LAST MAY, A SUMATRAN ORANGUTAN in Indonesia’s Gunung Leuser National Park grabbed headlines when researchers revealed he’d used a plant to heal a wound under his eye. The orangutan, known as Rakus, repeatedly treated his wound with juice he had extracted from the chewed leaves of a vine. He also used the solid part of the plant mash as a dressing, apparently to protect the wound from flies. The plant he selected, Fibraurea tinctoria, is recognized for its antibacterial and anti-inflammatory properties in humans. For Rakus, infection never set in, says Isabelle Laumer, a primatologist with the Max Planck Institute of Animal Behavior in Germany, and within five days, his wound closed. A month later, Rakus was completely healed, Laumer and her colleagues wrote in the journal Scientific Reports.

Most scientists studying self-medication focused first on big-brained primates, assuming that it takes some smarts to figure out how to use medicinal plants. “But insects are showing us that’s simply not the case,” says Emory University biologist Jaap de Roode. De Roode has discovered that if given a choice, monarch butterflies infected with a parasite that hampers their ability to fly will lay 68 percent of their eggs on milkweed with high cardenolide compounds. The same toxic chemicals that protect monarchs from predators, cardenolides also have antiparasitic powers. Mother monarchs seem to know how to give their offspring a fighting chance by laying eggs on what de Roode terms “medicinal milkweed.” When their caterpillars hatch and start eating the leaves, the larvae ingest the compounds that fend off parasites.