false eyes
June 14, 2010 12:35 PM   Subscribe

Fake Eyes "To small tropical birds foraging on the rainforest floor, those two scowling eyes peering back at them from between the leaves could be a predator. But they also could belong to one of the hundreds of caterpillar species that have evolved eyelike spots and patterns to trick feasting birds."
posted by dhruva (33 comments total) 10 users marked this as a favorite
They are also working on painting convincing-looking tunnels on the sides of mountains.
posted by Danf at 12:43 PM on June 14, 2010 [3 favorites]

Those caterpillars will tease you. And unease you. All the better just to please you.
posted by GuyZero at 1:11 PM on June 14, 2010 [2 favorites]

Oh, wonderful! I'm a big fan of the elephant hawk moth caterpillar myself.
posted by Catseye at 1:28 PM on June 14, 2010

Wow, the snake-imitating ones are amazing.
posted by cryptozoology at 2:21 PM on June 14, 2010 [1 favorite]

Related: the owl butterfly, which I saw in the Santa Elena Cloud Forest in Costa Rica. From one angle it looks like an owl's head from the side and from another it looks like a snake's head.
posted by Mental Wimp at 2:57 PM on June 14, 2010 [1 favorite]

Burhanistan: over many many generations. You start with a plain butterfly. You get a butterfly that has a spot that looks vaguely like an eye. That variation breeds true and the children are marginally less likely to get eaten. It's amazing how quickly a very tiny bonus to survival rates will lead to dominance of the gene pool. Now you have a butterfly population that has black spots on the wings. Repeat for many many millions of years and you get some pretty amazing mimicry.
posted by aspo at 3:20 PM on June 14, 2010

Cool post. I did field work in the ACG in the early 2000s and lived next door to Dan Janzen. It was fun interacting with the gusaneros (except the one that stole all of my beer).
posted by special-k at 3:38 PM on June 14, 2010

As a young boy (about 7 or 8) I was freaked out by an infestation of tomato hornworms in my uncle's extensive garden. If you want to squick yourself out google it.

I, on the other hand, will not open any of these links. Instead I will find puppy videos on YouTube.
posted by Splunge at 3:38 PM on June 14, 2010

aspo: how does a butterfly do that? You've supplied a plausible theory for the mechanics, but that doesn't explain the awareness that would be needed to actually instigate it.

Can you elaborate on the "awareness" part? It's not clear to me how that figures in to selection. I thought that the original mutation would be just random and nonlethal.
posted by Mental Wimp at 4:31 PM on June 14, 2010

Some caterpillars that don't grow such spots are known to be victims of merciless teasing, where other caterpillars call them "Two Eyes".
posted by bwg at 5:03 PM on June 14, 2010 [1 favorite]

"Awareness" wouldn't enter into this any more than with any other advantageous trait. That is, not at all.
posted by brundlefly at 5:39 PM on June 14, 2010

This is the first you've heard of how evolution works?
posted by GuyZero at 5:53 PM on June 14, 2010

It's not random. The first butterfly to have a spot (caused by random mutation) on its wing that vaguely resembled an eye had a better chance of surviving and having butterfly babies that also had spots. So that goes on for a while, spotted butterflies increasing their proportion of the population, and then one day one of them pops out with a spot surrounded by a spot that looks even *more* like an eye.

Repeat as necessary over millions of years. You'll note the ones with bright fuchsia arrows didn't make it through the process, and were probably quite tasty.

Alternately: a wizard did it.
posted by billybunny at 8:21 PM on June 14, 2010 [4 favorites]

No, no, no. The mutations themselves are random occurrences. They happen all the time, but usually they lead to the non-viability of the organism and so don't persist. The persistence of the mutation is what is driven by selection: a mutation by chance confers a survival and hence reproductive advantage. More random mutations in genes determining coloration patterns lead to refinement of the mimicry over time. Over a looonnnnngggg time. The same way any other genetic pattern gets selected naturally.
posted by Mental Wimp at 8:36 PM on June 14, 2010

Burhanistan, why are you tall? You're probably taller than you great grandfather and almost certainly taller than his grandfather. What did you do to receive this -- what awareness instigated it? Here's what: taller people (ostensibly) reproduce more and more successfully. Boom, the human population gets taller on average.
posted by zpousman at 8:42 PM on June 14, 2010

Don't think about the single item, such as you or a single butterfly. Think about the hundreds of millions of them that have existed. It's definitely *NOT* like finding a city grid. That's a one time thing -- population genetics is a many-many-many-many repeated trials kind of phenomenon.
posted by zpousman at 8:44 PM on June 14, 2010

Burhanistan: think of it like this. If you see a gigantic canyon and a tiny stream running at the bottom, it's hard to believe that the canyon was formed by the action of water. It takes a lot of time for the canyon to be formed by erosion. It's the same thing with the spots. Tiny, incremental changes (with lots of going back and forth) over millions of years can lead to dramatic changes. If a caterpillar with spots no matter how tiny has a small advantage in repelling predators, then its offspring have a better chance at survival, and that is carried on.
posted by dhruva at 9:04 PM on June 14, 2010 [1 favorite]

In the first link it mentions that even the merest suggestion of eyes (two black dots) is enough to scare off birds. That's a pretty puny adaptation with pretty clear advantages. From that point on it's a gentle slope to more and more effective mimicry.

Also, it's worth considering that the spots may have initially been an advantage for completely unrelated reasons and were only later repurposed for their eye-ishness. Maybe they served as traditional camouflage or affected the radiation of heat?
posted by brundlefly at 10:24 PM on June 14, 2010

Does anyone have a link to that game where you can select for traits? I've got it somewhere but I can't recall the name. I thought it was a good illustration of the process.
posted by Trochanter at 10:28 PM on June 14, 2010

It's quite old. The critters are black on white pixel patches, kind of like Rorschach images. The point it makes is that the pixel patches aren't doing anything. The Player (or the one who's running the sim) is doing the selecting...
posted by Trochanter at 10:36 PM on June 14, 2010

But they also could belong to one of the hundreds of caterpillar species that have evolved eyelike spots and patterns to trick feasting birds.

Wording like that doesn't really help the situation. "To trick" implies intention. "That trick" seems more fitting.
posted by Soupisgoodfood at 10:56 PM on June 14, 2010 [1 favorite]

Burhanistan: If I understand you correctly, you are questioning how these caterpillars and butterflies can evolve spots that resemble an owl's eye to such a great extent (down to the finest detail). The reason we see this incredible imitation, is that there's a separate evolutionary process going on in the mind of the caterpillar's predator trying to debunk the mimicry. Check out arms race on Wikipedia.
posted by Jonnings at 10:56 PM on June 14, 2010

It's super hard to talk about evolution without using terms that imply intent. Even Gould would slip. TV presenters do it all the time.
posted by Trochanter at 11:12 PM on June 14, 2010 [1 favorite]

Spicebush Swallowtails do have quite an interesting set of eye spots, but that's nothing compared to the erectile horns and chemical weaponry. I didn't find any pictures of a Spicebush Swallowtail, but the osmeterium in the picture of the Giant Swallowtail larva on this page is fairly representative of the genus as a whole.
posted by plinth at 7:23 AM on June 15, 2010

There are some nice photos of the Spicebush Swallowtail here.
posted by dhruva at 8:05 AM on June 15, 2010

I saw an eyed click beetle in my backyard last month.
posted by any major dude at 8:10 AM on June 15, 2010

And that, my best beloved, is how the caterpillar got his eye-spots.

(Karloff reads Kipling -- worth checking out. (please ignore the video -- just look away))
posted by Trochanter at 10:29 AM on June 15, 2010

Here's the Simulation I was talking about. It's a java applet, if that scares anyone.
posted by Trochanter at 11:10 AM on June 15, 2010

Okay, let's try this. 1 bazillion years ago a random mutation caused a spot to appear on the (already symmetrically patterned) wings of a butterfly. These spots, from a distance, looked to a small predator who likes butterflies like a larger predator's eyes, so this butterfly was not eaten because the predator went elsewhere to eat its counterpart without the spots. This "spotted" butterfly went on to pass this mutation down to its progeny, who were then less likely to be eaten as the original scenario repeated itself. This phenotype came to dominate over time because of this advantage. Now another random mutation comes along in a few millennia and causes a more featherlike look to the mottling around the eye. Another comes along in another few millennia and causes a yellowish ring around the spot, and now it comes to more closely resemble an owl's eye and head. Now, at the same time there are many, many other mutations occurring in the species that cause non-viability or have no effect on survival and reproduction, but as they have no advantage they do not dominate. Over many generations, small mutations happen that make small, incremental improvements to the optical illusion of the owl eye, and each confers an advantage to the possessor and its progeny, because it fools more predators more often. Small mutations accumulate to create a better and better illusion, until it is almost perfect.

Does this make more sense? Admittedly this is speculation, but is consistent with the way we know mutations occur, with how genotypes relate to phenotypes, how selection functions to change the frequency of genotypes, and how incremental mutations can radically change appearance and function. Think about how varied dogs are and that the specific characteristics were selected for by breeders who wanted [pick your favorite dog trait]. These differences were created in a fairly short amount of time by the "pressure" of this artificial selection. Evolution through natural selection has a much longer time period to work with, but must depend upon random new mutations instead of existing ones.
posted by Mental Wimp at 11:16 AM on June 15, 2010

A recent example of natural selection is the Peppered Moth. Although this type of adaptation doesn't include any genetic mutation, it does present a good example of how environmental pressures can effect changes in a relatively short time span.
posted by Krapulous at 2:42 PM on June 15, 2010 [1 favorite]

Evolutionary theories aside, the little bastards devour my lime tree.

Your lime tree, which by the same process has come to package its seeds in delicious lime flavoured bundles of endosperm so some brachiating, or fluttering, or lumbering beast will bear it a certain distance before doing what the pope does in the woods.

Miracles all up in this bitch.
posted by Trochanter at 4:45 PM on June 15, 2010

I'd like to change that to "bear its alleles a certain distance", but I can't.
posted by Trochanter at 4:51 PM on June 15, 2010

That actually holds a bit more water than the linear notions of gradual adaptation.

Yes, the scenario I used above to illustrate how natural selection would work to improve the mimicry assumes a static predator state, but only for simplicity. If one assumes a static prey phenotype, the predator would undergo a similar natural selection of mutations that improve its ability to distinguish mimicry from the real thing. Introducing simultaneous evolution in the pair ( the "arms race") creates a non-linear dynamic (recursive) relation, which can greatly accelerate the selection process if there is a high rate of reproduction. If the rate is low (or the population small), then a major improvement in one could lead to extinction of the other and - poof - no more evolution in those directions. But I hope it's not too hazy how natural selection can plausibly produce some pretty amazing mimicry results only through random mutation and competitive pressures. One need not introduce awareness or any third party, although some may want to a priori. Only Occam's razor stands in their way.
posted by Mental Wimp at 4:18 PM on June 17, 2010

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