Spider-tailed Adders

This species was brought to my attention about two years ago by a friend who, like me, was working on completing her Master's thesis at that time. In the post-script of her message, titled 'Probably the coolest thing I've learned in weeks', she wrote "PS I swear this started out as a legitimate search for information for my thesis." In addition to being a welcome distraction from my writing, the story of the Spider-tailed Horned Viper, Pseudocerastes urarachnoides, is, in my opinion, one of the most interesting recent discoveries in herpetology.

The first specimen of P. urarachnoides was collected in 1968 by the Second Street Expedition, mounted on behalf of Chicago's Field Museum of Natural History by a retired businessman and activist couple, William and Janice Street. The primary purpose of the expedition was to collect mammal specimens, but reptiles were also collected, including the first specimen (also known as a type specimen or holotype) of P. urarachnoides. Because only a single specimen was collected, its unusual tail morphology was thought at first to be a solfugid clinging to the tail. Solfugids (also called solpugids, camel spiders, wind scorpions, or sun spiders) are members of the same arthropod class, the Arachnida, as spiders and scorpions, although they are neither spiders nor scorpions. Upon closer examination, the Field Museum's Steven Anderson found that the tail of the snake bore a peculiar structure with an uncanny resemblance to a solfugid that could have been a tumor, congenital defect, or growth caused by a parasite. The snake was identified as Pseudocerastes persicus, the Persian Horned Viper, and entered into the Field Museum collection, where it was almost, but not quite, forgotten.

The story ended there, until 2001, when Hamid Bostanchi collected a second specimen with identical tail morphology to the first. A third specimen was later discovered in the collection of the Poisonous Animal Section of the Razi Institute in Karaj, Iran, in 2008; it had been misidentified as a Desert Horned Viper, Cerastes cerastes. Together with Anderson, who had described the first specimen, and their colleagues Haji Gholi Kami of Gorgan University and Ted Papenfuss of the Berkeley Museum of Vertebrate Zoology, they described the new species in 2006, naming it Pseudocerastes urarachnoides, from the Greek ura (tail), arachno (spider) and ides (similar to). In their paper, Bostanchi et al. described the structure of the tail, which is formed of the last pair of subcaudal scales, much enlarged, and a single enlarged dorsal scale. The elongated components are modified lateral scales. X-rays taken by the team showed that the caudal vertebrae extend well into this structure and are not deformed or modified. Bostanchi et al. also speculated that the function of the modified tail might be to augment caudal luring behavior exhibited by many vipers. By mimicking a solfugid, birds or other would-be solfugid predators could be enticed to approach within the viper's striking distance.

Behavioral observations made in 2008 of a live P. urarachnoides captured in western Iran and maintained in captivity confirm these ideas. Closed-circuit video was used to record behavior, and the results published in the Russian Journal of Herpetology by Behzad Fathinia of Razi University and his colleagues. They observed the snake, a juvenile male that regurgitated a Crested Lark, using its caudal lure to attract sparrows and baby chickens that they introduced into its enclosure. When the birds approached and pecked the tail, the snake struck and envenomated the birds, a process taking less than one half second. A bird was also found in the stomach of the paratype specimen, further evidence that this species might feed heavily on birds in the wild with the aid of its spectacular caudal lure. The tail of P. urarachnoides probably represents the most elaborate morphological caudal ornamentation known in any snake, with the possible exception of the sound-producing rattles of rattlesnakes.