Monday, January 27, 2014

The wacky world of mimicry

A showy Monarch butterfly, Danaus plexippus, nectars in a patch of Shale-barren Aster, Symphyotrichum oblongifolium.

This morning, in a fevered bid to conjure warmer times, I posted a photo of an Orange-patched Smoky Moth, and lamented the seemingly unending Arctic air mass that has cloaked our state. It didn't work; the temperature is hovering near 0 F as I write this piece. On the up side, the frigid temperatures and a proper mixture of snow underlain with ice have spawned an absolutely remarkable bounty of a very odd snow formation. I will photograph some of these snowy artworks tomorrow, and post about it here.

Anyway, that Orange-patched Smoky Moth got me thinking about mimicry. The Monarch above was long considered a consummate example of Batesian mimicry. This form of flattery is quite pragmatic. Named for its describer, English naturalist Henry Walter Bates, Batesian mimicry refers to a (more or less) harmless or nontoxic organism that mimics the appearance of a toxic or otherwise dangerous animal. Monarchs are foul-tasting at the least, as their caterpillars ingest and sequester nasty cardiac glycosides from their host milkweed plants. Birds and other predators quickly learn to shun the butterflies (and their caterpillars).
 
More than a few people have misidentified this butterfly with the Monarch. The mistake would be a reasonable one - the similarities are astonishing. This is a Viceroy, Limenitis archippus, and despite its close appearance, it is quite distinct from the Monarch.

Monarchs and Viceroys were long held up as an example of Batesian mimicry. The harmless Viceroy evolved much of the morphological characters of the toxic Monarch in order to fool predators, and thus gain a significant measure of protection from its foes.

Well, scientists have come to learn that Viceroy butterflies are not so inert after all. In fact, Viceroys are thought to be as foul if not more so than the model of its mimicry. Viceroy caterpillars feed on willows and other trees in the Salicaceae family. These plants are infused with salicylic acid, and the caterpillar concentrates this compound in its body, much of which apparently is passed to the adult stage. Thus, the butterfly is bitter and distasteful.

So much for Batesian mimicry, in this case. Enter Mullerian mimicry, which leads us to the aforementioned Orange-patched Smoky Moth.

Here it is again, the beautiful Orange-patched Smoky Moth, Pyromorpha dimidiata.This insect does not really look like a moth, and if you are into bugs, its appearance might remind you of something else...

A Net-winged Beetle! We have a few species of these pretty black and tangerine bugs in Ohio, and this one is Calopteron terminale, which is common throughout the state. The similarity between this beetle and the utterly unrelated moth is fairly startling, and their close resemblance is probably not a coincidence.

The beetle-moth relationship would seem to be a good example of Mullerian mimicry.

Named after German naturalist Fritz Muller, this form of imitation describes poisonous species that have evolved a similar appearance - quite different than Batesian mimicry, in which one of the organisms (the mimic) is more or less harmless and takes on the looks of a dangerous organism (the model).

In Mullerian mimicry, both mimic and model theoretically benefit - this would be an example of mutualism. In essence, each species is aiding the other by adding to the collection of similar-appearing creatures that predators learn to avoid by sight because of their toxicity. Of course, a predator must learn the hard way - at least until avoidance becomes encoded in its genetics - and thus there must be sacrifices by the Mullerian mimics. If both moth and beetle serve equally well as an example of what not to bite into, there should in theory be fewer sacrificial individuals given up from each species' population in order to train predators.

Net-winged beetles contain toxic compounds, as might be guessed by their black-and-orange warpaint, which serves as a visual advertisement of toxicity (although predators still apparently must sometimes learn this firsthand). The smoky moth also is know to be infused with a nasty substance, hydrogen cyanide.

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