Scientists look inside the ‘spider’ growing on the tip of a rare viper’s tail

(CNN) — In the arid, inhospitable climate of western Iran’s Zagros Mountains, life has found a remarkable way to survive.

Circling above, a bird glimpses what at first glance looks like a plump spider climbing on the rocks. But as the avian predator swoops in, it becomes the prey as something that initially appeared to be a rock moves to snap it up.

The roughened rock is actually the bumpy surface of a viper with hornlike features on its head — and the “spider” is the tip of its tail, flicking with tantalizing movement.

When scientists collected a specimen of the spider-tailed horned viper in Iran in 1968, they disregarded the arachnid-shaped tail tip.

“Everybody who looks at the tail thinks it has a tumor or some sort of growth or a parasite on it, and that it’s some sort of anomaly,” said Sara Ruane, associate curator of herpetology and director of core laboratories at the Field Museum’s Negaunee Integrative Research Center in Chicago. “And it wasn’t until another individual was collected in 2003 that somebody thought, ‘You know, maybe this is the norm for this species.’”

The 1968 specimen remains within the Field Museum’s collections as the holotype, or the individual that scientists reference to establish the species’ physical characteristics, Ruane said. Researchers used this holotype in 2006 to describe the snake as a newly identified species, Pseudocerastes urarachnoides.

Now, Ruane and her colleagues have used X-ray imaging to get a detailed look inside the vertebrae at the tip of the tail. The findings were published Thursday in the scientific journal The Anatomical Record.

“In some other snakes, like sand boas and rubber boas, the vertebrae at the tip of the tail have extreme shapes that look really alien,” said lead study author Dr. Georgios Georgalis, an associate professor in the Institute of Systematics and Evolution of Animals at the Polish Academy of Sciences in Kraków. “We wanted to investigate if the spider-tailed horn viper’s tail was similarly strange inside.”

Instead, the researchers uncovered something unexpected — and it’s a lesson in why skeletons don’t always tell the complete story of animals, both living and long extinct.

A snake that lives on the edge

While a doctoral student, Ruane first saw a “mind-blowing” video in 2008 of a spider-tailed horned viper moving its unusual tail to lure birds.

Other vipers are known to flick their tails, especially baby vipers with bright green or neon yellow tail tips that might appear like a wiggly worm to unsuspecting frogs.

“But the apparatus on the tip of the spider tail viper is on a whole other level,” Ruane said.

The viper is elusive, using its musculature to live along dramatic cliff faces. The creature’s extreme habitat makes it challenging to study or collect in the wild.

“Some of the video I’ve seen of these snakes in action shows them in tiny crevices that you would never think any animal is able to cling onto very well, let alone a snake that doesn’t have any limbs,” Ruane said. “When it actually grabs a bird, sometimes the whole snake goes flying off the rock face along with the bird. The snake’s whole style of life is also pretty wild.”

When Ruane and Georgalis met at a conference in 2023, he expressed an interest in studying the viper’s holotype. As a paleontologist, Georgalis studies modern snakes to understand the fragmented fossil remains of ancient extinct ones.

Ruane recalled thinking the timing was excellent because the Field Museum was getting a new X-ray computed tomography, or XCT, lab. The technology enables the scanning of objects to create thousands of X-rays, resulting in 3D renderings that can peer inside structures without damaging specimens — such as the inside of a snake’s tail.

Snake skeletons have hundreds of vertebrae, which means that snake vertebral columns have been poorly studied in the past, said Dr. Kurt Schwenk, professor emeritus in the department of ecology and evolutionary biology at the University of Connecticut in Storrs. Schwenk was not involved in the study.

The holotype was one of the first specimens to be scanned once the lab was up and running in late 2024. The scans revealed that the vertebrae within the tail were normal and typical of most vipers, Georgalis said.

Instead, the fleshy appendage is composed of elongated and modified scales, he said, with no bony element present.

“If we had these remains only as fossils today, we would not be able to reconstruct or imagine its spider-like tail morphology,” Georgalis wrote in an email. “This is a ‘painful’ reminder that fossil remains in paleontology do not afford the ‘absolute truth’ on how magnificent were once extinct reptiles and other now lost animals.”

As a reptile morphologist, Schwenk didn’t expect that the lure would include anything other than soft tissue.

“Modifications of the scales into various structures, such as spines, horns, knobs, etc., are fairly common in lizards and snakes and outside of the head, rarely, if ever, involve the underlying skeleton,” Schwenk wrote in an email.

While snake vertebrae are typically shorter in the tail, Schwenk noticed that the tail vertebrae seemed unusually slight in the spider-tailed horned viper.

“Having shorter vertebrae in the tail makes it especially flexible,” Schwenk said. “This would enhance the tail’s ability to make the tight, sinuous bends used to create the realistic, spider-like movements of the lure.”

A ‘spectacular’ adaptation

Questions remain about the spider-tailed horned viper, such as how it evolved this interesting feature, known as a caudal lure, on the end of its tail.

“If you look at the two species that are most closely related to the spider-tailed viper, they’ve got funny-looking tails,” Ruane said of the Persian horned viper and the Field’s horned viper.

“It seems smaller than it should be, and very keratinized,” Ruane said of the hard, dry outer coating of the tail, “and at least on the specimens I’ve seen, it’s a different color. The snake is sort of a sandy brown or gray, and then the tail is black.”

Many snakes twitch their tails, whether to scare off predators or lure prey, but the spider-tailed horned viper may be an extreme example, such as rattlesnakes, of how features evolve over time out of certain behaviors, she said.

Future research could focus on what genetic mechanisms may have been turned on or off to make the spider-like appendage appear, Ruane added.

Schwenk said he believes the behavior was likely present in the viper’s ancestors.

“The spider-tailed viper has taken this ancestral behavior and modified it into something really spectacular,” Schwenk said. “The spider-like structure in combination with sinuous motions of the tail create an impression of a moving spider that is so realistic that the first time I saw a video of the behavior I was completely fooled. It’s a wonderful example of the power of natural selection to shape animal form and function.”

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