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.