Butterfly in the Sky

Mimicry is Not Always Straightforward

Monarch by John Flannery, Flickr

The monarch, Danaus plexippus, is one of the most recognizable North American butterflies, ranging from southern Canada through the United States and Mexico into Central and northern South America. It can also be found in a variety of Atlantic and Pacific islands. It is a beautiful orange and black with white spots. This coloration serves as a warning signal, similar to the ones mentioned in the first two stories this season. The bright colors are an indication for predators that the monarch is unpalatable.

The chemical defense of the monarch is slightly different from those earlier examples. Firstly, coral snakes and bees produce a toxin that is injected into another organism. This makes them venomous. The butterfly has no injection mechanism, but rather the toxin takes effect when it is eaten, so it is poisonous. Also, the monarch does not produce this poison itself. The caterpillar stage of development feeds on milkweed plants, genus Asclepias, after the eggs are laid on the underside of milkweed leaves.

Common Milkweed by Hardyplants, Wikimedia Commons

These plants contain steroid compounds that the caterpillars store in their bodies and retain even after their metamorphosis. Ingesting these compounds can cause cardiac arrest in potential predators, though there are some bird species that have developed a tolerance towards these compounds and will still predate on the butterfly.

The monarch is also a well-known example of mimicry with another butterfly in the same Family, the viceroy, Limenitis archippus. This species is not as widespread as the monarch, but can still be found throughout much of North America. The wing coloration of the two species is nearly identical, making distinctions difficult for predators.

Limenitis archippus by Benny Mazur, Wikimedia Commons

For a long time, it was believed that these two butterflies exhibited a similar type of mimicry to those described previously this season. In this case, the monarch would be the unpalatable model and the viceroy would be the mimic taking advantage of its similarity to such a species. However, more recent research has led to a minor dispute among scientists about the exact form of mimicry displayed by these butterflies.

There are a variety of factors that have contributed to this discussion. Firstly, monarch adults exhibit varying toxicity based on the species and amount of milkweed plants consumed during their caterpillar stage. This makes the protection of the warning coloration less useful if a predator first attacks a less toxic individual. Furthermore, research into the viceroy has shown that it may be much less palatable than originally thought. When exposed to butterfly abdomens (the central part, lacking the wings), birds would reject viceroy abdomens as food after a single peck and display distress behaviors.

The viceroy likely acquires it unpalatable nature in the same manner as the monarch. As caterpillars, they mainly feed on plants in the willow family (willows, poplars, and cottonwoods). The caterpillars can store salicylic acid from these plants in its body. Those of you who are familiar with the history of aspirin and its derivation from willow bark may recognize that name. Storing large amounts of this acid makes the caterpillar and butterfly taste highly bitter and can cause stomach upset in predators.

The fact that predators would also avoid the viceroy as unpalatable makes it and the monarch co-mimics, instead of a mimic and a model. The mimicry displayed by these butterfly species is slightly different than other types, since each of the two would have converged on the similar appearance and received a benefit from it. Both would have increased survival chances against predators because a potential predator would only have to encounter an individual from either species in order for it to avoid both in the future.

As with other defensive mimicry stories, imagine yourself as a predator that eats butterflies. You encounter a bright orange and black individual, but have an unpleasant experience when you try to eat it. Going forward, are you going to be able to tell the difference between that butterfly, whichever species it was, and the other species? Would you be willing to risk another bad experience to try? In this way, both the viceroy and the monarch gain the use of their color signaling with fewer predator attacks on either.

As I said in my introduction to Season 4, there are many different types of mimicry, each with a different name and often with only subtle differences between their mechanisms. I wanted to share this particular story to demonstrate the diverse nature that mimicry can take as well as show that our understanding of the natural world is always changing and improving. Going back to my initial post on why I was starting this series, science is not just about learning facts. It is about discovery and constantly testing, learning, and refining our understanding. I hope that my stories inspire some of you to keep watching, listening, and learning because there is always something fascinating and new to see.

If you like my stories, please like, subscribe, and share them. If you have an idea for a topic for a future season, let me know and if I use it, I’ll give you a shout out in my introductory post.