“I didn’t start working with monarchs because I liked them,” says evolutionary biologist Jaap de Roode of Emory University. “I came to them because they have a really cool parasite.” That parasite, called Ophryocystis elektroscirrha
, normally pokes holes in the butterflies’ skin, causing them to leak bodily fluids. But de Roode noticed that monarchs that ate the tropical milkweed plant did not suffer from parasitic infections as much as monarchs eating swamp milkweed did. This led him to suggest to his colleagues that the monarchs were self-medicating.