A poisonous diet gives these animals their own toxic defense

By Brian Handwerk | Smithsonian Magazine |

SMITHSONIAN MAGAZINE - You are what you eat, the old saying goes, and that holds true for many animals that regularly ingest poison. For certain species that feed on toxic fare like plants and insects, not only do the poisonous meals do these creatures no harm, but the consumers actually co-opt the toxins. They become poisonous themselves for protection from parasites or predators.

Monarch butterflies are beautiful backyard favorites—but they’re also toxic. Monarchs lay their eggs on milkweed plants, where caterpillars devour leaves containing poisonous cardiac glycosides compounds. The monarchs store these toxins in their own bodies to deter predators—and the insects’ appealing black and orange color signals this toxicity.

The monarchs evolved to digest the toxic plant through just a few key genetic mutations—ones that scientists have recently uncovered. Unfortunately for the butterflies, some of their predators, including black-headed grosbeaks and eastern deer mice, have also evolved mutations enabling them to devour the insects and tolerate the toxins.

“It looks like in this situation there might be an evolutionary arms race up the food chain,” says Simon Cornelis “Niels” Groen, an evolutionary systems biologist at the University of California, Riverside.

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