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.

Mimicry, Shmimicry. It's the Fear, Stupid

Mimicry, Schmimicry. It's the Fear, Stupid.

What’s the reason for  the copy, or, perhaps as Yeats said, how do we know the dancer from the dance?

Two posts ago, I discussed mimicry in caterpillars and adult Lepidoptera again, mainly focusing on adaptations that allow these yummy bugs to hide in plain sight from all the creatures that want to eat them.  But this is really only half of the story. The other half – using mimicry as a kind of offensive weapon – is really more to the point when considering the case of the 5^th instar hickory horned devils and the spider mimicry that I have proposed their horns are helping them accomplish.

If camouflage is one way caterpillars and moths keep safe from birds and other predators, another is to develop a visual association with things these animals actively avoid – things that they find aggressive, unpleasant, or are outright life-threatening. If done effectively,  this is an even better strategy, because when the predator succeeds in discovering the bug, it still works. Birds and other predators actively avoid things that they are scared of. I say “if done effectively,” because mimicking a threat animal may, in fact, be harder to evolve working adaptations for than developing camouflage. Nonetheless, there are a lot of examples of effective fear-based  mimicry in nature, so it can’t be that hard.

A common kind of offensive mimicry in caterpillars is to evolve color spots that look like eyes, so as (obviously, due to their long, cylindrical shape) to look a bit like snakes.  Birds really do not like snakes, for good reason – snakes are one of their few predators that can get up in the trees. Here are a few examples of caterpillar-snake mimicry:

Eye spots are apparently relatively simple to evolve through changes in the genes that affect surface coloration, which may contribute to this being a popular (commonly arrived at through the forces of natural selection, that is) survival trick.  Eye spots also occur a lot in adult moths and butterflies, especially in larger ones – for instance among the giant silk moths. Here are two different North American examples:

Io Moth

Polyphemus Moth

Obviously moth eye spots are not there to mimic snakes, so what is the function? Well, birds have other predators as well in the trees – climbing mammals such as raccoons and opossums (and squirrels, which, believe it or not, sometimes eat bird), and especially hawks and owls. Behavioral biologists generally assume that the wing spots on moths are there because when a disturbed moth flashes them it gives an attacking bird the shock of thinking that it has stumbled on an owl.  If you are a small bird, always on the lookout for ambush attacks from these winged predators, this would be a very nasty surprise and an effective scare.

“Avoidance,” which we humans think of as a rational, learned thing, is actually more a behavioral adaptation in nature, and an unseen property in ecosystems. Creatures living in the natural environment have a day-to-day struggle to stay alive: to not be eaten, to not starve, or to not fall into a state (through lack of food, injury, sickness, etc.) where they cannot maintain the strenuous activities they absolutely must perform daily to keep living. Generally, they have no backup, no support system, the way humans do with our human society. Consequently, when there’s something that is pretty constantly bad/dangerous/risky for them in their natural environment, natural selection tends to evolve an automatic behavior in the animal’s population that causes them to avoid the risk – natural avoidance . Let’s call it an instinctive fear.  Humans can’t see these instinctive fears in nature directly (you’d have to be inside the bird’s head), but it can be a real thing, with really obvious signs – a pattern in the ecosystem, if you like – that you can detect indirectly.

Take the strange pattern of the commonness of blue-black butterflies in the American South.  Here are a few examples – notice the common pattern:

How do we explain the commonness of this pattern of colors? Okay, you might say, these are all swallowtail butterflies – they are all related to each other, so maybe they’ve all just inherited the pattern from a common ancestor. But then consider these two butterflies, which actually come in two different color forms,  a blue-black form, and one that is not:

Red spotted purple (Limenitis arthemis)

Banded purple (Limenitis arthemis)

Eastern Tiger Swallowtail (dark form)

Eastern Tiger Swallowtail

Notice first, that the red spotted purple is not related to the swallowtails (it belongs to the brush-footed butterflies, a very different group) and that the different forms of the two species are really different in appearance. One form of each is blue-black, while the other has vertical stripes. (Vertical stripes are an interesting defensive adaptation that helps flying butterflies survive bird attacks by making it difficult for the bird to distinguish between the butterfly’s body and wing when it is in motion – if the bird pecks the wing, the butterfly gets away.)

A key piece of information to note is that the blue-black forms of these two butterflies are not common everywhere in each butterfly’s range: they are only common in the south. When I collected butterflies as a kid in Ithaca, NY,  I never saw a red spotted purple or a dark form tiger swallowtail though banded purples and regular tiger swallowtails were very common.  But when my family traveled about 100 miles south (say, to Long Island or Pennsylvania ) I did finally see these elusive insects and collected them excitedly. I was really confused why they were so common a relatively short drive away.

There is a reason – the pipevine swallowtail (the first photo in the series) feeds pretty exclusively on the toxic southern plant pipevine and, with that plant, has a range whose northern limits are about at Long Island and central Pennsylvania. The poison the butterfly gets from its food plant is, apparently, very toxic to birds – so toxic, in fact,  that birds are really afraid of it (avoidance again), and other butterflies (none of which are poisonous themselves) have adapted (though natural selection) to take advantage of this fear through mimicry of the pipevine swallowtail.  This is a kind of mimicry that biologists call “Batesian Mimicry.”

I’ve read descriptions of how this works that go basically this way: “any bird that tastes a pipevine swallowtail has such an unpleasant experience that it doesn’t ever forget it and any butterfly that looks like the noxious one gets left alone as well.”  Is this what is happening?

As a human, it’s natural to imagine it this way, since we learn most of the avoidance we practice – like learning that the flame on the stove is hot, etc.  For birds to learn the poisonousness of butterflies this way however, seems somewhat unlikely – it would likely take a little while for the association of sickness with the specific insect to be learned, and birds only live a year or two. It seems likely that only a small part of the bird population would be experienced enough to practice avoidance. There’s also the issue of how clear a lesson birds are being taught, given all these harmless, tasty mimics that are perfectly good to eat also flitting around the birds. In my observations, both here and in other parts of the south, pipevine swallowtails are not rare, but they are by no means the most common of all these insects. If you were a bird regularly eating blue-black butterflies, and only occasionally ate one that made you sick, how long would it take for you to get the message? A while, I think. If each bird has to learn for itself not to eat a blue-black butterfly, the result seems hardly likely to be effective enough to make mimicry an effective adaptation – which it must be.

So let me propose a slightly different hypothesis that I think makes a little more sense, given all the evidence of mimicry: I propose that the toxin that is in the pipevine swallowtail can be seriously dangerous to birds’ health, having a significant impact on their ability to survive.  If the toxin is this harmful, natural selection would favor the development of an instinctive fear (natural avoidance again) of butterflies that look like that. This instinctive fear is really a simple, built-in behavioral rule that keeps the birds away from that particular form of harm (like a built-in fear of snakes and owls, or like my fear of spiders).  This behavioral rule is powerful and widespread (it evolves in many species for the same reason), so natural selection (in turn) favors many other butterfly species adapting to take advantage of its invisible presence. It doesn’t matter that most of them are perfectly good to eat – natural selection has hardwired in a dislike for black butterflies as a general principle, so if they evolve black coloration the avoid being eaten.

Pipevine -- a pretty plant, but the source of birds fearing
black butterflies.

My basic point here is that there are features in the landscape you can see directly (watch an owl eat a songbird), but there are also important forces out there that exist invisibly (in behavioral adaptations, like fear of black butterflies for example) that you can’t see directly. You can, however, still come to understand what these forces are by looking for patterns (common forms of mimicry, in this case) that provide clues.  In some ways,  I think it can be more exciting in nature study to see the invisible than it is the visible.  Who would have known that the innocent little pipevine plant could be responsible for  a significant piece of predator-prey behavior in eastern North America or for making many butterflies in the south black? Perhaps something similar is responsible for the bright coloration of the adult regal moth, but I’ll leave that for a later post.

Who Cares How You Look?

The birds do, Mr. Doo-doo.

After my last post, I have to say that the hickory horned devils are now second instar and, ahem,  big enough to be outside now, and everyone is happy about this, especially the hickory horned devils.

(I’ll talk about the arcane practice of “sleeving” caterpillars in a subsequent post.)

Each time a caterpillar sheds its skin to grow larger, it is said to have entered the next “instar.” The hickory horned devils are now in the second instar out of five. Some caterpillars look pretty similar in every instar (lunas, for example), but some change their appearances fairly significantly. Hickory horned devils are in the latter group. They are about an inch long and look like this:

What do you notice about the appearance of second instar hickory horned devils? Well, to birds they look like bird poo… or so entomologists think (who really knows what birds think?).

A fairly typical bird dropping. Yes this is gross – but the demands, of science, etc. If you look carefully at tree leaves, you see this all the time because a lot of birds perch in trees.

 Why do they see that in this harmless little caterpillar? Well, the splotchy pattern and the squiggly curve of the body does look a bit like the dropping photo, but this is also something that they have seen before in the bug world. Here are some other examples from both moth and butterfly caterpillars:

Viceroy butterfly caterpillar

 "Orange Dog" -- caterpillar of the giant swallowtail butterfly

Moonseed moth caterpillar

Ruddy daggerwing butterfly caterpillar

Sure, these caterpillars look pretty different from each other (and from the hickory horned devil), but you see the overall pattern – bumpy shape, white splotches mixed almost randomly with other colors, especially dark browns. Behaviorally, each of these caterpillars tends to rest curled in a questionmark shape. You can see the resemblance with the bird poo and with the hickory horned devils (though my photos aren't great).

Why do so many caterpillars use this kind of camouflage? (This is actually a form of what we call “mimicry” – but much more about that in later posts.) Well, there are some obvious answers. First, it’s a common pattern on tree leaves, so it really is a good pattern to copy if you want to blend in. Second, to birds it looks like… poop. Most creatures have a built-in dislike for putting that stuff in their mouths (or even getting near it) because being in contact with it is a great way to catch diseases from your fellow creatures. Think about your own disgust – it’s hardwired in us by evolution.  Though not all species share this built-in disgust  or “aversion” (dogs come to mind as an counter-example) to feces from their own kind, it’s common enough to assume it’s likely to be present in many birds.  

So that’s why this appearance pattern works as a survival “strategy,” but how did so many different kinds of caterpillars come up with it? (I’ve only shown a few examples – there are hundreds, if not thousands  of others out there.) You need to remember here that bugs don’t actually consciously “come up with” these natural tricks that help them survive, any more than you “came up with” your natural hair or eye color to make you look cool and attractive to other people.  The caterpillars’ appearance was, originally, the result of a random set of mutations -- or a random new combination of existing genes -- that accidntally happened to create this appearance. But the pattern happened to work for the bug that first was gifted with it by chance, and it survived ... and made a lot of similar bugs that also survived, and so the genes – the genetic instructions for how to look like bird poo – got passed on and became common. Because the trick works, the genes then stay common, even as the species evolved and passed on its genes to many other species, its ancestors.  This is basically how natural selection works on genes over time – genes that have a lot of usefulness stay in species’ genomes because they keep coming in handy in staying alive.

From the examples I showed above, you might have caught the fact that there are both butterflies and moths that use this pattern.  Think about what this means – biologists know pretty much for certain that all butterflies and moths are relatives – they have evolved from common ancestors, with butterflies first evolving from moths about  100 million years ago (this is a guess, of course – the oldest fossils of butterflies are about 48 million years old, but these fossils are very modern butterfly-like, indicating that the split from moths had to be earlier).  This means that genes that are shared between moth and butterfly species would have to be pretty ancient – at least 100 million years old.  I would guess that the basic genetic package for the bird poop-camouflage trick has to go way back  and still be hidden out there in the genes of most butterflies and moths for it to currently be so common in this group of animals.

Of course there is another possible explanation – it’s called “convergent evolution” – where different species independently “come up with” the same common set of features because, well, it works. This is possible here. The thing is though, for so many different varieties of moth and butterfly to come up with this same pattern there would still have to be some common underlying feature (a broken white/color pattern for example) in order for it to come up again and again and again. A better bet is that the bird poop imitation was so successful for the moth/butterfly ancestors that it stayed around long enough to eventually drift together on the genome as a kind of package -- a big and complex group of genes that is a fundamental tool for survival through the bird poop trick.

What’s fascinating to me about this kind of thing in caterpillars is that you can literally see the animal taking advantage of different parts of its genetic heritage as it grows up and its situation changes. When the caterpillar is really little (and too small to be mistaken for bird poop), it doesn’t look like this at all, as we have seen.  When it gets bigger, it’s too big to be mistaken for bird poop, and so its appearance changes again. But when the caterpillar is in its second instar, it is really just about the right size, so the time is right for natural selection to again pull these genes out of an ancient bag of tricks. Pretty cool, I think, the way you can see all kinds of pieces of ancient history coming out and showing itself to you in a living bug.

Bibliography

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

Scott, James A. 1986. The Butterflies of North America. Stanford University Press, Stanford.

Food Loyalty in Bugs

In which I talk about picky eating in bugs, and decide to try an experiment…

Most bugs are picky eaters. Photo © Men’s Health

Another unusual thing about the Hickory Horned Devil (the regal moth caterpillar) is that it eats a whole lot of things besides hickory. According to The Wild Silk Moths of North America (WSMNA), it eats basically any nut tree (Juglandaceae – hickories, walnuts, butternut, pecan), sweet gum, persimmon, all sumacs, sourwood, ashes, sycamore, lilac and cotton. Other sources note that it will also eat oaks and cherries. This is an unusually broad range of food plants for a moth or butterfly caterpillar, and even a broad range for  a giant silk moth caterpillar (a group which often does eat a large number of deciduous tree species), and it notably includes leaves that are pretty different in taxonomy and texture and smell, which means that they are probably chemically different (I’m guessing).

Some of the regal moth's foodplants.

Why are many caterpillars only found on one or two foodplants? Well, science has worked out that it almost certainly has to do with past history. Imagine hundreds of thousands (or perhaps millions) of years ago, a species of moth found that a specific plant or group of plants suited its dietary needs – it had leaves with materials in them that the caterpillars were capable of chewing… could digest… had nutrients that the caterpillar needed to grow, and contained no poisons that could make the caterpillar sick. Perfect! However, if it was too perfect a food, this could be a problem for the plant because the caterpillar populations would thrive, eat a lot of the plants and quickly it would start threatening the plant’s survival. This growing catastrophe would then strongly favor the chance survival of individual varieties of this plant that happened to be a little different genetically so that they contained some different chemicals making those varieties more toxic – in other words, the leaves tasted worse -- to the caterpillars. The worse the plants tasted, the more plants would survive (perhaps this a defense mechanism the broccoli tribe has used against human children). In this way, over a lot generations, the plant would evolve a chemical defense against the caterpillar that was eating it until it wasn’t threatened any more. It’s an evolutionary process called natural selection – perhaps you’ve heard of it.

The back-and-forth “arms race” between plants and caterpillars.

However, natural selection also works for the caterpillar – since the plant was otherwise a good food, lucky caterpillars that happened to have genes making them less sensitive to the plant’s new toxins would tend to survive better, so after a while all the moth’s caterpillars might develop immunity to the new toxin (bad-tasting poison) in the plant. They might even evolve to use the toxin in their own chemistry to help them grow and survive. So then there would be a problem again for the plant, and varieties that had new toxins that the caterpillars didn’t like would survive better… and then the caterpillars… and then the plants… you get the idea. It’s an arms race between plant and caterpillar, and  the two begin evolving together, so the caterpillar’s chemistry is very finely tuned to eat a specific kind of plant that contains a lot of nasty chemicals. It may have even  evolved to use those chemicals to defend itself against caterpillar predators (like birds), because something that is toxic to caterpillars is often also toxic to animals that eat caterpillars. A familiar example is the monarch butterfly, which feeds exclusively on milkweeds, which contain some nasty poisons, which the monarch caterpillar (and butterfly) use to make it poisonous to birds.

 Toxic monarch caterpillar on toxic milkweed plant

Plant-eating insects like moths and butterflies have evolved this way with specific plants for a long time, which makes most of them picky eaters… which is why a non-picky insect like the royal moth is somewhat unusual. How would a moth come to be this way? My best guess (another “hypothesis” of mine) comes from something that I was talking about two posts ago – the fact that regal moths and their relatives seem to be relatively recent “invaders” from the south, taking advantage of ecosystems that are relatively “new,” thanks to past ice ages. In ecology, scientists note that the species that can best take advantage of a new place (where there are not a lot of established, well-balanced relationships between species) are creatures they call “generalists.” Generalists are species that can eat lots of different things,  aren’t fussy about living arrangements, etc. and thus are able to make do in new situations better than other organisms that depend on established relationships with each other. In older ecosystems where all the organisms have long worked out their relationships with each other, these “generalists” may not do as well, but in less-established situations they do better. Curiously, this points to the fact that the regal moth, apparently a relatively recent invader, is somewhat of a “generalist,” despite its unusual appearance (which looks very specialized) and freakishly large size. It’s well-suited to survive in a variety of conditions by eating a variety of foods.

 The thing is, evolution is still going on. WSMNA notes that some studies have found that for regal moth caterpillars “the effectiveness of host plant assimilation varies from population to population.” In other words, regal moths in different areas appear to be evolving preferences for different species of foodplant. The authors go on to say: “A study of localized preferences in this species, with its broad array of natural host plants, would be an interesting topic for further investigation.” Suggestion accepted: I think I will attempt a small (and poorly designed, since I’m winging it) experiment.

 What I am doing is picking two different foodplants -- pignut hickory and sweet gum -- and testing whether one seems in any way to be a preferred choice for the caterpillars I am raising. Do they accept one readily but not the other? Do a larger percentage survive when fed one than when fed the other? Do caterpillars grow at different rates on the different plants? (etc.) I picked these two plants because both are really common here, and both are on the list of “most common host plants.” They are not closely related plants, so they are likely to have significantly different toxins in their leaves.

Hickory horned devil caterpillars from Charlotte, NC seem to prefer glass to hickory leaves. This being the South, maybe I should have offered them Chick-fil-A,

I will try to gather some data relating to some of the questions above, but don’t expect too much from my informal experimenting here… I may produce some evidence for a local food preference, but it sure won’t be conclusive. Think of this as a test run for a much-better-designed experiment that you might think up and run.

Bibliography:

Tuskes, Paul M., Tuttle, James P. and Collins, Michael M. 1996. The Wild Silk Moths of North America. Cornell University Press, Ithaca, NY.

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