Why are monarch butterflies distasteful to many vertebrate predators?

Monarch butterflies are distasteful to vertebrate predators because of their diet during the larval stage. Monarch caterpillars feed exclusively on milkweed plants, which contain toxic compounds known as cardenolides.12

The caterpillars store these toxins in their bodies, retaining them through the pupal stage and into adulthood. Cardenolides are cardiac glycosides that have a specific toxic effect on the vertebrate heart and trigger the brain's vomiting center. Because the dose required to induce vomiting is lower than the lethal dose, predators that sample a monarch typically survive and learn to avoid them in the future.3

To reinforce this defense, monarchs utilize aposematism, displaying bright orange and black wing patterns that serve as a visual warning of their toxicity. This strategy is further supported by Müllerian mimicry, where monarchs and other unpalatable species, such as the viceroy butterfly, share similar coloration to help predators more easily recognize and avoid the danger.145