For most animals, the milkweed plant is far from appetizing: It contains nasty toxins called cardenolides that can make the creatures vomit and, should they ingest enough, cause their hearts to beat out of control.
Yet some insects appear entirely unfazed by the powerful poison. The monarch butterfly's colorful caterpillars, for example, devour milkweed with gusto — in fact, it is the only thing they ever eat.
They can tolerate this food source because of a peculiarity in a crucial protein in their bodies, a sodium pump, that the cardenolide toxins usually interfere with.
All animals have this pump. It's essential for physiological recovery after heart muscle cells contract or nerve cells fire — events that are triggered when sodium floods into the cells, causing an electrical discharge. After the firing and contracting is done, the cells must clean up, and so they turn on their sodium pumps and expel the sodium. This restores the electrical balance and resets the cell to its usual state, ready again for action.
Cardenolides are noxious because they bind to key parts of these pumps and prevent them from doing their job. This makes animal hearts beat stronger and stronger, often ending in cardiac arrest.
But since animals are under constant competition for food sources, the ability to eat plants that are toxic to others offers a fantastic opportunity, and many insects have evolved ways to do so.
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