Animals as Medicine Makers: Monarch Butterflies, Milkweed and the Evolution of Animal Pharmacology

Summary

In this Royal Institution talk, Jaap De Rode shows that animals can be knowledgeable about medicine, using plants and behavior to influence infections and health. He uses monarch butterflies and their milkweed diet to illustrate how toxins can act as medicines against parasites, and describes greenhouse experiments that test whether mothers or offspring choose medicinal plants. The talk also reveals parallel examples from woolly bear caterpillars, sheep and goats, chimpanzees, and domestic pets. It ties these natural practices to human medicine, including the origin of aspirin from willow bark and plants that reduce mosquito bites in cats. The message is that nature can inspire new drugs and therapeutic strategies for human use.

• Monarch milkweed toxins may reduce parasite infection in caterpillars
• Medicine can be instinctive, learned or socially transmitted in animals
• Traditional knowledge across species underpins modern medicine
• Human medicines often reflect observed animal and plant remedies

Introduction

The Royal Institution talk by Jaap De Rode examines a provocative idea in biology and medicine: animals may know how to use medicines from their environment. Through the lens of monarch butterflies, De Rode explains how caterpillars feed exclusively on milkweed and how the plant’s toxins can influence infection by parasites. This leads to a broader discussion of how animals might use medicinal compounds to combat disease and how that could inform future drug discovery for humans.

Monarchs and Milkweed

De Rode details experiments in which monarch caterpillars are rear ed on different milkweed species, ranging from highly toxic to less toxic. The results show that more toxic milkweed reduces the likelihood of infection and, when infection occurs, disease severity is lower. The core idea is that the toxins obtained from milkweed can function as a natural medicine for the butterflies, potentially helping them weather parasite pressure during migration. Importantly, monarch mothers are careful about where to lay eggs, a behavior that could influence the medicinal environment of the next generation. Tests in greenhouse flight cages reveal that infected versus uninfected mothers can influence offspring exposure to these medicinal plants, while caterpillars themselves show no clear preference when given a choice, suggesting that maternal input is crucial for provisioning potential medicines to offspring.

Three Ways Animals Use Medicine

The talk outlines three general pathways by which animals may use medicine. First is instinctive or genetic programming that drives certain medicinal behaviors. Second is individual learning, where animals associate relief from illness with certain foods, such as tannin rich plants for gut parasites in grazing livestock. Third is social learning, where animals observe and imitate others, as seen in chimpanzees using bitter plant pith or rough leaves to expel parasites from their guts. Each pathway highlights how animals interact with their environment to mitigate illness without human intervention.

Additional Illustrative Examples

Beyond monarchs, De Rode discusses woolly bear caterpillars that increase intake of alkaloid rich plants when parasitized, and the neural basis for this taste modulation discovered through neural recordings. He also covers livestock examples where tannins provide relief from gastrointestinal worms through learned associations, and Jane Goodall era observations of chimpanzees swallowing folded leaves to physically irritate the gut and expel parasites. These examples emphasize that medicine can be encoded in behavior, not only in human pharmacology.

Domestic Animals and Everyday Medicine

The discussion turns to dogs and cats that engage in medicinal behaviors such as eating grass or rolling in certain plants. While not always a direct response to worms, these actions appear to reflect discomfort relief and ancestral ingrained behavior from wild ancestors. De Rode cautions that selecting grasses free of pesticides is important when allowing pets to engage in self medication, but he argues that animal instincts and experiences can reveal valuable insights about health management in homes.

Aspirin and the Translational Arc

The talk closes with a reflection on aspirin as an example of animal and plant knowledge guiding human medicine. Willow bark, used by ancient humans for pain relief due to salicylic acid, led to the modern drug discovery process sponsored by the pharmaceutical industry. De Rode notes that traditional healing systems and shamans have long observed animal strategies for health and that contemporary medicine has repeatedly rediscovered these natural sources of insight. He also highlights recent work on mosquito repellent chemistry inspired by cat and other animal responses, underscoring the ongoing collaboration between animal behavior and human health needs.

Implications for the Future

De Rode argues that accepting animals as knowledgeable about medicine broadens our approach to discovering new pharmaceuticals. This perspective does not diminish the need for scientific validation, but it broadens the search space for medicines, including compounds from milkweed and other plants, as well as understanding how maternal provisioning influences offspring health. The talk invites researchers and clinicians to consider animal wisdom as a source of inspiration and a partner in the ongoing quest to develop safer, more effective medicines for human use.

Conclusion

The overarching message is that nature holds a wealth of information about medicine. By studying animals and their interactions with medicinal plants, we can uncover new chemicals and strategies that may lead to novel therapies for humans. The talk underscores the value of cross species learning and the integration of traditional knowledge into modern pharmacology.