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.

Conjuring Nature’s Ghosts

Conjuring Nature’s Ghosts

In my last post, I pondered what survival strategies  the hickory horned devil caterpillar could be using in the middle stages of its larval life. On the one hand, they are not huge and threatening yet, and clearly not toxic to predators (no toxic foodplant, no stinging spines). They also are an inconvenient color (brown) among the green leaves they feed on during the day. This should be a recipe for disaster…

But they do have these freaky looking horns – harmless spines that are kind of like antlers. These can’t be easy to handle when you’re a caterpillar crawling around a tree… which means there must be some advantage to having them.

Looking at them a bit more, I remembered something from my own past experience. I actually had seen a hickory horned devil just once before raising them. Seven years ago, I was new to Charlotte, and out walking my dog in my neighborhood.  Watching the dog sniff things, I noticed some large caterpillar droppings on the pavement. I looked up, and there, just above my head, was a fifth instar hickory horned devil clinging to some sweet gum leaves. I jumped back, which, for me, was an odd reaction to spotting a spectacular caterpillar. As you’ve probably figured out from reading this blog, I think caterpillars are cool and I’m not at all frightened of them.

But I am, inexplicably, frightened of spiders (I always have been) and there was something spiderlike about this caterpillar. My spider fear is literally instinctual (I say it’s “hard-wired”) – I generally have no fear of other bugs, even ones that bite. This is actually a common human fear (psychologists call it “arachnophobia”) and I’ve read scientists who theorize that many of us come with it built into our brains because, in the distant past, our ancestors lived in places where spiders were frequently dangerous, so evolution has favored a built-in fear. I believe this, because it is definitely something primal in my brain, like a fear of barking dogs or of snakes.

So, look again at the photo of the fourth instar caterpillar above and look at these orb-weaving spider photos:

Notice the similarities between the spiders’ conspicuous legs and the caterpillar’s spines? There are eight large spines, and even in the fourth instar, they look distinctively like these native, long-legged spiders. Yuck.

And I can tell you (with an inner quiver) that we have a lot of these spiders in Charlotte, hanging their webs from tree branches. In August, about the time the hickory horned devil caterpillars reach their fourth and fifth instars, orb weavers also mature and come down out of the treetops where they have been growing in obscurity and hang their webs out in the open. At this time the hickory horned devils are becoming so large that they too become conspicuous.

Why would a caterpillar mimic a spider? Spiders bite, but these spiders are not dangerously venomous like, say, the black widow spider… one wouldn’t expect birds to be afraid of them because of that. Of course, they are dangerous to other insects, particularly to flying insects…

So here may be our answer – you may remember that I mentioned in earlier posts that parasitic wasps are one of the biggest dangers that giant silk moth caterpillars face, destroying large percentages of every brood. A big spider is a dangerous predator to a little wasp. The hickory horned devil’s spider mimicry may frighten away the parasites.

But, thinking about it, there still may be an effect here on bird predators as well. I’ve always found it puzzling that big orb weaving spiders (like those whose photos you see above) are so conspicuous – large, brightly colored, hanging in the middle of the web with no cover, where a flying bird could easily pluck them.  Yet birds don’t. I did a search of the literature on bird predation on orb weavers and found that biologists find that bird predation appears to be uncommon. In nature, bright coloration is sometimes a sign that an insect is toxic and not good to eat. Yet again, there is no mention of this in the literature – surely someone has looked into these big arachnids  being bad tasting… They are not poisonous in bite or taste, yet they advertise their presence… hmmm.

So let me propose an alternative –perhaps birds leave orb weavers alone because they are hard-wired to be instinctively frightened of large spiders just like I am, though there is no longer a real threat in these particular bugs.  In the tropics there are large orb weavers that are more venomous and sometimes prey on small birds. In other words, evolutionary history has given large spiders a fearsome reputation, and, though they may not still deserve the fear, they are still using it for protection. And so does the hickory horned devil… Deep instinctive fear is a force and it has echoes.

See -- totally harmless!

History has meaning to humans because we learn lessons from it.  In the battle for survival that we call nature, some lessons have value for other species as well, and they too find ways to preserve the history.

Anyway, it’s a hypothesis.