If you need to identify a snake, try the Snake Identification Facebook group.
For professional, respectful, and non-lethal snake removal and consultation services in your town, try Wildlife Removal USA.

Saturday, May 28, 2016

Rattlesnake Roundups Revisited

This article will soon be available in Spanish

A chalkboard at the 2016 Sweetwater Rattlesnake Roundup,
showing that a record number of pounds of snake had
already been bought and sold by the second day, and that
commerce was suspended on the third and fourth days of the
event due to the massive surplus.
Photo source unknown.
At the 58th annual Sweetwater Rattlesnake Roundup this March, a record 24,481 pounds of rattlesnakes (about 25,000 individuals), primarily Western Diamond-backed Rattlesnakes (Crotalus atrox), were slaughtered. That's over four times the all-time average and about five times the recent average, breaking from a trajectory of slow decline at the few remaining rattlesnake roundups. The Sweetwater Jaycees attribute this year’s record catch to heavy rains, an explanation which might hold some water, but another probable contributing factor is the possibility of an impending Texas Parks & Wildlife ban on using gasoline fumes to collect rattlesnakes, which was discussed this week at a meeting in Austin on May 25th, 2016. The Texas Parks and Wildlife Commission decided to begin developing language for a new rule either prohibiting or further regulating this practice in the state. The rule is still far from going into effect, and would include a two-year delay on the effective date. It won't be reviewed again until November 2016 (at which time, watch this space for a link to an opportunity for a public comment, if available). TPWD's Snake Harvest Working Group recommended earlier this year that Texas join 29 other states in banning this environmentally-harmful practice, which has been shown to kill numerous non-target species and has been compared with other unsportsmanlike methods of hunting, such as shooting at an out-of-range bird or fishing with dynamite. The state wildlife agency has been moving slowly but steadily to regulate rattlesnake collection in Texas because of the economic importance of rattlesnake roundups to towns like Sweetwater (e.g., over 25,000 people contributed over $8 million to the local economy in 2015, although the TPWD report found that the weather and the diversity of other events had stronger associations with profits than the number of rattlesnakes at an event).

Locations of the remaining rattlesnake roundups,
including non-lethal festivals.
From TPWD Report Reference Document (p. 22)
Ironically, this year's surplus of snakes drove the price of rattlesnake down so much (historically as high as $10.00 per pound, this year the price fell below $0.50/lb. despite efforts to maintain higher prices) that only about a quarter of the rattlesnakes collected were purchased for their meat, rattles, and skins before all demand had been exhausted. Rattlesnakes collected using gassing are no longer purchased by the antivenom industry, because of their short lifespan and poor health (as well as a more nuanced understanding of the importance of geographic variation in venom composition, emphasizing the necessity of knowing the geographic origin of each snake used in venom research). The fate of the rattlesnakes left unsold after Sweetwater (which some have speculated as being up to 75,000) has not been made public, although reports suggest that prices are also down at other roundups in Texas and Oklahoma, possibly as a result of vendors trying to sell their snakes there. Anyone who has gone to great expense to collect snakes in this manner and now cannot find a buyer is at risk of losing their investment. Claims about the impacts on snakebites to humans and livestock if these snakes were to be released are unsubstantiatable and untrue, considering that the survival of wild snakes captured and released elsewhere is greatly reduced (not to mention the dubiousness of the link between rattlesnake abundance and snakebite frequency in the first place).

Trajectory of profit (red, blue), number of snakes (purple), and
weather conditions (green) at the Sweetwater Roundup over the last decade.
Chart prepared by Rob Denkhaus, TPWD Wildlife Diversity Advisory Committee
and presented in TPWD Report Reference Document (p. 64)
I am hopeful that eventually all stakeholders can overcome the cognitive dissonance between the flawed concept of predator population control (which was the original impetus behind rattlesnake roundups) and the implicit economic reasons behind their persistence. Although rattlesnake roundups are inarguably sensational and exploitative, claims about the sustainability of the wild rattlesnake harvest cannot currently be independently evaluated (I encourage anyone interested in the subject to read my previous article and check out this well-researched book). But, increasing oversight by Texas wildlife agencies could allow them or others to monitor the effect of the harvest on rattlesnakes, which could lead to valuable insights into snake biology and help prevent economic and environmental disasters like this year's Sweetwater roundup. This week's decision inches us towards the hopeful possibility of a sustainable snake harvest that could, over time, change the relationship between humans and western diamondbacks into a positive one, similar to our view of white-tailed deer, bobwhite quail, or largemouth bass. It's a non-traditional model for snake conservation, to be sure, but the efforts of the TPWD Snake Harvest Working Group combined with actions being taken by some unlikely allies, such as roundup organizer Jackie Bibby, will hopefully continue to move us towards a common goal of respectfully managing rattlesnakes as either game or non-game wildlife and not as pests. The best part: we can help people in the process (e.g., by providing healthier products with stable prices, such as rattlesnake meat untainted with gasoline).

Percentage of time radio-tracked Burmese Pythons spent
fully concealed (black), partly visible (gray), and mostly visible (white).
In nineteen 30-minute searches of a 30 x 25 m enclosure containing
ten pythons, only two pythons were detected out of
190 possible detection opportunities.
From Dorcas & Willson 2013
And—as if the irony weren't already thick enough—compare the above totals with the ~2000 lbs. of Burmese Python (106 snakes) collected in Florida this year as part of an Florida Fish and Wildlife Conservation Commission-sponsored contest to control a snake whose populations actually do need to be "controlled" (despite the near-total impossibility of doing so). Among the several reasons for the difference include the lack of cultural inertia promoting snake hunting in Florida, the challenging habitat of the Everglades, and the snakes' biology—pythons don't aggregate the way rattlesnakes do. If gassing is banned in Texas, flushing rattlesnakes out of their hibernacula en masse will no longer be a legal hunting strategy. Does this mean that rattlesnake roundup totals will become more like those of the Python Challenge? Not necessarily—the TPWD report references alternative strategies already in use in other parts of the country that can still yield hundreds of pounds of rattlesnakes. Would a change in the hunting methods allowed have positive effects on snakes and other wildlife? Almost certainly. What would be the impacts on the roundup? I think it's worth pointing out that many former roundups, such as the Claxton Rattlesnake Festival in Claxton, Georgia, hosted by the Evans County Wildlife Club, and the Fitzgerald Wild Chicken Festival in Fitzgerald, Georgia, still generate economic opportunity for their towns without collecting and killing wild snakes. I think it's quite likely that events like the Sweetwater Rattlesnake Roundup could continue to bring benefits to their communities without using gas to extract rattlesnakes from their dens.


Thanks to Ray Autry and Dale Burton from the Rise Against Rattlesnake Roundups Facebook group for pointing me to some resources about the 2016 Sweetwater Roundup.


Adams, C.E. and J.K. Thomas. 2008. Texas Rattlesnake Roundups. Texas A&M University Press, College Station, Texas <link>

Arena, P. C., C. Warwick, and D. Duvall. 1995. Rattlesnake Round-ups. Pages 313-324 in R. L. Knight and K. Gutzwiller, editors. Wildlife and Recreationists. Island Press, Washington, DC <link>

Campbell, J. A., D. R. Formanowicz Jr, and E. D. Brodie Jr. 1989. Potential impact of rattlesnake roundups on natural populations. Texas Journal of Science 41:301-317.

Dorcas, M. E., and J. D. Willson. 2013. Hidden giants: problems associated with studying secretive invasive pythons. Pages 367-385 in W. I. Lutterschmidt, editor. Reptiles in Research. Nova Biomedical, New York, New York <link>

Elliott, W. R. 2000. Conservation of the North American cave and karst biota. Pages 665-689 in H. Wilkens, D. Culver, and W. Humphreys, editors. Subterranean Ecosystems. Elsevier, Amsterdam.

Jackley, A. M. 1939. Rattlesnake Control and Conservation. South Dakota Conservation Digest 6:11.

Margres, M. J., J. J. McGivern, M. Seavy, K. P. Wray, J. Facente, and D. R. Rokyta. 2015. Contrasting modes and tempos of venom expression evolution in two snake species. Genetics 199:165-176 <link>

Reinert, H., and R. Rupert. 1999. Impacts of translocation on behavior and survival of Timber Rattlesnakes, Crotalus horridus. Journal of Herpetology 33:45-61 <link>

Texas Parks and Wildlife Department. 2016. Snake Harvest Working Group Final Report <link> <references> <summary>

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

Tuesday, April 26, 2016

Even snakes have their charismatic megafauna

This post will soon become available in Spanish.

Bitis harenna
From Gower et al. 2016
Last year, I wrote about the 10,000th reptile and the 3,500th snake species to be described by scientists. The pace has not slowed down—as of its most recent update last week, The Reptile Database currently lists 3,596 species of snakes out of a total of 10,391 species of (non-avian) reptiles. A few weeks ago, the March 21st issue of the frequently-published journal Zootaxa (volume 4093, issue 1) included descriptions of three of these new snake species. What's interesting is that I initially looked this issue up because I saw one of them being shared a lot on social media—a new large species of viper. The other two, a pipesnake and a blindsnake, hadn't received as much attention. Zootaxa tweets all of their new species, and an examination of their feed shows that the viper tweet received 4 retweets and 2 likes, whereas the pipesnake and the blindsnake received 2 retweets and one like each (even though the pipesnake had a photo1 and was on the cover). Even though that's a small sample size, I think it's telling that even snakes have their charismatic megafauna.

A bongo (Tragelaphus eurycerus, top)
and a tiger (Panthera tigris, bottom).
You only needed a caption for one
It seems backwards, in a way, that the dangerously venomous viper should be more popular than the innocuous pipesnake. One conservation blogger, Corey Bradshaw, put it nicely by saying that "the only thing worse than being labelled deadly is not being called anything at all". Bradshaw pointed out that drawing attention to the potential for a species to cause harm to humans is not necessarily bad for the species in question. Even though snake biologists often decry these claims as exaggerated (usually because they are), Bradshaw wondered whether they are really very harmful. He suggested that people are generally more fascinated with animals that could kill us (even if they rarely do) than they are with entire groups of benign species, such as skinks or plethodontid salamanders, which are often considered boring (if a person is even aware of their existence). Compare tigers with, say, bongos. Both are critically endangered, inarguably gorgeous animals from exotic places. Tigers sometimes kill and eat people. Everyone knows a tiger. Most people think a bongo is a drum. Or, if you want a snake example, take rattlesnakes. Rattlesnakes are the Bald Eagles of snakes. They are distinctly North American. Everybody in North America knows them. One was on our flag. In contrast, the USA has never had a Smooth Greensnake (Opheodrys vernalis) on its flag, even though they are beautiful and North American and eat spiders. Perhaps the idea that any publicity is good publicity applies to conservation as well. Then again, perhaps not—many residents of Massachusetts are needlessly worried about a Timber Rattlesnake reintroduction plan on an island in the Quabbin Reservoir, probably in part because of the bad PR that rattlesnakes get on a regular basis. If the Massachusetts Division of Fisheries & Wildlife were reintroducing Smooth Greensnakes, I doubt that most people would care (and it certainly wouldn't have been the subject of such venomous debate in the media). Indeed, Illinois's Lincoln Park Zoo is reintroducing Smooth Greensnakes in Chicago, and nobody is writing letters to the editor about it (and, in a way, that's a shame, because it's an interesting and worthwhile effort).

Letheobia mbeerensis
From Malonza et al. 2016
Anyway, I wanted to give some well-deserved press to the two less-publicized new snakes. The blindsnake, Letheobia mbeerensis, is pink with tiny, barely visible eyes. It was described from a single specimen collected southeast of Mt. Kenya in April of 2014 by a local farmer, who found it while tilling his fields. This person, whose name was not known to the scientists who wrote the article, made a considerable effort to get the snake identified—he traveled 125 miles from Siakago to Nairobi, where he gave the specimen to the Nairobi Snake Park, who forwarded it to herpetologists at the National Museums of Kenya. It is unique in having a relatively long tail (for a blindsnake), and in being found in a moist inland savanna. The other two Kenyan species of Letheobia, one of which was just described in 2007, are found in coastal lowlands with sandy soils. It is the 24th species of blindsnake known from Kenya, but I can guarantee that it won't be the last.

Historical drawings of Cylindrophis ruffus
Illustrations A-C from Scheuchzer 1735
D-E from 
Seba 1735
From Kieckbusch et al. 2016
The story of the new pipesnake is even more interesting, and I suspect the paper in which it is described will ultimately be the most read and most cited of the three snake papers in this issue. This is because, in addition to describing the new species, it contains "an overview of the tangled taxonomic history of C[ylindrophis] ruffus", a widespread species commonly known as the Red-tailed or Common Pipe or Cylinder Snake. The fourteen species of Asian Pipesnakes (family Cylindrophiidae) are secretive and semifossorial snakes with small eyes, bodies that barely taper at all, and ventral scales only slightly larger than or equal in size to their dorsal scales. Many have contrasting light and dark ventral blotching with conspicuous bright coloration on the underside of their short tail, which they expose when threatened. Scientific knowledge of these snakes predates modern biological nomenclature. One is pictured in Albertus Seba's Thesaurus, which was one of Linnaeus's main sources, although Linnaeus didn't include C. ruffus in either the 1758 or the 1766 edition of his Systema Naturae—instead, its first post-Linnaean description was written by Laurenti in 1768. Compared with other CylindrophisC. ruffus has a much larger distribution than any other species of Asian pipesnake. It's one of those species that is really a species complex—a group of closely related species that are very similar in appearance, to the point that the boundaries between them are often unclear. Other well-known examples include African House Snakes (Boaedon fuliginosus, formerly Lamprophis fuliginosus) and American Milksnakes (Lampropeltis triangulum). Often unusual populations of these species are described as separate species, but without extensive rangewide sampling it's easy to miss more subtle, clinal variation, especially when that variation is genetic rather than morphological. A recent revision of milksnakes split this wide-ranging species into several, and researchers have been working on African House Snakes as well. But no one has really examined Red-tailed Pipesnakes. Last year, a group of European and Indonesian researchers examined a large number of Cylindrophis museum specimens and discovered several specimens which did not fit any recognized species. But many of these specimens are old and some of their locations are uncertain. We don't have a lot of molecular data, and we have no specimens at all from many areas. And, no one has yet carried out a totally comprehensive review of the species complex (which really should encompass the entire genus, since the milksnake researchers found that some "milksnakes" were actually more closely related to mountain kingsnakes than they were to other milksnakes).

Cylindrophis ruffus raising its tail "flag"
Despite its re-description in 2015, Cylindrophis ruffus is still a species complex that suffers from a lot of complexity. Its morphology is highly variable. Its geographic range limits are unsettled. There is no type specimen. The original type locality (“Surinami”) is a hemisphere away, obviously an error, which complicates decisions about which populations of C. ruffus should get to keep that name and which should change. The 2015 paper, as the authors of this month's paper delicately put it, "contain[s] some inaccuracies, including descriptive errors, which unfortunately increase the complexity of an already intricate taxonomic situation". The researchers state that they are currently undertaking the kind of comprehensive review that I called for above, but that in the process they discovered a morphologically distinct population from central Java, which they describe as Cylindrophis subocularis in this paper. But the real value of this paper, in my mind, is the step-by-step description of the history of this snake, starting with its first depiction in 1735 and continuing to present day. I'll leave the gory details for those who are really interested (the full-text is available here), but suffice it to say that the story of Cylindrophis ruffus is much more interesting than I ever knew (it took almost 100 years to get the geography right), and far from over.

1 Granted, it was a photo of a preserved specimen.


Thanks to M. A. MuinNigel Swales and Marcus Meissner for the use of their photos.


Amarasinghe, A. A. T., P. D. Campbell, J. Hallermann, I. Sidik, J. Supriatna, and I. Ineich. 2015. Two new species of the genus Cylindrophis Wagler, 1828 (Squamata: Cylindrophiidae) from Southeast Asia. Amphibian and Reptile Conservation 9:34-51 <link>

Gower, D. J., E. O. Z. Wade, S. Spawls, W. Böhme, E. R. Buechley, D. Sykes, and T. J. Colston. 2016. A new large species of Bitis Gray, 1842 (Serpentes: Viperidae) from the Bale Mountains of Ethiopia. Zootaxa 4093:41-63 <link>

Kieckbusch, M., S. Mecke, L. Hartmann, L. Ehrmantraut, M. O’Shea, and H. Kaiser. 2016. An inconspicuous, conspicuous new species of Asian pipesnake, genus Cylindrophis (Reptilia: Squamata: Cylindrophiidae), from the south coast of Jawa Tengah, Java, Indonesia, and an overview of the tangled taxonomic history of C. ruffus (Laurenti, 1768). Zootaxa 4093:1-25 <link>

Malonza, P. K., A. M. Bauer, and J. M. Ngwava. 2016. A new species of Letheobia (Serpentes: Typhlopidae) from central Kenya. Zootaxa 4093:143-150 <link>

Scheuchzer, J. J. 1735. Physica Sacra Iconibus Anaeis Illustrata, Procurante & Sumtus Suppeditante. Tomus IV. Augustae Vindelicorum et Ulmae, Ulm <link>

Seba, A. 1734-1765. Locupletissimi rerum naturalium thesauri accurata descriptio, et iconibus artificiosissimis expressio, per universam physices historiam :opus, cui, in hoc rerum genere, nullum par exstitit. Apud Janssonio-Waesbergios & J. Wetstenium & Gul. Smith, Amstelaedami <link>

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

Monday, March 28, 2016

State Snakes, Linnaean Names, and Other Recent Updates

As I wrote in December, the demands of completing my dissertation (and my new position as a science reporter with Utah Public Radio) haven't left me enough time to write the more in-depth long-form content that I (and readers, it seems) like so much. If all goes according to plan, I should return to those more elaborate articles towards the end of 2016, but in the meantime I wanted to highlight some recent and exciting updates to some of my older articles.

What the State Snakes Should Be

A Common Gartersnake (Thamnophis sirtalis)
eats a Woodhouse's Toad (Anaxyrus woodhousei)
In February the state of Virginia became the first state to officially designate a state snake. They chose the Common Gartersnake (Thamnophis sirtalis), despite being literally one of only two states in the nation to share their name with a genus of snake! If they had read my 2013 article, they might have gone with my pick of Virginia valeriae, the widespread Smooth Earthsnake, instead. But, perhaps there was already enough controversy: the gartersnake was proposed by 11-year-old Aiden Coleman of Williamsburg, but was put down by senators for being too wimpy. A couple of senators preferred the Timber Rattlesnake (Crotalus horridus), but the gartersnake was reinstated after Coleman asked one of them "just how much like West Virginia do you want us to be?"—unlike the legislators, Coleman already knew that the Timber Rattlesnake is the (very well-chosen) state reptile of West Virginia. The bill is now with the governor, whom some have suggested is the real snake.

The Linnaean Snakes

An Eastern Ribbonsnake from the panhandle of Florida
Heads up, taxonomy buffs—the scientific name of the Eastern Ribbonsnake (currently Thamnophis sauritus) is probably about to change to Thamnophis saurita, for some fairly technical linguistic reasons. Linnaeus named both this species and Thamnophis sirtalis, but because Linnaeus's description for sirtalis better matched sauritus, the two names were for decades confusingly interchanged. All seemed to be settled by a 1956 ICZN ruling, but in March a new paper in the journal Herpetological Review pointed out that Saurita, the original spelling used by Linnaeus, was capitalized and that its –a ending did not match the masculine gender of his genus Coluber. According to the grammatical rules of species naming that Linnaeus followed and which we still follow, this means that he meant "Saurita" to be a noun, rather than an adjective, and so the ending should not change to match the gender of the genus. The common assertion that "The specific name sauritus is New Latin, meaning lizardlike" is incorrect: sauros is Greek, not Latin, and the suffix –ita does not mean "like", but "little" (in Spanish). An obscure 5th-Century Greek dictionary by the lexicographer Hesychius, which is famous for being the only remaining source for a lot of ancient Greek words and would have been available to Linnaeus, lists "Saurita" as "a kind of serpent", settling the issue.

The Truth About Snakebite

Close-up of part of Liz Nixon's infographic
Fear of snakes made the New York Times op-ed section this week in an insightful article about the way humans assess the relative risks of terrorism and climate change. Although I completely agree with the article's point, in my opinion the author missed an opportunity to emphasize how our fear of snakes, like our fear of terrorism, is way beyond the risk posed by either (especially in the USA). It was a bit frustrating for me to read an article that came so close to making the analogy that we fear snakes even though they are unlikely to do us harm, but instead used fear of snakes as an example of an urgent fear distracting us from more gradual, but ultimately more dangerous threats. It's a tricky subject, but I did like the comparison between the number of deaths in the USA from falling in the bathtub (464/year) vs. from a terrorist attack (17/year)—both more likely than death from venomous snakebite (5/year). Also, if you haven't seen it, check out the awesome infographic that scientific illustrator Liz Nixon made using some of the data in my snakebite post.

Tetrodotoxin-resistant Snakes

An Eastern Hog-nosed Snake eats a toad
I rarely reference my own research on this blog, but last year I collaborated with Dr. Butch Brodie and members of his lab to publish some data on tetrodotoxin resistance in hog-nosed snakes (genus Heterodon). These snakes are well-known toad-eaters, but the few records of them eating newts were scattered until I brought them together in our new paper. Combined with molecular and whole-body resistance data, we showed that Eastern Hog-nosed Snakes from parts of upstate New York are more resistant to tetrodotoxin (TTX) than even the most resistant gartersnakes. But, Eastern Hog-nosed Snakes elsewhere are not as TTX-resistant, and Western Hog-nosed Snakes do not appear to be TTX-resistant at all. Most interesting, the mechanism of resistance appears to be something quite distinct from the conserved mutations in gartersnakes and other newt-eating snakes, and so far unknown.


Thanks to David HerasimtschukPatti and Jack Sandow, and Pierson Hill for the use of their photos.


Feldman, C. R., E. D. Brodie, and M. E. Pfrender. 2012. Constraint shapes convergence in tetrodotoxin-resistant sodium channels of snakes. Proceedings of the National Academy of Sciences 106:13415-13420 <link>

Feldman, C. R., A. M. Durso, C. T. Hanifin, M. E. Pfrender, P. K. Ducey, A. N. Stokes, K. E. Barnett, E. Brodie III, and E. Brodie Jr. 2016. Is there more than one way to skin a newt? North American snakes with convergent feeding adaptations do not share a common genetic mechanism. Heredity 116:84-91 <link>

Kraus, F. and H. D. Cameron. 2016. A note on the proper nomenclature for the snake currently known as Thamnophis sauritus. Herpetological Review 47:74-75

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

Tuesday, February 23, 2016

Dragonsnakes and Filesnakes Revisited

I've written about both filesnakes (family Acrochordidae) and dragonsnakes (part of the family Xenodermidae1) before. Traditional snake taxonomy suggests that, although they branch off from the main stem of the snake family tree at about the same time, they're not very closely related. But, new evidence emphasizes the uniqueness of dragonsnakes and thickens the plot in the unfolding story of the evolution of snakes.

Two hypotheses about the relationships of the major groups of snakes.
Left: tree based on nuclear genes, showing Acrochordidae and Xenodermidae
as successive outgroups to core Colubroidea
Right: tree based on mitochondrial genes, showing a sister relationship
between Acrochordidae and Xenodermidae
From Oguiura et al. 2009
Most phylogenetic analyses are pretty consistent in classifying both filesnakes and dragonsnakes as caenophidians, or "advanced" snakes. But, they differ in their placement of dragonsnakes and other xenodermids, including the truly strange and obscure odd-scaled snakes (Achalinus), bearded snakes (Fimbrios), stream, earth, or red snakes (Stoliczkia2), wood, mountain, or narrow-headed snakes (Xylophis), and a new genus, just described in 2015 and still without a common name, Parafimbrios. Most analyses group xenodermids with the colubroids (pareids1, vipers, homalopsids, colubrids, lamprophiids, and elapids), albeit as the most basal branch. Many textbooks actually define Caenophidia as Colubroidea + Acrochordidae (aka Acrochordoidea), distinctly separating the colubroids from the filesnakes on the basis of shared, derived characteristics such as wide ventral scales, as well as features of the skull, hemipenes, and the muscles, cartilages, and arteries between the ribs. However, several recent trees based on DNA sequences suggest instead that filesnakes and dragonsnakes might be one another's closest living relatives.

Acrochordid-Xenodermid Relationship
How many species?
What data were used?
4 mitochondrial genes
not reported
7 nuclear genes
20 nuclear genes
12 nuclear genes
2 mitochondrial genes + 1 nuclear gene
3 mitochondrial genes + 2 nuclear genes
141 extant +
51 extinct
610 morphological characters
44 nuclear genes
5 mitochondrial genes + 7 nuclear genes
333 nuclear loci
with 100% coverage
5 mitochondrial genes + 47 nuclear genes
A selection of studies that have examined the relationship between acrochordids and xenodermids.
X+A means that the two are each other's closest relatives; A,X means that acrochordids are more distantly
related to colubroids than xenodermids; X,A means that xenodermids are more distant
*Relationships differed depending on which methods were used

Arafura Filesnake (Acrochordus arafurae)
For example, the first study to use DNA to examine the relationships of these two groups of snakes found some support for each hypothesis, concluding that the "potential sister-taxon relationship of acrochordids and xenodermines [is] a reasonable hypothesis requiring future testing." In 2003, data from three more mitochondrial genes resulted in the same relationship, causing the authors to suggest that xenodermids should be excluded from Colubroidea. However, since that time, numerous studies have not repeated this result. In 2009, one research group predicted that "these differences...are due to taxonomic sampling issues", predicting that as DNA was collected from more species of snakes, the basal position of Acrochordus would be confirmed.

Dragonsnake (Xenodermus javanicus)
So, it was a real surprise when a 2013 analysis, the largest yet, including samples from 80% of all snake genera, placed Acrochordidae and Xenodermidae as sister groups. Neither a follow-up analysis combining that dataset with one containing data from many more genes nor an analysis using only the most complete data have settled the issue. The latter study compared several methods for generating phylogenetic trees and found that the relationship between acrochordids and xenodermids depended a lot on which methods were used. Part of the problem is that, even if they are each others' closest relatives, they still diverged between 70 and 80 million years ago, making them susceptible to a problem in phylogenetics known as long-branch attraction, which happens when the amount of evolutionary change within a lineage causes that lineage to appear similar (and thus closely related) to another long-branched lineage, solely because they have both undergone a lot of change, rather than because they are actually related.

Bearded Snake (Fimbrios klossi)
The truth is that both acrochordids and xenodermids are obscure snakes, and we don't have that much data on either one of them. They are both found in areas of the world that are hard to get to. Morphologically, they appear superficially similar, and an association between them was first hypothesized in 1893But, even the most comprehensive morphological trait database for snakes is missing crucial data on their anatomy, such as whether or not their hemipenial spines are mineralized. This would be helpful to know because  the hemipenial spines of basal snakes such as boas and pythons are not mineralized, whereas those of definitive colubroids are heavily mineralized.

Parafimbrios lao
From Teynié et al. 2015
Within the past year, two new studies on the chromosomes of dragonsnakes (Xenodermus javanicus) have been published. In the first, the karyotype (the number of chromosomes and their shape) of dragonsnakes was reported for the first time. In humans, each cell normally contains 23 pairs of chromosomes, for a total of 46. In most snakes, each cell normally contains 18 pairs of chromosomes, for a total of 36. Usually, eight of these pairs are relatively large (called macrochromosomes), and the other ten are somewhat small (called microchromosomes). Dragonsnakes have 16 pairs of chromosomes, for a total of 32, of which seven are large and nine are small. The dragonsnake karyotype probably evolved by two fusion events, one of two macrochromosomes and the other between a macrochromosome with a microchromosome. There are some other exceptions to the 18-pair pattern; some snakes have as few as 12 or as many as 25 pairs, including the only other xenodermid to have been karyotyped, the Sichuan Odd-scaled Snake (Achalinus meiguensis), which has just 12 pairs of chromosomes.

Amami Odd-scaled Snake (Achalinus werneri)
From the 1960s to the 1980s, before DNA sequencing became cheap and easy, scientists invested heavily in collecting karyotypes from a diversity of species for comparative purposes, so we can say with pretty good certainty that the ancestral state for all snakes is 36 (18 pairs). That's the number in filesnakes, pareids, most vipers, homalopsids, and many "crown colubroids" (colubrids, lamprophiids, and elapids, although there are lots of exceptions in these three groups).  The fusions in xenodermids emphasize their uniqueness, but unfortunately don't shed any new light on their phylogenetic placement.

Stoliczkia borneensis
The other study focused on the sex chromosomes. In humans, sex is determined by which combination of sex chromosomes a baby receives from its parents: two X chromosomes make a female, whereas an X and a Y chromosome make a male. It's pretty similar in snakes, with a twist: the sex chromosomes are called Z and W instead of X any Y, and females are the heterogametic sex (meaning that a Z and a W chromosome make a female, and two Z chromosomes make a male). Birds and many other reptiles also have ZW sex determination. In many colubroid snakes, the W chromosome is about twice the size of the Z,  and it is often unusual in other ways as well, such as having sections of highly condensed chromatin or a different centromere position. In contrast, filesnakes, boids, and other more basal snakes have morphologically indistinguishable Z and W chromosomes, although they still contain different genes and perform different functions.

Perrotet's Narrow-headed Snake (Xylophis perroteti)
Are members of this genus really xenodermids? Or, like the
former xenodermids Oxyrhabdium and Nothopsis, will they
prove to be more closely related to something else?
One reason the W chromosome looks so different from the Z in colubroids is that it contains repetitive elements called Bkm ('banded krait minorsatellite') repeats, which consist of the sequence "GATA" (sometimes "GACA") repeated thousands of times. Mammalian X chromosomes and avian W chromosomes also have these repeats. Cell biologists think that these repeats function to inactivate all the genes on the W chromosome except for those that determine sex3. Both mammalian X chromosomes and snake W chromosomes become very dense in body cells, so that none of the genes on them can be expressed. They only decondense and plays their brief, female-determining roles, in maturing eggs that are destined to become females. Unlike in mammals, the sex chromosomes of snakes span the gamut from completely identical to markedly differentiated, allowing biologists to study the evolution of chromosomal sex determination. The new study showed that female dragonsnakes have two different-looking sex chromosomes, with many Bkm repeats in the W, whereas the two Z sex chromosomes of male dragonsnakes look similar and lacked Bkm repeats, bolstering the relationship between xenodermids and other colubroids and diminishing the relationship between xenodermids and filesnakes.

The other major finding of the new study is the documentation that at least part of the sex chromosomes are homologous across all families of caenophidian snakes, suggesting that snake sex chromosomes emerged in the common ancestor of Caenophidia some 60-80 million years ago. One gene that is only on the Z chromosome in all caenophidians, including dragonsnakes, is also found on the W chromosome in filesnakes. The Z-chromosome-specific genes in caenophidians were on both the Z and W chromosomes in boas, pythons, and sunbeam snakes (Xenopeltidae), as well as in bearded dragons and anoles. Other toxicoferan lizards with ZW sex chromosomes, including chameleons and monitor lizards, seem to have evolved them independently.

1 A recent article in the journal Herpetological Review pointed out that the grammatical rules for structuring family and subfamily names from genus names have recently been incorrectly applied in two cases involving snakes which concern this article: 1) Xenodermatidae/inae for the family/subfamily containing Xenodermus, the root of which is "dermus", a masculine noun with which the masculine specific epithet javanicus is correctly coupled (not the neuter javanicum; in contrast think of the neuter Heloderma horridum in family Helodermatidae). The correct family or subfamily name is thus Xenodermidae/inae. 2) Pareatidae or Pareatinae for the family containing Pareas, which is also masculine, making the correct family/subfamily name Pareidae/inae.

2 Don't confuse this snake genus (Stoliczkia) with a genus of extinct ammonite (Stoliczkaia), both named for Czech biologist Ferdinand Stoliczka. The extra "a" was added to the original spelling of the snake genus by Boulenger in 1899, probably by accident, and this genus is still widely misspelled today (e.g., on GenBank and on Wikipedia before I fixed it while writing this article).

3 It's also thought that "GATA" is a particularly potent regulatory sequence, with the power to turn nearby genes on and off. In a way, the sex genes have essentially 'hijacked' the W chromosome, turning off all its other genes, and simultaneously creating a concentrated source of mutation-causing elements. Chromosomal sex determination may therefore constitute a unique and potentially very powerful genotypic mechanism for abruptly enhancing evolutionary rates, which might have contributed to the explosive radiations of species in clades with chromosomal sex determination, such as mammals, birds, squamates, and certain groups of insects.


Thanks to Thomas CalameSam HowardKonrad MebertZeeshan MirzaTakehito Sato, and Stephen Zozaya for the use of their photos.


Boulenger, G. A. 1893. Catalogue of the Snakes in the British Museum (Natural History). Volume I., containing the families Typhlopidae, Glauconiidae, Boidae, Ilysiidae, Uropeltidae, Xenopeltidae, and Colubridae Aglyphae, Part. Trustees of the British Museum, London. <link>

Boulenger, G. A. 1899. Description of three new reptiles and a new batrachian from Mt. Kina Balu, North Borneo. Annals and Magazine of Natural History 7:451-453 <link>

Gauthier, J. A., M. Kearney, J. A. Maisano, O. Rieppel, and A. D. B. Behlke. 2012. Assembling the squamate Tree of Life: perspectives from the phenotype and the fossil record. Bulletin of the Peabody Museum of Natural History 53:3-308 <link>

Jerdon, T. C. 1870. Notes on Indian Herpetology. Proceedings of the Asiatic Society of Bengal 1870:66-85 <link>

Jones, K., and L. Singh. 1985. Snakes and the evolution of sex chromosomes. Trends in Genetics 1:55-61 <link>

Lawson, R., J. B. Slowinski, B. I. Crother, and F. T. Burbrink. 2005. Phylogeny of the Colubroidea (Serpentes): new evidence from mitochondrial and nuclear genes. Molecular Phylogenetics and Evolution 37:581-601 <link>

Kelly, C. M. R., N. P. Barker, and M. H. Villet. 2003. Phylogenetics of advanced snakes (Caenophidia) based on four mitochondrial genes. Systematic Biology 52:439-459 <link>

Kraus, F., and W. M. Brown. 1998. Phylogenetic relationships of colubroid snakes based on mitochondrial DNA sequences. Zoological Journal of the Linnean Society 122:455-487 <link>

Oguiura, N., H. Ferrarezzi, and R. Batistic. 2009. Cytogenetics and molecular data in snakes: a phylogenetic approach. Cytogenetic and Genome Research 127:128-142 <link>

O’Meally, D., H. R. Patel, R. Stiglec, S. D. Sarre, A. Georges, J. A. M. Graves, and T. Ezaz. 2010. Non-homologous sex chromosomes of birds and snakes share repetitive sequences. Chromosome Research 18:787-800 <link>

Pokorna, M., and L. Kratochvíl. 2009. Phylogeny of sex‐determining mechanisms in squamate reptiles: are sex chromosomes an evolutionary trap? Zoological Journal of the Linnean Society 156:168-183 <link>

Pyron, R. A., F. T. Burbrink, G. R. Colli, A. N. M. de Oca, L. J. Vitt, C. A. Kuczynski, and J. J. Wiens. 2011. The phylogeny of advanced snakes (Colubroidea), with discovery of a new subfamily and comparison of support methods for likelihood trees. Molecular Phylogenetics and Evolution 58:329-342 <link>

Pyron, R. A., F. Burbrink, and J. J. Wiens. 2013. A phylogeny and revised classification of Squamata, including 4161 species of lizards and snakes. BMC Biology 13:53 <link>

Pyron, R. A., C. R. Hendry, V. M. Chou, E. M. Lemmon, A. R. Lemmon, and F. T. Burbrink. 2014. Effectiveness of phylogenomic data and coalescent species-tree methods for resolving difficult nodes in the phylogeny of advanced snakes (Serpentes: Caenophidia). Molecular Phylogenetics and Evolution 81:221-231 <link>

Rovatsos, M., M. Johnson Pokorná, and L. Kratochvíl. 2015. Differentiation of sex chromosomes and karyotype characterisation in the Dragonsnake Xenodermus javanicus (Squamata: Xenodermatidae). Cytogenetic and Genome Research 147:48-54 <link>

Rovatsos, M., J. Vukić, P. Lymberakis, and L. Kratochvíl. 2015. Evolutionary stability of sex chromosomes in snakes. Proceedings of the Royal Society B: Biological Sciences 282:20151992 <link>

Savage, J. M. 2015. What are the correct family names for the taxa that include the snake genera Xenodermus, Pareas, and Calamaria? Herpetological Review 46:664-665 <link>

Sharma, G., and U. Nakhasi. 1980. Karyological studies on six species of Indian snakes (Colubridae: Reptilia). Cytobios 27:177-192 link>

Teynié, A., P. David, A. Lottier, M. D. Le, N. Visal, and T. Q. Nguyan. 2015. A new genus and species of xenodermatid snake (Squamata: Caenophidia: Xenodermatidae) from northern Lao People’s Democratic Republic. Zootaxa 3926:523-540 <link>

Vicoso, B., J. Emerson, Y. Zektser, S. Mahajan, and D. Bachtrog. 2013. Comparative sex chromosome genomics in snakes: differentiation, evolutionary strata, and lack of global dosage compensation. PLoS Biology 11:e1001643 <link>

Vidal, N., A. S. Delmas, P. David, C. Cruaud, A. Couloux, and S. B. Hedges. 2007. The phylogeny and classification of caenophidian snakes inferred from seven nuclear protein-coding genes. Comptes Rendus Biologies 330:182-187 <link>

Wang, G., S. He, S. Huang, M. He, and E. Zhao. 2009. The complete mitochondrial DNA sequence and the phylogenetic position of Achalinus meiguensis (Reptilia: Squamata). Chinese Science Bulletin 54:1713-1724 <link>

Wiens, J. J., C. A. Kuczynski, S. A. Smith, D. G. Mulcahy, J. W. Sites, T. M. Townsend, and T. W. Reeder. 2008. Branch lengths, support, and congruence: testing the phylogenomic approach with 20 nuclear loci in snakes. Systematic Biology 57:420-431 <link>

Wiens, J. J., C. R. Hutter, D. G. Mulcahy, B. P. Noonan, T. M. Townsend, J. W. Sites, and T. W. Reeder. 2012. Resolving the phylogeny of lizards and snakes (Squamata) with extensive sampling of genes and species. Biology Letters 8:1043-1046 <link>

Zaher, H., F. G. Grazziotin, J. E. Cadle, R. W. Murphy, J. C. Moura-Leite, and S. L. Bonatto. 2009. Molecular phylogeny of advanced snakes (Serpentes, Caenophidia) with an emphasis on South American Xenodontines: A revised classification and descriptions of new taxa. Papeis Avulsos de Zoologia (Sao Paulo) 49:115-153 <link>

Zheng, Y., and J. J. Wiens. 2016. Combining phylogenomic and supermatrix approaches, and a time-calibrated phylogeny for squamate reptiles (lizards and snakes) based on 52 genes and 4162 species. Molecular Phylogenetics and Evolution 94:537-547 <link>

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