Spider-tailed horned viper

The spider-tailed horned viper (Pseudocerastes urarachnoides) is a species of viper, a venomous snake, in the family Viperidae and genus Pseudocerastes. The genus is commonly known as "false-horned vipers".

The species is endemic to western Iran and over the border region with Iraq. It was originally described by scientists as Pseudocerastes persicus, attributing the tail to either a parasite, deformity, or tumors. Another specimen was found in 2003. P. urarachnoides was officially described in 2006. The head looks very similar to that of other Pseudocerastes species in the region, but the spider-tailed horned viper has a unique tail with a bulb-like end that is bordered by long drooping scales that give it the appearance of a spider. The tail tip is waved around and used to lure insectivorous birds to within striking range.

Etymology
The specific name, urarachnoides, is derived from Ancient Greek (οὐρά tail + ἀράχνη spider + οειδής like), and refers to this snake's spider-like tail-tip, as does the common name, spider-tailed horned viper.

Description
Like other vipers in the genus Pseudocerastes, the scales above the eyes rise up to give P. urarachnoides a horned appearance. A specimen had been collected in 1968 as part of the Second Street Expedition to Iran and deposited in the Field Museum of Natural History at Chicago, identified as Pseudocerastes persicus. It was, however, found to be distinct, and it was described as a new species in 2006. The species is distinguished by a number of characteristics. There are about 16 to 17 scales between the horns, and the scales on the body above are rougher than on other species in the genus. There are 15 pairs of subcaudal scales, and the scales on the sides of the tail are elongated and appear like appendages of an arthropod. The tip of the tail is inflated into a bulb-like shape.

Distribution and habitat
The species is found in the western parts of Iran and over the border into the eastern part of Iraq. It is found in the Zagros Mountains, more commonly on the western side of the mountains. Its current range is relatively small and getting smaller. This species lives in higher elevations that are primarily composed of gypsum. It prefers deep cracks and holes within the rock. They use these areas because they trap humidity and moisture during the hot summer months. During the day, it is most commonly found in the shade of a bush.

It overlaps in distribution with P. fieldi in Gilan-e Gharb, next to Qasr-e Shirin, Kermanshah, and with P. persicus in Bina and Bijar, Ilam Province.

Mimicry and behavior
The snake is a superb mimic. The tail resembles a spider or other arachnid, and the authors who described the species speculated that it was used as a lure to attract birds, as a digested lark had been found in the stomach of the paratype specimen.

The tip of the tail is used as a lure in several other species of snake, including the horned adder (Bitis caudalis), sidewinder (Crotalus cerastes), eastern massasauga (Sistrurus catenatus), eastern copperhead (Agkistrodon contortrix), common death-adder (Acanthophis antarcticus), northern death-adder (Acanthophis praelongus), and green tree python (Morelia viridis), but none of these examples has the unique elongated scales that give it the appearance of arthropod appendages. The actual use of the tail to lure birds (an example of caudal luring) has been confirmed in field studies; the tail is moved in a figure-of-eight pattern.

Footage of the spider-tailed horned viper using its tail to lure a migrating bird featured in the Asia episode of the BBC series Seven Worlds, One Planet narrated by David Attenborough.

Venom
The venom of Pseudocerastes urarachnoides mainly acts as a cytotoxin. It targets cells and destroys them. Unlike its sister species, this cytotoxin destroys a wide variety of cell types. It has minor neurotoxic effects. This venom works as a procoagulant, with a remarkable coagulation speed, with reports of about 13 seconds before coagulation. Compared to its sister species, this venom is highly-specialized for birds. The venoms of the sister species, P. fieldi and P. persicus, do not have significant effects on birds and are more suited for mammals and amphibians. P. urarachnoides has a significant effect on birds, mammals, and amphibians. There is speculation that the venom affecting amphibians, toads specifically, could be a sign that these are the main diet of the young snakes. Though more information about the juveniles still needs to be conducted. In a study by Brouw (2021), it was found that the venom of this species does have a coagulant effect on humans and affects the human's Factor X. However, there has never been a recorded case of human envenomation in the wild. The severity of the effects that this venom would have on humans is not known.

Taxonomy
Molecular studies based on cytochrome b show it to be closer to the Persian horned viper (Pseudocerastes persicus) than to Field's horned viper (Pseudocerastes fieldi).