Monarch caterpillar on common milkweedFLICKR, USFWS MIDWESTMonarch butterfly caterpillars have evolved the ability to store toxins known as cardenolides, obtained from their milkweed diet, specifically to make themselves poisonous to birds, as has at least one other species of milkweed-munching caterpillar, according to a study published Wednesday (November 4) in Proceedings of the Royal Society B.
“This finding is fascinating and novel,” Stephen Malcolm, a professor at Western Michigan University who studies cardenolides but was not involved in the new research, wrote in an email to The Scientist. “It is exciting to have evidence for the importance of top-down influences from predators.”
Scientists have long known that milkweed cardenolides, which in most animals disable a vital sodium-potassium pump enzyme if they are absorbed into the blood, serve to make caterpillars and butterflies dangerous meals for their predators, but whether that acquired toxicity was a side effect of an adaptation that allowed monarchs to eat milkweed or had developed separately as a defensive mechanism was unclear. To distinguish between these possibilities, Cornell University’s Anurag Agrawal and Georg Petschenka tested three milkweed-loving species ...
