Imitation Isn’t Always Flattery

Jeff Mitton

Natural Selections (appeared July 28, 2006 in the Daily Camera)

            Appearances can be deceiving, particularly when it comes to butterflies.

            Some butterflies, such as the eastern tiger swallowtail, are a tasty source of nutrition for a wide variety of birds. Butterflies tend to be brightly colored, and they fly slowly, so they are easy prey for birds.

            Many species of butterflies are unpalatable. For example, the monarch butterfly specializes on milkweeds, which are loaded with plant defensive compounds called cardiac glycosides. Monarch caterpillars feed on milkweed leaves, and sequester glycosides into their tissues. Adult monarchs consume nectar, but they are protected by the glycosides they consumed as caterpillars.

            Birds, like other animals, learn what is good to eat, and what is not. When birds consume monarch butterflies, they encounter a nasty taste, and the glycosides are emetic, meaning they induce vomiting. That bird will never attack a monarch again, and furthermore, it will avoid butterflies that resemble monarchs.

            Batesian mimicry is the evolutionary convergence in appearance of a palatable species to an unpalatable species. Mimicry probably begins as a fortuitous, vague resemblance that protects the palatable butterflies that most resemble the unpalatable species, called the model. The lucky few that are most resemble the model escape predation, and their offspring benefit from their inherited resemblance to the model. As time, predation, and natural selection proceed, the palatable species, called the mimic, evolves to resemble the model more precisely.

            The eastern swallowtail butterfly, Papilio glaucus, demonstrates some of the intricacies and the evolutionary dynamics of a mimicry system. Eastern swallowtails are large, four to five inches across, and they are colorful. The males are yellow, with black stripes on both the forewings and the hindwings. Most of the females are quite similar, but have brilliant blue spots on the trailing edges of their hindwings. However, some of the females look completely different.

            The dark morph of the eastern swallowtail is mostly black, with small yellow dots on the trailing edges of the forewings and hindwings, and bright blue and orange on the hindwings. The black females do not occur throughout the full range of the eastern swallowtail, but are common within the range of another black butterfly, the pipevine swallowtail.

            Pipevine swallowtails specialize on plants called pipevines, genus Aristolochia, which are defended by chemical compounds that make them distasteful if not poisonous. Larval pipevine swallowtails consume and sequester the compounds, so birds learn to avoid both pipevine caterpillars and butterflies.

            Where eastern tiger and pipevine swallowtails are sympatric, the dark forms of the eastern tigers are avoided by any birds that have tried to eat pipevine swallowtails. In contrast, all of the male and the yellow female tiger swallowtails are eaten with relish and impunity. Consequently, the black females become common where pipevine butterflies teach bird to avoid black butterflies. In areas where there are no pipevine butterflies, the black females have no advantage, and are rare or absent.

            The protection from predation provided by resemblance to pipevine butterflies has driven evolution of mimicry in other species as well. Eastern black swallowtails, spicebush swallowtails, and red-spotted purples all mimic pipevine butterflies.

Photo legend:

This black form of the eastern tiger swallowtail is a mimic of the poisonous pipevine swallowtail.

Photo by Jeff Mitton