Window on Nature
By Lowell & Kaye Christie, F47246
Look-alikes in nature find safety in sameness.
We spend a lot of our time outdoors trying to identify what we see, and sometimes this can be quite difficult. During spring migration, look-alike birds give us fits. Mushroom hunting can cause similar problems, but, fortunately, we’re usually after photographs, not dinner.
Once we’ve made a difficult identification, our next question is usually, “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 mid-1800s English naturalist Henry Bates proposed an answer to this question that is most often illustrated by comparing the life cycle of monarch and viceroy butterflies. These two butterflies are quite similar in appearance, both in color and markings. Upon close examination, the viceroy has a narrow black line on its hind wing that is missing on the monarch. And in flight the smaller viceroy flutters, while the larger monarch flaps and glides. But unless you know these specific differences, the two are easily mistaken, both by humans and by their common predators: 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 mistaken identity. In the case of the monarch, during the caterpillar stage it feeds on the milkweed plant, which contains a toxic substance that remains in the butterfly when it becomes an adult. A bird that eats a monarch gets a very disagreeable-tasting snack and, in some cases, will actually become sick.
If the bird is smart, it won’t repeat the experience. And if it sees a look-alike viceroy, it probably won’t be tempted to try its luck with that butterfly either, even though 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 one’s knowledge about the world is acquired secondhand 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 B.C. book Historia Animalia, said that spiders had six legs. This became common knowledge and was repeated for centuries. Finally, another authority actually took the time to count the legs and found out there were eight. Oops!
Today most scientific publications require peer review before printing new ideas, so as to avoid this type of mistake. If several respected sources independently come up with the same answers, it is much more likely to be true. And when someone finally double-checked the monarch/viceroy mimicry idea, the results weren’t quite as expected.
In 1991 David Ritland and Lincoln Brower conducted an experiment with red-winged blackbirds and the monarch and viceroy butterflies. The blackbird likes to dine on butterflies, so the scientists offered the birds the abdomens (identifying wings removed) of various butterfly species. It turned out that the blackbirds didn’t really care for either of the mimic pair (eating only 40 percent of them), while they ate 98 percent of the nontoxic species used as an experimental control.
Evidently, like the monarch, the look-alike viceroy doesn’t taste very good. Further checking brought some more conflicting facts to light. Milkweed plants in the north are much less toxic than those farther south, and older monarch butterflies that sip nectar are much less toxic than younger members of the species. The whole situation, like many things in nature, is far more complex than it looks at first sight.
The entire monarch/viceroy issue actually may 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 are similar in appearance, both will benefit. Birds will learn to associate a disagreeable experience with a visual pattern and avoid repeating it. What this means is that a bird would only need to eat a monarch or a viceroy, not one of each, before realizing its distaste for a butterfly that has the same appearance. In effect, this would reduce the mortality of each type of butterfly.
Some researchers now go so far as to say that mimicry itself doesn’t exist, but we think that’s a bit 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 back in the 1980s called “Insect Self-Defense” described the mimicry of the monarch and the . . . oops! Scratch that.