Which chemical compounds make monarch butterflies distasteful to many predators?

Cardenolides make monarch butterflies distasteful to many predators. Monarch caterpillars obtain these cardiac-glycoside compounds by feeding on milkweed plants, which contain cardenolides in varying concentrations.

The chemicals can remain in the monarch’s body through metamorphosis, so the adult butterfly may retain the plant-derived defense. Predators such as some birds can become ill after eating monarchs, encouraging them to avoid the butterfly’s conspicuous orange-and-black pattern.

This relationship is a classic example of chemical defense and warning coloration. It is not accurate to assume that every monarch contains exactly the same amount of cardenolides: milkweed species, plant condition, caterpillar behavior, and the individual insect all affect the final concentration.

Monarchs are also involved in mimicry discussions because the viceroy butterfly has a similar orange-and-black appearance. The precise balance between shared warning signals and independent defenses has been studied extensively by biologists.