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.”
Parece una araña dando vueltas sin sentido, pero es una serpiente Pseudocerastes urarachnoides moviendo su cola como señuelo para atraer a los pájaros que forman parte de su dieta. Si queréis verla en acción, aquí podéis ver una captura: https://t.co/vRhh0JJlza. #naturaleza pic.twitter.com/7wJ0LjxPeV
— ✍️ Leonardo D'Anchiano (@HdAnchiano) January 18, 2020
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).