Monarch butterflies (Danaus plexippus) are a common sight in Colorado during the summer months. These bright orange and black butterflies specialize on milkweeds (Asclepias species, family Asclepiadaceae); females lay their eggs and caterpillars feed only on these plants. Milkweeds contain chemical compounds, called cardiac glycosides or cardenolides, that are toxic to most animals, but not to monarchs. Not only can the monarchs tolerate these compounds in their food, but they also have the ability to store these compounds; thereby making themselves poisonous and bad tasting to their own predators. Both the caterpillars and the adults advertise their bad taste by being very brightly colored (warning coloration): caterpillars are striped black, yellow and white; adult butterflies are orange and black.
Cardenolides, such as digitalis, affect the vertebrate heart. They also affect the vomiting center in the brain. Humans use digitalis for heart problems and dosages must be carefully calculated so that they have the appropriate effect on the heart but do not cause vomiting. For predators of the monarch, such as birds, the cardenolides stored by the monarch also induce vomiting; otherwise, the dose of cardenolides contained in a single butterfly could kill such predators.
Monarchs are easily mistaken for another butterfly species, the viceroy (Limenitis archippus). Viceroy butterflies look very similar to the Monarchs and they too taste terrible to predators. The caterpillars do not feed on milkweeds, however; they feed on willows and poplars and store a different kind of chemical compound from these plants. When two different species that are toxic have similar coloration, it is called Mullerian mimicry.
Another fascinating part of the biology of Monarchs is that they migrate long distances in the spring and fall. Monarchs cannot survive cold winters, thus every fall they migrate from as far north as Canada down to overwintering sites in Mexico or the coast of California. Individual butterflies may travel as far as 1800 miles! Each fall, the Monarchs start their southward journey; overwinter; and in February and March, begin the northward migration. Overwintering butterflies from Mexico migrate north and repopulate the southern United States, lay eggs, and die. The next generation then moves further north, and the next generation even further. Monarchs are the only butterfly species that migrates for such long distances.
For more information about monarch migration and to find out how you can help scientists understand this remarkable migration, check out Monarch Watch
In the Museum's BioLounge
exhibit, you can learn more about the monarch butterfly and see actual specimens.