Our Window on Nature

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Almost Twins

Filed under: Bugs — Lowell Christie -- June 21, 2015 @ 1:03 pm

Viceroy_ButterflyWe spend a lot of our time outdoors trying to identify what we see, and sometimes this can be quite difficult. During spring migration the look-alike flycatchers give us fits. Mushroom hunting can cause similar problems, but fortunately we’re usually after photographs, not dinner.

But once we’ve made a difficult identification our next question is usually “Why?” Why do distantly related plants and animals often seem to mimic each other? We thought we had learned most of the reasons many years ago, but recent research is making us rethink our understanding of certain types of mimicry in nature.

In the late 1800’s the English naturalist Henry Bates proposed an answer to this question that is most often illustrated by comparing the life cycle of the Monarch and the Viceroy butterflies. These two butterflies are quite similar in appearance in both color and markings. Upon close examination the Viceroy has a narrow black line on its hind wing that is missing in the Monarch. And in flight the smaller Viceroy flutters, while the larger Monarch flaps and glides. But unless you know these field marks, the two are easily mistaken, both by humans and by their common predators, the birds.

According to the theory of Batesian mimicry, one member of a look-alike pair develops a defense mechanism that gives it some protection from predators. The other member of the pair benefits from a case of mistaken identity. In the case of the Monarch, during the caterpillar stage it feeds on the milkweed plant which contains a toxic substance. A bird eating a Monarch that develops from that caterpillar gets a very disagreeable tasting snack, or in some cases actually gets sick.

If the bird is smart, it won’t repeat the experience. And if it sees a Viceroy look-alike, it probably won’t be tempted to try its luck there either. The Viceroy caterpillar doesn’t eat milkweed, depending instead upon the willow. But because the Viceroy mimics the appearance of the Monarch, it benefits from what the birds have learned. This is just so logical it has to be true. Or so we were taught.

Much of the knowledge in the world is acquired second-hand through books, lectures, or word-of-mouth (and magazine columns such as this one.) There just isn’t time to find out everything by personal experimentation, so we tend to rely on “experts” for much of what we “know.” One of the most famous examples of an expert being wrong is when Aristotle, in his 350 BC book Historia Animalia, said that spiders had six legs. This became common knowledge and was repeated for centuries. Finally another authority actually counted legs and found out there were eight. Oops!

Today most scientific publications require peer review before printing new ideas, trying to avoid this situation. If several respected sources independently come up with the same answers, it is much more likely to be true. But when someone finally double-checked the Monarch/Viceroy mimicry, the results weren’t quite what was expected.

In 1991 David Ritland and Lincoln Brower conducted some experiments with red-winged blackbirds and the Monarch and Viceroy butterflies. The red-wing likes to dine on butterflies, so the experimenters offered the birds the abdomens (diagnostic wings removed) of various butterfly species. It turns out that the red-wings don’t really care for either of the mimic pair (only eating 40% of them), while it ate 98% of the non-toxic species used as an experimental control.

Further checking brought some more conflicting facts to light. Evidently milkweed plants in the north are much less toxic than those further south, and older Monarch butterflies (that sip nectar) are much less toxic than younger members of the species. And evidently the look-alike Viceroy really doesn’t taste very good itself. The whole situation, like many things in nature, is much more complex than it looks at first sight.

The entire Monarch/Viceroy issue may actually be a case of Mullerian mimicry, named after the 19th century German zoologist Fritz Muller. In this case the term mimicry may be a poor choice, but again the logic seems reasonable. If two species have a similar defense mechanism (bad taste) and similar looks, both will benefit. Birds will learn to associate a disagreeable experience with a visual pattern and avoid repeating it.

Some researchers now go so far as to say that mimicry itself doesn’t exit, but we think that’s being extreme. In looking over our past columns, we once wrote about a case of mimicry in the firefly. It turns out that one species of female firefly mimics the flashing of another species. This attracts the male – which she then proceeds to eat.

And then a column we wrote clear back in the 80’s called Insect Self-Defense explained about the mimicry of the Monarch and the …. Oops! Scratch that.


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