Research: viceroys pack a punch too
Ever since I can recall, I've had a childlike fascination with things around me. Be it bugs, birds, plants, whatever, I've possessed an inner passion to know more about what creeps, crawls, flies or walks around me. In this light, I had a visitor to my back yard about three weeks ago I quickly recognized as a viceroy butterfly (Limenitis archippus). It's not that viceroys are uncommon--they're not--but it was the first one I had seen in my yard this year. I remember taking an entomology class at NDSU (was it really over 25 years ago?) and this particular species was held up as a classic example of something called Batesian mimicry. Turns out, they were wrong.
Viceroys are common butterflies over a very large range stretching from the Northwest Territories to central Mexico. They fly about on wings spanning roughly three inches, feeding on carrion, dung, fungus and flowers. Here in North Dakota the species is bivoltine, meaning there are two generations during the season. One flies in late June to early July while the second generation is evident in August through early September.
What makes the adult viceroy more noticeable than others is its striking coloration. It's a dead-ringing imitator of the celebrated monarch butterfly, with rich orange wings webbed with black veins; and black wing edges peppered with white spots. Separating the two species is a matter of noticing size (the viceroy is smaller), underwing intensity (viceroy has rich colors whereas the monarch is noticeably duller underneath), and a black line across the hindwing which the monarch lacks.
Monarchs are well-known for being rather unpalatable to predators. It picks up cardiac glycosides in the larval (or caterpillar) stage by feeding exclusively on milkweeds. It was thought that viceroys mimicked the appearance of monarchs, taking advantage of its toxicity, thereby avoiding predators too. This fit neatly into the Batesian mimicry mold.
I'll try not to get too technical but for a primer on mimicry I turned to a friend, Dr. Gerald Fauske, a research entomologist at NDSU and the manager of the state insect collection. He reminded me that there are several types of mimicry but all share three elements: a model, a mimic, and a dupe. The model, he said, "must have some property such that resembling it is advantageous to the mimic." (e.g. monarch butterfly's toxicity). The mimic simply looks like the model (e.g. viceroy butterfly). The dupe, Fauske said, "is the critter that is being deceived by the mimic (e.g. a potential predator such as a bird).
In Batesian mimicry, the mimic enjoys the protection of the model's noxious property without possessing it itself. "This is the typical monarch/viceroy system as originally described," said Fauske.
Recent research, however, has blazed a new trail. The viceroy's preferred host (plant on which its caterpillars feed) is the willow. Guess where humans first started getting aspirin. That's right, willow bark. Testing has shown that viceroys can contain a "significant amount of salicylic acid (aspirin) and are rejected by birds on their own merits," said Fauske. Hmmm. That doesn't fit the Batesian mimicry definition.
What is taking place instead is Mullerian mimicry. This, according to Fauske, means "multiple models converging on the same color pattern so that any given species is simultaneously a model and a mimic." In such a case, both species benefit and are more likely to survive in a sort of mutualism.
This may sound rather complex but believe me when I say this mimicry business is way more complicated than what is briefly described here. It is interesting, however, to get a taste of what Fauske describes as "one of the many evolutionary mechanisms driving natural selection."
Viceroys, by now, are about done for the year. Unlike the migrating monarchs, viceroy caterpillars are now weaving themselves with silk into curled leaves where they will spend the winter. The little larvae concentrate ethylene glycol (sound familiar? - it's antifreeze) in their bodies, thereby lowering their freezing point.
Look for the adults again next summer. And check out any "monarch" you see for a small one with a black line on the hindwings. That will be a viceroy. Like a monarch, it won't taste very good to predating birds either.