A stunning case of mimicry

January 21, 2020 • 9:00 am

I don’t remember encountering this case of mimicry, but it’s so amazing that, when I became aware of it from a tweet (yes, Twitter has its uses), I decided to give it a post of its own.

First the tweet, sent to me by Matthew. He added, “This is the Iranian viper, as featured in Seven Worlds, One Planet, made by the BBC. Amazing.”

You don’t need to translate the Spanish, though, as the video below tells all. I swear that when I first watched it, I thought there was a real spider crawling on the snake’s back.

The snake is the spider-tailed horned viper, Pseudocerastes urarachnoides, which has a small range in Western Iran (map from Wikipedia):

It wasn’t described as a new species until 2006 in the paper below (free access); before that it was thought to be the already-describe Persian horned viper. (I guess they overlooked the tail ornament.)

Here’s a photo of the tail “spider” from the paper; the one below that is from Wikipedia. The resemblance may not be precise, but (as you see above), when the ornament is moved about, it looks remarkably like a spider—certainly good enough to fool birds.

In that paper, the authors didn’t know how the tail ornament was used, but were impressed at its spider-like appearance. And they guessed accurately:

This raises the question of the elaborate and sophisticated appearance of the caudal appendage in our new species, as the waving or wriggling motion of a distinctively colored tail tip seems perfectly adequate to attract lizard and anuran prey. We can only speculate that in the case of the present species, the caudal lure serves to deceive a more specific kind of prey, such as shrews or birds. Indeed, ZMGU 1300 [the specimen number] contains an undigested, unidentified passerine bird in the stomach (the feet protruding through the body wall).

Only later, using live captive specimens, did researchers see that the ornament did indeed attract birds that the snake caught and consumed, as in the video above.

Any biologist who sees this is immediately impressed by the ability of natural selection to mold not only morphology, but the behavior of the snake: the twitching of its tail so that the spider ornament appears to “walk.”  But any adaptation like this ornament must have incipient stages, and each subsequent modification must improve the adaptation—that is, it much give the snake possessing the “improved” improvement a reproductive advantage. (That advantage would derive from the better nutrition of a snake who caught more birds, and thus might have more offspring, increasing the proportion of genes for more spider-like ornaments.)

My own guess was that the ornament started with the simple twitching of the tail of an immobile snake, a twitching that might attract predators and, moreover, is already known in several snakes. After that, any mutation that modified the tail, making it look more like a spider, would give the snake a further reproductive advantage. And so we get the spider ornament, which might of course still be evolving. Concurrent with the evolution of the ornament itself would be the evolution of the snake’s tail-twitching behavior, which makes the caudal appendage resemble a spider nearly perfectly.

It turns out, of course, that I’m not the first person to think of this scenario. Discover Magazine wrote about this snake last spring, and speculated about its evolution:

“The evolution of luring is more complex than contrasting color or simple shaking — the movement is precisely adapted to duplicate prey movement frequencies, amplitudes and directions, at least in specialized cases.”It’s not uncommon for many snakes to do something similar with their tails to deceive prey. The common death adder of Australia buries itself in leaves, then writhes its tail like a worm to catch lizards and frogs. The Saharan sand viper conceals itself in sand with only its eyes and nostrils visible. When a lizard comes along, it sticks its tail out from the dirt, making it squirm like an insect larvae.  The behavior — and the elaborate body modifications that can accompany it — likely arose from a behavior common to many reptiles, Schwenk explains. When they are about to strike prey, any lizards and snakes enter a hyper-alert pose. The reptiles will focus their vision by cocking their heads to the side, arching their backs, and certain species will commonly vibrate their tail tip against the ground. This can distract the prey, which will shift its attention to the vibrating tail, ignoring the reptile mouth opening to grab them.“This simple pattern leads to selection causing refining of the tail form and motion to be more attractive to such prey by more accurately mimicking actual prey movements,” Schwenk theorizes. “The other ancestral condition that could have led to caudal luring, or possibly an intermediate step in the process, is the use of tail vibration for prey distraction rather than for luring.” Indeed, those most famous tail shakers, the rattlesnakes, sometimes also use caudal luring. For example, juvenile dusky pygmy rattlesnakes, whose rattle is so small it barely makes noise, wiggle their tails to attract prey. The behavior, in fact, may be key to how rattlesnakes evolved their distinctive rears, although this theory is somewhat controversial. “Like many other apparently simple things in biology, there is a lot of complexity to caudal luring that has barely been explored,” Schwenk says. “Much of this has been considered in a piecemeal fashion, but a thorough review and synthesis … has not been attempted.”

Now we’re not sure if this is the correct evolutionary pathway, but constructing a plausible step-by-step scenario like this, and showing that the intermediate “stages” occur as adaptations among existing species, is sufficient to refute the creationist claim that structures like the spider ornament could not have evolved and thus much have been created by God (or a “designer”, which means the same thing). The same kind of argument was used by Darwin in The Origin to refute Paley’s argument that the camera eye must have been created by God. Dawkins discusses it in the video below (and, as I recall, in his book The Blind Watchmaker).


44 thoughts on “A stunning case of mimicry

  1. This seems like such a useful adaptation, one wonders why it has not evolved more frequently. Fascinating to watch!

  2. Fabulous! It’s especially striking how the “legs” seem to be moving, but I’m fairly certain they are scales not under muscular control. I hope Harry Greene sees this!

    Here’s a quick translation of the Spanish:

    It seems to be a spider aimlessly doing laps, but it’s a snake, Pseudocerastes urarachnoides, moving its tail as a lure to attract the birds that are part of its diet. If you would like to see it in action, you can see a capture here:

  3. So it wasn’t that the ancestor of the spider-tailed snake “knew” that there were other spiders in the area. Caudal wagging just happened to be prominent among snakes and some random mutation allowed the tail to develop leg-like outgrowths in various stages up to the modern-day spider-tailed snake?

    1. That is the… I will play it cautious and say hypothesis. There are indications that this sort of thing is common in evolution. Here, reptiles often have a distinctly colored tail tip which is widely used to distract larger predators toward pecking their tail rather than their head. From there some may start to use the tail as a lure to attract prey simply b/c some prey are also attracted to the brightly colored tail. The direction this can go is fairly unpredictable, and the above example is still pretty surprising.
      A term that gets used around here is ‘exaptation’, where a structure that is useful for one purpose just happens to also be useful for a different purpose. That secondary purpose can become a kind of ‘hook’ for natural selection to remodel structure, eventually to dramatic effect.
      So it is that the drill used by many female insects to lay eggs happens to be a thing that can poke into a substrate (for egg laying). In some insects, this drill is secondarily used as a weapon of defense. In wasps and their relatives (bees and ants), this structure is now fully repurposed as their stinger. Stingers are highly modified egg laying tools, and that is why only female bees, wasps, and ants can sting.

      1. The tail does not necessarily need to be brightly coloured. In the early 1960s I had a captive Coachwhip Snake (Masticophis flagellum testaceus)(long story. When it saw prey it rattled its dull tail in and above the substrate. The mouse stopped to look at where it heard the sound – not a wise thing to do.

  4. “And so we get the spider ornament, which might of course still be evolving.”

    I’m sure but where could evolution possibly take it next? Two spiders mating? A spider and a cockroach? It boggles.

  5. Spectacular!

    It completely fooled me for the first couple of seconds. Then your eyes readjust and you can see the viper’s tail moving it as well as it’s head in the background. But of course, that’s with the benefit of a stable camera shot and forewarning as to what I’m seeing. Walking or moving through the brush? Or a bird flying overhead? I bet it’s convincing enough.

    I’m a bit reminded of my cat, though, in that she likes to take her time and watch her prey a while before pouncing. As the snake’s lure shows, patient ambushing can also be a valuable adaptation.

    1. Yes it fooled us. Yet, I’m surprised a bird, with it’s superior eyesight, was lured too. A much more impressive feat.

        1. Indeed – I thought it got away.. it’s what made it chilling to me : seemed to just get nicked, but I’m a flash – life was over. Geahhhh…

        2. Yes, that struck me too. You’d think the bird would take fright after the first attempt, wouldn’t you?


  6. I am curious.

    Does a commentator have any information about what year Richard Dawkins’ video is from? And where it was broadcast?

    1. //www.rigb.org/christmas-lectures/watch/1991/growing-up-in-the-universe/climbing-mount-improbable

      Royal Institution Christmas Lecture (#3) 1991. The whole series is available at the link.

      “The CHRISTMAS LECTURES have been inspiring children and adults alike since 1825. The Lectures were initiated by Michael Faraday at a time when organised education for young people was scarce. He presented 19 series himself, establishing an exciting new way of presenting science to young people.

      The CHRISTMAS LECTURES have continued annually since the 1825 series, stopping only for four years during World War II. Many world-famous scientists have given the Lectures including Nobel Prize winners William and Lawrence Bragg, Sir David Attenborough, Carl Sagan and Dame Nancy Rothwell.

      The CHRISTMAS LECTURES were first broadcast on television by the BBC in 1936 which makes them the world’s oldest science TV series.”

      1. Oops – that was supposed to be a playlist. Oh well.

        Douglas Adams appears in The Ultraviolet Garden episode- a treat!

      2. Yes, that furry, slightly bovine looking animal with watery eyes, that wants to be the main dish in the restaurant at the end of the universe, I always found most fascinating.
        Sadly, in the film ‘The Hitchhiker’s Guide to the Galaxy’ , albeit advised by Douglas Adams himself, the passage is missing.

      3. … with guest appearance by DNA – Douglas Adams – himself!

        My favourite Adams quote is his refutation of the creationist argument – I forget its name – that the world was obviously designed for us.

        “Imagine a puddle waking up one morning and thinking, ‘This is an interesting world I find myself in, an interesting hole I find myself in, fits me rather neatly, doesn’t it? In fact it fits me staggeringly well, must have been made to have me in it!’ This is such a powerful idea that as the sun rises in the sky and the air heats up and as, gradually, the puddle gets smaller and smaller, it’s still frantically hanging on to the notion that everything’s going to be alright, because this world was meant to have him in it, was built to have him in it; so the moment he disappears catches him rather by surprise. I think this may be something we need to be on the watch out for.’”


    2. They are the Royal Institution Christmas lectures in 1991, titled “Growing up in the Universe”. The whole series is well worth watching and informative, even now 30 years. It was designed for children, and is very easy to understand, but without sacrificing much from the enjoyment that adults would get from it.

  7. Although complex eyes originated at least 40 (if not many more) times, and they evolve in the blink of an eye as Richard (well, Dan Nilsson*) showed, the actual photoreceptors appear to come in only 2 types, rhabdomeric and ciliary. Possibly originating only twice:
    Arendt also proposes that our retinal ganglion, amacrine and horizontal nerve cells derived from rhabdomeric receptors, while our photoreceptors are ciliary. A path in chordates (deuterostomes) different from the one in the protostomes.

    *I note that Dan Nilsson (and his student Susanne Pelger) always insisted it was a mathematical model, not a ‘computer’ model, as Richard appears to imply.

    1. Come to think of it, all multi-cellulars, nay eukaryotes, use carotenoids (retinal-like molecules) for photoreception. Only some cyanobacteria use phycobilins for light reception, and they really do not have eyes.
      Moreover, all use opsin-like proteins in the second step.
      Maybe photoreception has a single origin in eukaryotes. (note, this is all AFAIK)

  8. One of the most incredible adaptations I’ve seen. And it’s my new favorite snake.

    There’s a nice animated description of the camera eye’s evolution in the series “Cosmos” narrated by N. deGrasse Tyson.

  9. That’s really convincing mimicry. I wonder if the snake understands why it moves its tail the way it does. Or is it an involuntary movement that just happens when the snake is waiting in ambush. Not sure how you could figure that out though.

    1. I doubt the snake “understands” what it is doing. It’s more likely it was just programmed by natural selection to move its tail, and then programmed to develop a spider-like ornament on the tail tip. But it needn’t be an autonomic movement, either: the snake could just move its tail tip when its resting and hungry.

      1. I wonder if the snake moves its tail only when it knows prey is nearby and not any other animal that could actually bite the snake’s “spider” … I think perhaps this is the amazing nature of mimicry, that no animal exists that would bite the “spider” and survive to reproduce… I need to read about mimicry…

  10. This reminds me of that caterpillar who can minikin Viper I think the snake is called? I don’t remember exactly what it’s called though.

  11. I just had a dream with a number of terrifying animals in it, undoubtedly because of this post…. and sci-fi movies too, but you see I haven’t seen any of that kind of sci-fi for a long time…

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