Animal Self Medication: Monarch Butterflies, Chimpanzees, Medicine

Jaap de Roode, a biologist at Emory University, specializes in infectious diseases and parasites, particularly in monarch butterflies and honeybees. His work delves into how animals, including insects, utilize medicinal properties from plants, challenging the long-held belief that such knowledge is exclusive to humans.

Monarch Butterflies and Medicinal Milkweed

Monarch butterflies are famous for their spectacular migration from Canada and the United States to Mexico. During this journey, they fuel up on nectar from various plants, such as saltbush. Like humans and pets, monarch butterflies are susceptible to infectious diseases, including one caused by a parasite similar to human malaria parasites.

De Roode's research began when he observed that monarch butterfly caterpillars exclusively feed on milkweed. Milkweed species vary in toxicity, with some being highly poisonous and others less so. Traditionally, scientists believed these toxins primarily protected monarchs from predators, with their bright colors signaling toxicity. However, de Roode, having previously studied malaria, hypothesized that these toxins might also act as medicine.

His team conducted experiments where caterpillars were reared on both toxic and non-toxic milkweed. They discovered that caterpillars consuming more toxic milkweed were less likely to become infected by parasites and, if infected, experienced less severe illness. This suggested that monarchs might be using these plants medicinally.

Monarch butterflies are highly selective when choosing where to lay their eggs. Females taste plants using their antennae, feet, and specialized legs to determine suitability. De Roode's team investigated whether infected female butterflies would preferentially lay eggs on medicinal, toxic milkweed to benefit their offspring. They found that while caterpillars showed no preference, infected mothers did choose to lay eggs on the medicinal milkweed, effectively providing medicine to their future offspring. This remarkable behavior highlights a form of "maternal medicine."

Challenging Anthropocentric Views of Medicine

Initially, the idea that butterflies, with their tiny brains, could possess medicinal knowledge was met with skepticism. This skepticism stemmed from the prevailing scientific view that medicine, like language and tool use, was a uniquely human trait.

The earliest evidence of animals using medicine emerged in the 1980s with chimpanzees. Mike Huffman, a researcher from Kyoto University, along with a traditional healer and park ranger in Tanzania, observed a chimpanzee named Chausiku exhibiting unusual behavior indicative of illness. They witnessed her consuming the inner pith of a specific shrub. The traditional healer recognized this plant as one used in human medicine for infectious diseases. Chausiku recovered within 24 hours, the same timeframe traditional healers expected the medicine to act.

Subsequent studies confirmed that this plant contained toxins effective against parasitic worms. Chimpanzees with intestinal worms were observed to consume these plants more frequently, especially during the wet season when worm infestations are higher. This evidence solidified the understanding that chimpanzees were indeed self-medicating. This discovery was controversial, as it challenged the notion of human uniqueness in Western science.

Three Ways Animals Use Medicine

De Roode identifies three primary ways animals utilize medicine:

1. Instinct (Genetic Predisposition): This is an innate response, often without conscious awareness of illness or the medicinal properties of the plant.

   • Example: Woolly Bear Caterpillars: These caterpillars are susceptible to parasitoid flies, which lay eggs that hatch into maggots, consuming the caterpillar from the inside. Scientists found that when infected, these caterpillars specifically seek out and consume plants rich in alkaloids, which kill the fly maggots. Experiments with tiny electrodes in the caterpillars' taste receptors revealed that when infected, their neurons respond more strongly to alkaloids, making these compounds "taste better." This leads them to consume more, effectively self-curing without needing to understand the process.

2. Individual Learning: Animals learn through experience that certain plants alleviate their symptoms.

   • Example: Sheep, Goats, and Cattle: Research at Utah State University has shown that these animals, when suffering from intestinal worms, consume foods rich in tannins. Tannins, also found in red wine, combat these parasites. When animals experience relief after consuming tannins, they form an association. The next time they feel sick, they actively seek out plants containing tannins.

3. Social Learning: Animals learn medicinal practices from each other, passing knowledge down through generations.

   • Example: Chimpanzees and Leaf Swallowing: Jane Goodall first observed chimpanzees consuming whole, undigested leaves in the 1960s. Later, Mike Huffman and colleagues discovered that chimpanzees fold and swallow rough leaves, which scrape parasites from their guts, inducing diarrhea to expel the worms. Different chimpanzee groups exhibit distinct leaf-folding methods, indicating social learning. Infants learn these techniques from adults, ensuring the knowledge is passed on.

Pets and Wild Carnivores: Grass Eating

Even domestic pets exhibit self-medicating behaviors. Many dog owners observe their dogs eating grass. While research on this specific behavior in pets is limited, observations of wild carnivores like mountain lions, wolves, and coyotes provide clues. These animals, despite being primarily meat-eaters, often consume grass. Field biologists have noted that after eating grass, these carnivores frequently vomit or pass feces containing undigested grass and parasitic worms. This suggests a similar mechanism to the chimpanzees' leaf swallowing, where grass helps expel intestinal parasites.

Domestic dogs and cats, having descended from wild ancestors, likely inherited this behavior. While modern pets often receive deworming medication, de Roode suggests that grass eating in pets might be a response to general discomfort rather than just worms. He recounts an instance where his dog, after swallowing half a rubber band, ate grass and later vomited the object, demonstrating a self-medicating response to discomfort.

Animals as Inspiration for Human Medicine

The study of animal self-medication is not merely a scientific curiosity; it has profound implications for human medicine. De Roode notes that many "discoveries" in modern medicine are actually "rediscoveries," as traditional healers and shamans have long observed animals for medicinal insights.

• Aspirin: The origins of aspirin can be traced back to bears. After hibernation, bears, feeling stiff and sore, consume the bark of willow trees. Willow bark contains salicylic acid, the active chemical in aspirin. Humans in the northern hemisphere observed this behavior and began using willow bark for similar ailments. In the 1700s, this knowledge was rediscovered, leading to the purification of salicylic acid and the development of aspirin by the pharmaceutical industry.

• Mosquito Repellent: More recently, scientists have looked to cats for new medicinal compounds. Domestic cats famously roll in catnip and silver vine, plants that induce a euphoric response. Japanese scientists discovered that this behavior coats cats in chemicals that act as a potent mosquito repellent. This protective behavior is not limited to domestic cats; large cats in zoos also exhibit it, suggesting a behavior evolved over millions of years. The chemicals found on cats were tested on humans and proved effective as a mosquito repellent, leading to a patent for a new human mosquito repellent.

These examples underscore the vast, untapped knowledge within the animal kingdom. By acknowledging animals' sophisticated understanding of medicine, humans can continue to discover new chemicals and develop novel drugs for their own use.