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Many of our medicines originate in plants. But we’re not the only ones who tap into nature.There’s increasing evidence that some animals seek out specific plants for their medicinal qualities. A new study shows that monarch¹ butterflies may be among them.

Monarch butterflies—famous for their U.S.–Mexico migration—feed on dozens of species of milkweed. Some milkweeds have a higher concentration of chemicals called cardenolides, which make the butterflies toxic² to predators³. Research had focused on whether the monarc’s chose different milkweeds to hurt their enemies.

But Emory University’s Jaap de Roode wondered if the choice might be related to a parasite⁴ that’s passed from mother to offspring, with fatal results: “What we do find is that the monarchs⁵ prefer to lay their eggs on the medicinal species when they are infected. However, when they are not infected with the parasite, they do not prefer this species over this one, they lay their eggs equally between these two species. So somehow they know that they’re infected and they know what to do about it.” The research was published in the journal Ecology Letters.

Co-author Mark Hunter notes that watching animal choices could offer clues about what plants might contain potential medicines for us.