journal PLOS One, suggested that chimps could help us with the search for new medicines.
They tested 17 samples from 13 different plant species used by injured or ill chimps in the wild and found that almost 90 per cent of the extracts inhibited bacterial growth. A third had anti-inflammatory properties.
Some might argue that our modern chemistry and technology should equip us well enough to come up with new drugs from scratch.
But consider this: over the past 40 years, more than half of the new antibacterial drugs and 45 per cent of the antiparasitic drugs that hit the market have been derived from natural products.
These include compounds from plants, bacteria, and fungi – all of which are used by all sorts of animals to fight infections and alleviate disease.
People have long looked at animals to gain medicinal wisdom.
According to legend, in what is now Ethiopia in 850AD a herder noted his goats became friskier after consuming coffee berries.
This led an abbot at a nearby monastery to roast the berries and brew them into a drink that many of us now happily depend upon to get through the day.
Similarly, you may have heard of horny goat weed, a herb in the genus Epimedium. Legend has it that a Chinese goat herder noticed that goats eating this herb became, well, you can guess.
Indeed, laboratory studies have shown that when rats feed on icariin, a flavonoid chemical extracted from this herb, they produce more sperm.
Even rats that have been castrated, and therefore should be impotent, manage to have proper erections when given icariin, according to lab studies by scientists at Peking University First Hospital, and Chongqing Medical University, both in China.
The herb was first recorded as a treatment for impotence in China 400 years ago. And although there are few clinical trials behind it, it’s now sold as a herbal supplement for libido.
We know that animals have beaten us to discovering some of the world’s most important drugs.
For thousands of years, it appears that bears the world over have been using a common painkiller when waking up achy after long months of hibernation.
In spring, when they leave their dens, they start eating willow bark, willow buds and meadowsweet shoots – which all contain salicylic acid, the chemical compound that’s the basis of aspirin.
As well as reducing pain caused by lying still for so long, salicylic acid can flush out painful excess uric acid that’s built up in their blood and tissues. Modern civilisation discovered salicylic acid only when Edward Stone, an English vicar, reported that willow bark could be an effective anti-fever agent in 1763. The German pharmaceutical company Bayer eventually launched the wonder drug aspirin in 1899.
So what else do animals know that could help us?
Much closer to home than wild bears, our domestic cat’s favourite drugs may offer us a new weapon in the fight against mosquito-borne diseases such as malaria, yellow and dengue fevers.
Cat lovers will be used to seeing their pets’ frenzied response to catnip (Nepeta cataria) and silver vine (Actinidia polygama). They will start rolling over and rubbing the leaves into their face and fur in joyful play.
At Iwate University in Japan, Masao Miyazaki, a professor of chemical biology, began to investigate why silver vine leaves excite cats so much.
In 2013, Professor Miyazaki extracted the chemicals in silver-vine leaves, separating them into six samples, each containing different mixes of constituent compounds.
Most did not send cats into a frenzy, but one elicited a very strong response. Further analysis suggested the compound responsible was a chemical named nepetalactol. Next, in 2018, Professor Miyazaki and student Reiko Uenoyama studied cats’ opioid systems – which regulate feelings of reward and euphoria in mammals.
The results showed that when cats are exposed to nepetalactol, their rolling and rubbing – as well as their levels of feelgood brain-chemicals – rose significantly.
Professor Miyazaki persuaded collaborators at the local zoo to run similar tests with a leopard, two jaguars and two lynx.
All of these big cats happily rubbed their faces on samples of nepetalactol and rolled around in them.
Domestic cats and wild big cats share a common ancestor that lived around ten million years ago. These results suggest that the response to nepetalactol evolved early on, suggesting it was important for the cats’ survival.
But why? Professor Miyazaki hypothesised that the cats rub the chemical on their skin as a way of protecting themselves from insect bites.
Mosquitoes, in particular, could transmit dangerous pathogens and were therefore the most likely threat to the cats’ survival.
In the wild, the blood-sucking insects transmit numerous parasites, including heartworms that cause potentially fatal infections in the lungs and heart.
Professor Miyazaki went on to find that cats that had rubbed nepetalactol on their fur suffered only half as many mosquito bites as unprotected cats.
He felt it only fair to repeat the experiment on humans – himself included. ‘We did the experiment with our own arms,’ he says. ‘Only one arm had nepetalactol. We put both arms in a cage filled with mosquitoes. We found that nepetalactol also protects our skin from mosquito bites.’
Convinced by their findings, Iwate University and Nagoya University filed a patent on their newly found substance.
They are now seeking a pharmaceutical company that can turn the cat-discovered chemicals into an effective mosquito repellent for humans.
As Professor Miyazaki and Reiko Uenoyama showed, by studying animal behaviours in detail we may find whole new types of chemicals that we humans can use to protect ourselves from some of the most dangerous diseases in the world.
- Adapted from Doctors by Nature by Jaap de Roode (Princeton University Press, £22), published on March 4.
- © Jaap de Roode 2025. To order a copy for £19.80 (offer valid to 11/03/25; UK P&P free on orders over £25) go to mailshop.co.uk/books or call 020 3176 2937.
