The Language of Warning

The Language of Warning

In which I go into a long discussion of the semiotics (big word -- look it up) of nature, but eventually get back to my favorite bugs. 

I haven’t put up a post on this blog recently because, frankly, my occasion for blogging – raising the hickory horned devil caterpillars – has come to its natural end with the caterpillars pupating.

Still, there are some things left to say here, though the bugs hurried through their life cycle before I could get them out. 

Now, two posts ago, I was talking about mimicry – the kind that happens when one bug learns how to do something defensive, and the others all learn to mimic it so as to share in the benefits. It’s kind of like hanging around with the big, bad kid, so the bullies think you are tough too and leave you alone.

I argued in that post that certain insects were using something like psychological warfare – using fear (in the case I cited, fear of being poisoned by the pipevine swallowtail) as a defensive mechanism against predators. I also argued that for this fear to really work for the fakers (the mimics), the fear had to be instinctual, not just learned by the bad experiences of individual birds.  I argued that an anti-predator weapon that makes the predator avoid the insect (“avoidance”) becomes an invisible force in nature that other insects can use too when it gets hard-wired into the behavior of the predators through natural selection.

Because birds are major predators of butterflies and moths, and because birds are notable for having excellent color vision, it makes sense that color becomes the main signal, the main way of triggering these instinctive fears. In a sense, it becomes the language that bird fear understands.

With the pipevine swallowtail and its mimics, I talked about a specific butterfly design – solid black wings with iridescent  blue highlights – but, in fact there are other color designs that apparently scream “Warning, Will Sparrow! Warning!” that are considerably more generalized. These color schemes approach being a language – an abstraction of warning – that strangely pops up all over.

Consider the following caterpillar that I found happily munching on parsley in my herb bed a week or so back:

This is the caterpillar of the black swallowtail, one of the black butterflies I used in my example in my last post. The beautiful striped markings on this caterpillar differ significantly from the markings of most other species of swallowtail (many have the eyespot markings I pictured in the same post), but the design does fall into a general pattern that you can find in many other, often distantly related, butterfly caterpillars, and, in fact, in a lot of moth caterpillars. Here are a few of thousands of other possible examples:

brown hooded owlet

monarch butterfly

milkweed tussock moth

silvered prominent

zebra caterpillar

Note the strong contrast in the stripes – black and red and yellow are common combinations. This pattern is an eye-catcher, and looks unlike anything likely to be surrounding leaves and twigs where the caterpillar lives. Remember that birds see colors really well and most of these caterpillars are desired prey. It’s almost as if the caterpillar is saying “Yoo hoo! Pay attention! Here I am! Come eat me!”

Also note that, though the color patterns of these caterpillars are similar and all involve some stripes,  there really isn’t a working attempt for some caterpillars to be carefully “mimicking” the look of another very dangerous caterpillar. These caterpillars clearly all look somewhat different from each other, unlike the pipevine swallowtail and its imitators. Some of these caterpillars are, in fact, poisonous (the monarch caterpillar, the milkweed tussock caterpillar and the black swallowtail caterpillar) but many are not.

So what is going on here? Why is the anti-camouflage of  yellow/red/black contrasting stripes, both vertical and horizontal, so popular in nature?

The answer appears to be that these strong color combinations,  put in a pattern that accentuates the contrast (stripes, always a favorite among “loud” dressers) are a way of saying “Look at me! Look at me!” and natural selection seems to have built into birds a natural distrust, in fact a fear, of any insect that calls attention to itself. Why? Because some dangerous animals in the distant past came upon this color combination as a way to be memorable and the general lesson get fixed into behavior by the selective process. You can almost hear the bird saying “now that just doesn’t look natural. No way I’m going to eat that."

Think I’m making this up? (It is just a hypothesis, so of course I may be.) Consider that the same pattern also occurs in snakes (poisonous and non-poisonous alike):

Arizona mountain king snake

coral snake (poisonous)

milk snake

scarlet snake

Remember that some of the main predators or snakes are birds (hawks and owls). Fear has a language in nature and it’s very colorful and very loud – which makes perfect sense. You don’t frighten someone off by whispering sweetly.

What does any of this have to do with the insects that I have been blogging about? Remember how I said many posts ago that a lot of species related to the regal moth are camouflage experts, often mimicking fall leaves? Well, you can’t say that about the regal moth can you?

The regal moth may not look like a leaf, but it’s bright color scheme does remind us of something else:

The monarch butterfly, as noted before, is toxic to birds. Big, bright orange wings kind of stand out like a sign that says “danger!” don’t they? Nature's stop sign for birds.


Bibliography

Wagner, David L. 2005. Caterpillars of Eastern North America. Princeton University Press, Princeton.