Birds do it. Bees do it. Butterflies and chimpanzees do it.
These animals and many others self-medicate, using plants and other surprising materials to improve not only their own health but also the health of their offspring.Monarch butterflies swarm a tree in Sierra Chincua, Mexico. Photograph by Joel Sartore, National Geographic
A video of capuchin monkeys at the Edinburgh Zoo shows them rubbing onions and limes on their skin and into their fur as an antiseptic and insect repellent. Biologists have noticed that parasite-infected female monarch butterflies are more likely to lay their eggs on anti-parasitic milkweed, giving their offspring instant medication, while uninfected females show no preference. And urban birds who incorporate cigarette butts into their nests may be doing so because chemical properties in the smoked cigarettes may repel parasites, according to a 2012 study.
While cigarette-butt wallpaper may not appeal to most of us, other ways that animals self-medicate might be worth watching. Mark Hunter, a University of Michigan ecologist who was involved in the monarch research, says there is plenty to be learned from observing the way animals use the entire outdoors like one big drugstore. It’s something our own species probably once did—and might do well to revisit with modern pharmaceutical engineering and computer modeling techniques.
“It’s not the only way, but it seems to me that a sensible way [to aid in human drug development] would be to watch what animals do in nature to see how they exploit the natural products, the pharmaceuticals that are available to them in the environment, and try to learn from them,” he says.
Earlier this year, Hunter spent time with people of the Shangaan tribe in South Africa.
“If you go for a walk with somebody, every plant you pass has a cultural or medicinal significance, and many of those have been learned from watching animals,” Hunter says. The bark of the black monkey thorn tree, for example, is used as a stomach medication, a choice based on watching how elephants behave.
Not long ago primates were thought to be the only animals smart enough to self-medicate. Mark Bowler, the San Diego Zoo Global Institute for Conservation Research postdoctoral fellow who made the capuchin video, says that chimpanzees use a range of medicinal plants, “including some that have a physical, not chemical action: They swallow wads of hairy leaves whole, and the leaf hairs appear to physically ‘brush’ certain parasites out of the gut. I tested some of this with Edinburgh Zoo’s chimps, and they seem to do it spontaneously—no learning process involved.”
The capuchins’ behavior in the video—rubbing the body with a particular chemical or scent—is called “anointing,” and other animals do it, too. Bowler is currently studying whether anointing behavior in some other primates is scent-marking or something else, but says that at least in capuchins, the purpose appears to be self-medication. Anointing can also be a form of self-defense: Ground squirrels chew rattlesnake skins and then lick their fur, a trick likely to deter that particular predator.
Insects have been found to be prolific self-medicators, too. Take the arresting case of the fruit fly Drosophilia melanogaster, which uses alcohol to protect itself against parasitic wasps. The wasps lay their eggs in the fruit fly larvae; the developing wasp grubs will eventually eat the flies from the inside out and burst forth from their dead bodies.
Larvae that consume high doses of alcohol from fermented fruits, however, are less likely to be infected—and if they are, the invading wasp grubs die quite nastily with their internal organs being ejected out of their anus. Moreover, fruit fly mothers who see female parasite wasps nearby will give their young instant protection by laying their eggs in alcohol-soaked environments—which means they see and remember their nemesis. (Related: “Flies Use Alcohol to Protect Their Young From Body Snatchers.”)
“Not a bad defense,” says Hunter, adding that this demonstrates the idea that “the cost we’re willing to pay for a medicine depends on the consequences of not using it.” While the alcohol isn’t necessarily good for the flies (though some species of Drosophilia melanogaster show a resistance to its ill effects), the flies will die if parasitized.
“The alcohol has worse effects on the parasites than it does on them. So it’s worth laying your eggs in a high-alcohol environment if it will save your offspring,” he says.
Honey bees self-medicate by protecting their home. The bees traditionally “line their nests with resins that they collect from plants, and those resins contain a wide variety of antimicrobial compounds,” Hunter says. The resulting mix of resins and beeswax is called propolis, and it’s been used as a traditional medication for centuries.
But because beekeepers didn’t want to deal with those sticky resins every time they opened a hive, Hunter says this resin deposition may have been selected out of most commercial bee populations over the years. After the bee genome was sequenced, scientists realized that bees have different immune systems than other insects—“probably because they’ve been relying on this resin,” says Hunter. Disease is one of the many suspected causes of bee die-offs, along with decreased plant diversity (and thus fewer places to get resins from) and pesticides, so encouraging propolis collection is probably a good idea.
Do animals learn to self-medicate, or is it pure instinct? Well, plenty of intelligent animals self-medicate, so it’s not always clear. But in the case of the monarch, Hunter points out, the mothers don’t hang around to see what happens to their babies, so there’s no learning involved. In this case, “the only possibility is that it’s a genetically determined behavior. It’s instinct.”
So the next time you’re on your way to the drugstore and pass a monarch hovering around a milkweed, or a bird who seems to have taken up a smoking habit, consider that they might actually be running an errand, just like you.