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Tuesday, April 30, 2013

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 tail in question
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.

Egyptian Giant Solfugid (Galeodes arabs)
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.

Within its restricted range in the mountainous terrain of western Iran, P. urarachnoides inhabits rock crevices in the gypsum formations that comprise its hilly, arid habitat. Adaptations of the genus Pseudocerastes to desert life include supralabials (upper lip scales) with a serrated lower margin and a groove to accommodate the lower lip, which provide complete closure of the mouth and prevent sand from entering. The nostrils also have a valvular prominence to the same effect. The other two species of Pseudocerastes, P. persicus and P. fieldi, share these characteristics. These two species are sometimes combined, although differences in venom chemistry and scalation, along with the fact that their ranges are separated by the Zagros Mountains, suggest that they are probably distinct species (and they are certainly distinct morphologically from P. urarachnoides). Both overlap in range with P. urarachnoides in places.

P. urarachnoides

Two other recent and noteworthy discoveries of Old World pitvipers are worth a mention. One, Protobothrops mangshanensis, is a large and beautiful pitviper discovered in 1990 in mountainous regions in southern Hunan and reputed to be the only non-cobra capable of spitting venom. The other, Atheris matildae, discovered in 2011, is a member of an especially popular genus in the pet trade (although this could be said of many of the most beautiful vipers). The exact type locality of A. matildae, in the southern highlands of Tanzania, was concealed in order to limit collection for the pet trade. In addition, a novel strategy is being tested: A. matildae is being bred at a facility in Tanzania and the first few dozen offspring are being given away to collectors in order to reduce the market for illegally collected specimens. Whether this strategy will succeed remains to be seen, but hopefully A. matildae can be saved from the same sad fate as the Lao Newt, Roti Island Snake-necked Turtle, Chinese Leopard Gecko, coelacanth, and other species that have been overcollected almost as soon as they were described.

Atheris matildae
Protobothrops mangshanensis

Check out another amazing new snake discovery at Greg Laden's blog: once thought to be a single deadly sea snake, Enhyrina schistosa is actually two!

Update 1/16/2015: Check out this video of a P. urarachnoides actually caudal luring and catching a bird!


Thanks to Heather Heinz for bringing P. urarachnoides to my attention, and to photographers and videographers Michael Kern, Behzad Fathinia, Michael & Patricia Fogden, Omid Mozaffari, and Alireza Shahrdari.


Bostanchi H, Anderson SC, Kami HG, Papenfuss TJ (2006) A new species of Pseudocerastes with elaborate tail ornamentation from western Iran (Squamata: Viperidae). Proceedings of the California Academy of Sciences 57:443-450 <link>

David P, Tong H (1997) Translations of recent descriptions of Chinese pitvipers of the Trimeresurus-complex (Serpentes, Viperidae), with a key to the complex in China and adjacent areas. Smithsonian Herpetological Information Service 112:1-31 <link>

Fathinia B, Anderson SC, Rastegar-Pouyani N, Jahani H, Mohamadi H (2009) Notes on the natural history of Pseudocerastes urarachnoides (Squamata: Viperidae). Russian Journal of Herpetology 16:134-138 <link>

Fathinia B, Rastegar-Pouyani N (2010) On the Species of Pseudocerastes (Ophidia: Viperidae) in Iran. Russian Journal of Herpetology 17:275-279 <link>

Menegon M, Davenport T, Howell K (2011) Description of a new and critically endangered species of Atheris (Serpentes: Viperidae) from the Southern Highlands of Tanzania, with an overview of the country’s tree viper fauna. Zootaxa 3120:43-54 <link>

Stuart BL, Rhodin AGJ, Grismer LL, Hansel T (2006) Scientific description can imperil species. Science 312:1137 <link>

Creative Commons License

Life is Short, but Snakes are Long by Andrew M. Durso is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.


Anonymous said...

Thanks for posting these amazing snakes! This one is particularly awesome.

Unknown said...

Wow. This is completely new to me...just wow.

Who is collecting coelacanths?

Anonymous said...

Great article, Andrew! Not only are we both snake enthusiasts, and USU Aggies, but we also blog about the same critters. I wrote this article on P. ururachnoides back in 2010. I write more for the general population, but prefer to read articles like yours. I do envy your access to the resources you have for research though. Nice to meet you, and hope we cross paths in the near future.


Jamison - Layton, UT

Andrew Durso said...

Thanks Jamison! Nice article yourself. If you're ever up in Logan, look me up.

Andrew Durso said...

Every museum wants a coelacanth specimen...


Andrew Durso said...

Thanks! I agree.

fcotterill said...

Splendid. Glad my original post on this fascinating snake has attracted follow ups in the attention this viper deserves !

Now, what a splendid example of coevolution, with which to pith a creationist :-) :-)

here is recent film footage of Pseudocerastes urarachnoides luring birds in the wild.


Andrew Durso said...

Awesome footage! Thanks for your original article, which was very informative to me when writing this post.

The Grail said...

I am curious to know more about it. How strong is their venom? I have read that they usually prey more on migratory birds due to a lot of the birds from that area apparently knowing better than to attack the tail.

Andrew Durso said...

Very good questions. I don't think anyone has looked at their venom, although other species of Pseudocerastes (persicus, fieldi) have venom that is quite different from one another, possibly because these species have been isolated from one another for a long time. Antivenom is made only for P. persicus, but the variation suggests that it would be of limited use for the other species. All three species have venom that is capable of causing medically significant bites in humans.

The detailed diet analysis done by Fathnia in 2015 did suggest that P. urarachnoides prey mostly on migratory birds rather than resident birds, although they also eat lizards, mammals, and arthropods.