If you like evolution and natural history, you should already be reading Ed Yong’s terrific site Not Exactly Rocket Science at National Geographic. In his short post “This snake has a tail that looks like a spider,” he describes a remarkable and newly found type of mimicry.
The snake is, appropriately, the spider-tailed viper (Pseudocerastes urarachnoides), first described formally only 7 years ago from Iran. It was known since the sixties, but the one specimen’s tail was dismissed as a tumor or deformity. We now know from other specimens that this is indeed a species-specific trait. As Yong notes:
The tail is bizarre. If you saw a close-up photo of it, you’d struggle to believe that there was a snake at the other end. There’s a large orange or grey bulb at the tip, and the scales just before that are bizarrely long and thin. Together, these features look a bit like the legs and abdomen of a spider or their close relatives, the solpugids or ‘camel spiders’.
A close-up photo:
This video shows the snake waving its tail, and the “appendages” of the mock spider look amazingly lifelike.
Now what is this bizarre appendage for? Two possibilities come to mind: the mock spider could be used to scare off potential predators, or it could be used as a lure to attract prey. Given that this viper has a pronounced threat display (see Yong’s piece), the latter seems more likely. And tests show that the “lure” hypothesis is probably correct:
And then there’s the tail. It’s probably a lure, like a fisherman’s fly. By resembling a tasty morsel, it draws potential prey into the snake’s striking range. Fathinia tested this idea by putting a chick into the same enclosure as his captive viper, which duly undulated its tail.
“It was very attractive and looked exactly like a spider moving rapidly,” Fathinia wrote. “After approximately half an hour, the chick went toward the tail and pecked the knob-like structure. The viper pulled the tail structure toward itself, struck and bit the chick in less than 0.5 seconds. The chick died after 1 hour.” A sparrow met the same fate.
Yong notes that “caudal lures” aren’t uncommon in snakes, but this one is extraordinarily elaborate. A more normal lure is present in the northern death adder from Australia (Acanthophis praelongus); it’s a wormlike tip of the tail that’s moved to attract prey while the rest of the snake remains absolutely still: