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.

Friday, December 30, 2016

Life is Short but Snakes are Long 2016 Milestones

Dear reader,

Screenshot from 15 November showing Blogger's estimate
that Life is Short but Snakes are Long reached one million
views, which is probably a bit too optimistic.
As I did last year, I want to thank you for your readership in 2016. Life is Short but Snakes are Long reached three-quarters of a million unique views on September 6th this year, by over 430,000 unique readers from nearly every country. We're currently over 860,000 views and on track to reach one million in 2017. The more liberal Blogger statistics show that we're already at one million, but I suspect that many of these are bots, and I'm sticking with the more conservative estimates provided by Google Analytics. I'm so happy to have reached so many people. Furthermore, at least 34 new species of snakes were described in 2016—another reason to celebrate!

In addition to defending my dissertation and moving to Germany in 2016, I also published 5 scientific papers and co-authored a book chapter for the new 3rd edition of Mader's Reptile Medicine and Surgery, on the behavior of reptiles and amphibians, which will be published in 2017. I became a lot more active in the Facebook Snake Identification and Wild Snakes: Education and Discussion groups, which are fantastic resources for quick, reputable answers to questions about snakes. I recently accepted a position as an Associate Editor of the Snake Natural History Notes section at the journal Herpetological Review, and I was invited to become a curator at the Encyclopedia of Life project, where I've written several short summaries of snake taxa.

Life is Short but Snakes are Long was voted one of
Bel-Rea Vet Tech College's Top 25 Reptile/Amphibian Blogs in 2016
Life is Short but Snakes are Long was voted one of Bel-Rea Vet Tech College's Top 25 Reptile/Amphibian Blogs. The students and staff wrote that they particularly appreciated my efforts to reference my sources, and I was really glad to know that others appreciate my efforts to provide verifiable information (apparently there's all too little of that on the Internet these days).

I was particularly glad that the BBC's Planet Earth II featured Galápagos Racers so prominently this year, generating Internet-wide buzz about snakes and their feeding habits, a topic close to my heart. Since I wrote about these interesting snakes back in 2013, a lot of curious people found my blog, inspiring me to write an update and include much more detailed information. I also revisited several other favorite topics, including the relationship between dragonsnakes and filesnakes, rattlesnake roundupssnake penises, and snakes as state/provincial symbols. I have some really good content planned to debut in 2017, including articles on the roles that snakes play in ecosystems, the nitty-gritty details of courtship, sex, and mating in snakes, the little-known and seldom-seen ecology of blindsnakes, profiles of some fossil snakes, and venomous bites from "non-venomous" snakes.

Life is Short but Snakes are Long would not be possible without support from volunteer translators Alvaro Pemartin & Estefania Carrillo, from Utah State University, particularly my advisor Susannah French and the Ecology Center, and from my loving girlfriend and editor Kendal Morris.

Thank you, and happy 2017!

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.

Sunday, November 27, 2016

Galápagos Racers: answers to your questions about the BBC Planet Earth II iguana chase scene

This post will soon become available in Spanish

Galápagos Racers (Pseudalsophis occidentalis)
on Fernandina Island, from the BBC's Planet Earth II footage
If you haven't seen the incredible footage of the "iguana chase scene" from the BBC's Planet Earth II Islands episode, I encourage you to watch it right away. In addition to being a highly dramatic cinematographic masterpiece, it raises a number of interesting questions about the biology of the snakes in the clip. For a few days after it aired, the Internet was buzzing with these questions, and I've cataloged the answers to some of the most popular ones below. If you have one that isn't listed, feel free to ask it in the comments! And, if you want to know more about the process I used to dig up some of this information, check out my tutorial for teaching oneself about obscure snakes.

What kind of snakes are they?

Throughout the clip, Attenborough calls them "racer snakes"1, but herpetologists would normally call the snakes on the screen Galápagos Racers. Although these snakes are called "racers", they're not closely related to North American racers (genus Coluber); it's been about 45 million years since these two snakes last shared a common ancestor.

Galápagos Racers belong to the genus Pseudalsophis. Depending on which sources you consult, there are between 4 and 7 species of Pseudalsophis in the Galápagos, as well as one in mainland South America.

Pseudalsophis slevini eating a gecko on Pinzón Island
Just like Galápagos tortoises, finches, and many other organisms, there are different species of Galápagos Racers on the different Galápagos Islands (one of the concepts that sparked Darwin's theory of evolution by natural selection). The film was made on Fernandina, the youngest, westernmost, and most volcanically-active island in the Galápagos. Fernandina has two species of snakes, Pseudalsophis slevini and Pseudalsophis occidentalis. The snakes in the film must be Pseudalsophis occidentalis, because they are too large and not boldly banded enough to be P. slevini. You can read the original descriptions of both species here.

None of the sources reporting which species is shown in the film are authoritative, but without exception when the species is given it is given as Pseudalsophis biserialis. This is not correct under any modern taxonomy, although there is also a good explanation for why it is mistakenly being used—P. occidentalis was briefly a subspecies of P. biserialis, but has mostly been and is now treated either as a subspecies of P. dorsalis or as its own species. See below for much more (probably too much) detail.

Why are there so many of them?

Galápagos Racer (P. dorsalis) among adult Marine Iguanas
on Santa Cruz, which are much too large for it to eat
Most snakes are not social, and because they must swallow their food whole they cannot share prey. These snakes are not found at such high densities year-round, but rather aggregate around consistent Marine Iguana nesting sites in May when the eggs are hatching.

Just as when baby sea turtles emerge from their nests, predators congregate at the temporary buffet, returning afterwards to their usual densities. Around the world, there are numerous examples of avian and snake predators exploiting emerging hatchling iguanas. Researchers working at other iguana nesting sites in the Bahamas, the West Indies, and Venezuela have hypothesized that snakes and other predators also converge on the nesting sites of these other iguanas to exploit the temporary food source. Another example of snakes congregating around abundant prey resources is that of Puerto Rican and Cuban boas, which aggregate around the openings of massive bat caves.

The rest of the year, Galápagos Racers eat lava lizards, geckos, insects, marine fishes, and hatchling birds, as well as introduced rats and mice.

Are they really hunting in a pack?

Almost certainly not. Again, most snakes are not social, and because they must swallow their food whole they cannot share prey. Pack-hunting behavior is unknown in snakes.

Two P. occidentalis trying to eat the same iguana
Jaw-walking is a fixed action pattern in snakes and they
may eat things that only vaguely resemble their food
once they start jaw-walking them.
From Planet Earth II Behind the Scenes
Some species have surprisingly social behaviors. It would be really interesting to examine social behavior in these snakes. To my knowledge no one has done so. Although they obviously cannot share a single food item, but if they are foraging in the same time and place on a limited resource, there might be an opportunity for the evolution of social cues. At least one paper suggested that this might be the case with a pit viper. Even though the BBC videographers saw snakes actively fighting over the same prey items and in some cases eating one another, it's possible that more closely-related snakes are less likely to fight over food or eat one another, or that males are less likely to compete with or try to eat females. These are testable hypotheses. However, these are not well-studied snakes. I don't think they are helping each other, but there's a lot that we don't know about snakes. Some snakes exhibit dominance hierarchies, and one study suggested that individual recognition occurs and persists over time in gartersnakes.

Few scientists are currently studying these snakes. It's a testament to the BBC that they are consistently able to film natural phenomena that are still unknown to science. Hopefully this tape will stimulate some research on this exact question, and on the ecology of Galápagos Racers. When I wrote about Galápagos Racers in 2013, not much was known about their ecology, and that's still the case. It's amazing that so little research has been done on these snakes, particularly in contrast to Galápagos tortoises and marine iguanas (not to mention finches and other non-avian reptiles).

Why don't the female Marine Iguanas just lay their eggs somewhere else, closer to the ocean maybe?

Fates of rock iguana hatchlings, over half of which were
eaten by Cubophis and Epicrates snake predators in their
first month of life. From Knapp et al. 2010
Marine Iguanas have to dig nests and lay their eggs in soft sand, away from the rocky, tidal foraging grounds of the adults. They choose protected lava reefs for this purpose, which are in short supply on most islands. One estimate suggested that the cost of migrating to their nesting sites represented half the reproductive effort of female Galápagos land iguanas.

Many species of reptiles nest in areas where they otherwise do not spend much time, especially aquatic species (reptile eggs need to "breathe" air and cannot be laid underwater). Female Marine Iguanas may all use the same nesting sites because those are the only sites available, or they may choose to nest near one another because, just like with sea turtles, synchronous hatching of the young increases their probability of survival.

In a study of Bahamian rock iguanas (Cyclura cychlura), snake predation was the most likely cause of mortality for newborn iguanas dispersing away from their nests. They estimated that about 20-30% of hatchling iguanas survived their first month, and those that moved quickly and linearly away from their nests were the most likely to survive, perhaps because predators had learned to hang around the nesting area. Another study of Galápagos land iguanas showed that predation attempts by Galápagos hawks were more than three times as likely to be successful when the body temperature of the iguana hatchlings was below 90°F. And, baby Galápagos marine iguanas that hung around their hatching area had about a 10% lower survival rate than those that moved to the coast, which the researchers attribute mostly to higher risk of predation at the nesting area.

Studies on the population biology of Marine Iguanas have shown that most of their mortality is caused by "predation, starvation (sometimes as a result of being trapped by a rock), crushing by a rock, being beaten against rocks by the sea, and suffocation in collapsed nest burrows. Animals may also die after being swept out to sea by offshore currents". So, actually, predation may be the best way for them to go. Besides Galápagos Racers, their other predators include Galápagos Hawks, Short-eared Owls, crabs, and Giant Hawk-fish.

Are they venomous/dangerous to humans?

No. Like many snakes, Galápagos Racers are rear-fanged. This means that, although technically they are venomous, they don't pose a danger to humans. Rear-fanged snakes mostly have grooved teeth (rather than hollow fangs) on the back of their upper jaw (as opposed to the front); they can use these teeth to get venom into their prey once they are biting it, but they cannot strike out and deliver venom the way a viper can. A small minority of rear-fanged snakes have delivered medically-significant bites to humans, but almost all of these take place in a captive setting. You can read more about the different types of snake fangs here.

I didn't know there were snakes in the Galápagos. How did they get there?

Map showing the estimated age of each of the
Galápagos Islands. From Ali & Aitchison 2014
Galápagos Racers colonized the Galápagos Islands from mainland South America, just like all of the other Galápagos fauna and flora. The modern Galápagos Islands formed from volcanoes over the past 4 to 5 million years, although some of them have been building beneath the ocean surface for up to 15 million years. It is thought that there have been islands in the Galápagos for at least 8 million years, but the oldest islands have eroded and are now back beneath the ocean surface.

Because the Galápagos Islands are located only six hundred miles off the coast of Ecuador, it is easier for them to be colonized by plants and animals from the mainland than for a more remote island chain such as Hawaii (which is >2,500 miles away from the nearest snake-inhabited landmass).

Molecular dating of the divergence time between Galápagos Racers and their closest mainland relative, Pseudalsophis elegans, suggests that it has been about 15 million years since they last shared a common ancestor. This suggests that the mainland ancestor of Galápagos Racers probably went extinct sometime over the last 15 million years, and that the ancestors of Galápagos Racers probably colonized the Galápagos Islands before any of the current islands existed (as is also the case for the Marine Iguanas). Until genetic work is done, we won't know how many times snakes colonized the Galápagos archipelago or how many distinct lineages there are. [Edit 12/30/2016: I have recently learned that Massey University ecologist Luis Ortiz-Catedral and his colleagues are working to understand the evolution of all the species in the genus Pseudalsophis and definitively answer this question.]

Could the film have been staged?

Obviously the scenes are spliced together, but in my opinion there's no chance the Galápagos National Park would allow something like this to be staged. They are among the strictest places in the world for researchers to conduct scientific work. However, more recent episodes of Planet Earth II have been criticized for incorporating fake sound effects.

One of the few phylogenies to include Galápagos Racers
Broadly, Pseudalsophis is nested within a large clade of Caribbean, Central, and South American xenodontine snakes including, among numerous others, the genus Alsophis, which once contained Galápagos Racers and after which their current genus is named. They have been in a variety of genera since their description, especially Dromicus, which is no longer in use, from 1876 to 1997.

In 1973, herpetologist Charles Myers wrote: "The classification of colubrid snakes in general, and of South American colubrids in particular, is in a notoriously unsatisfactory state." Unfortunately, we are not that much better off today when it comes to Galápagos Racers. It seems pretty clear that the nearest relative of P. biserialis, P. dorsalis, and P. occidentalis is Pseudalsophis elegans, the only species in the genus found on the mainland (in Ecuador, Peru, and extreme northern Chile). Beyond that, there isn't a lot of clarity about their next-closest relatives. They are possibly most closely related to obscure South American "groundsnakes" in the genus Psomophis, or to the even more obscure genus Saphenophis, which was described by Myers as "quite lacking in peculiar or unique features" and so named "in allusion to one incontrovertible fact about these snakes...from the Greek saphenes (evident truth, clear) + ophis (a serpent), meaning 'clearly a snake'". We don't really have a great hypothesis about how the different lineages of Galápagos Racers are related to one another, or even if they are all descended from a single common ancestor, because we only have DNA from one of them so far.

Hypothesized scenario for the evolution of Pseudalsophis snakes
So far, we have no DNA evidence that would support or refute this model
From Ali & Aitchison 2014
Two reviews based on morphology addressed this question in the late 1990s. The first (Thomas 1997) focused exclusively on Galápagos Racers and suggested that P. biserialis, P. dorsalis, and P. occidentalis are descended from a shared common ancestor with P. elegans, but that P. hoodensis is more closely related to the mainland species Philodryas chammissonis, and that P. slevini and P. steindachneri are most closely related to Caribbean species. The other study (Zaher 1999), which looked at hemipene morphology over a much larger group of snakes, disagreed, finding a shared derived character—an inflated papillate ridge, placed far medially, on the medial surface of the lobes—linking the Galápagos Racers together with the mainland species P. elegans. Statements that Galápagos Racers have “very similar hemipenes” notwithstanding, Zaher was criticized for not describing the specific characters uniting the Galápagos species to the exclusion of others.

Maglio (1970) noted that the tooth counts and arrangement and the and shape of the premaxilla bone was most similar among the three Galápagos species that he examined (P. biserialisP. dorsalis, and P. slevini), and different from the West Indian species that Taylor later suggested are P. slevini's closest relatives. More recently, a study led by Grazziotin claimed that they "unequivocally support...Zaher's (1999) hypothesis based on morphology that continental Pseudalsophis elegans is closely related to the Galápagos Island species of Xenodontinae (herein represented by Pseudalsophis dorsalis), rather than to West Indian Alsophis and Antillophis, and mainland Philodryas (Thomas, 1997)." However, they obviously didn't read Thomas's paper very carefully, because he also hypothesizes that P. dorsalis is closely related to P. elegans, and the Grazziotin paper didn't sequence any DNA from P. slevini, P. steindachneri, or P. hoodensis, and therefore didn't test any hypotheses about them.

As for whether or not the snakes in Planet Earth II should be called P. occidentalis or P. dorsalis occidentalis, that's really a lumper/splitter question. But, both the IUCN and the 2014 edition of Snakes of the World recognize P. occidentalis as a full species; it was originally described as such by Van Denburgh in 1912, sunk to a subspecies of P. dorsalis by Mertens in 1960, and re-elevated to a full species in a 1999 paper by Zaher that was not primarily concerned with taxonomy and appears to have subsequently been neglected. The Reptile Database is currently a holdout for the subspecies designation, which has not been disputed but which is also not explicitly supported by unambiguous data. Perhaps wisely, the official webpage of Galápagos National Park chooses not to use scientific names and refers to the Fernandina racers as the "western subspecies". The truth is that, until more research is done, we won't be able to settle on an accurate taxonomy for these snakes.

1 This sounds a bit redundant to a snake biologist, but it isn't incorrect. The one thing that I wish BBC programs would do is identify the species in them more precisely. I'm advocating for a "biologist mode" that can be activated which would show the location and identity of species in all clips, similar to the old MTV show Pop-up Video.


Thanks to Andy Kraemer and Jim Moulton for the use of their photographs.


Ali, J. R. and J. C. Aitchison. 2014. Exploring the combined role of eustasy and oceanic island thermal subsidence in shaping biodiversity on the Galápagos. Journal of Biogeography 41:1227-1241 <full-text>

Bisconti, M., W. Landini, G. Bianucci, G. Cantalamessa, G. Carnevale, L. Ragaini, and G. Valleri. 2001. Biogeographic relationships of the Galapagos terrestrial biota: parsimony analyses of endemicity based on reptiles, land birds and Scalesia land plants. Journal of Biogeography 28:495-510 <full-text>

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Grazziotin, F. G., H. Zaher, R. W. Murphy, G. Scrocchi, M. A. Benavides, Y.-P. Zhang, and S. L. Bonattoh. 2012. Molecular phylogeny of the New World Dipsadidae (Serpentes: Colubroidea): a reappraisal. Cladistics 28:437-459 <full-text>

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Hedges, S. B., A. Couloux, and N. Vidal. 2009. Molecular phylogeny, classification, and biogeography of West Indian racer snakes of the Tribe Alsophiini (Squamata, Dipsadidae, Xenodontinae). Zootaxa 2067:1-28 <full-text>

Knapp, C. R., S. Alvarez-Clare, and C. Perez-Heydrich. 2010. The influence of landscape heterogeneity and dispersal on survival of neonate insular iguanas. Copeia 2010:62-70 <full-text>

Laurie, W. and D. Brown. 1990. Population biology of marine iguanas (Amblyrhynchus cristatus). II. Changes in annual survival rates and the effects of size, sex, age and fecundity in a population crash. Journal of Animal Ecology 59:529-544 <full-text>

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Merlen, G. and R. A. Thomas. 2013. A Galapagos ectothermic terrestrial snake gambles a potential chilly bath for a protein-rich dish of fish. Herpetological Review 44:415-417 <full-text>

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Parent, C. E., A. Caccone, and K. Petren. 2008. Colonization and diversification of Galápagos terrestrial fauna: a phylogenetic and biogeographical synthesis. Philosophical Transactions of the Royal Society B: Biological Sciences 363:3347-3361 <full-text>

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Wallach, V. W., Kenneth J. and J. Boundy. 2014. Snakes of the World: A Catalogue of Living and Extinct Species. CRC Press, Boca Raton, Florida, USA <Google book>

Weinstein, S. A., D. A. Warrell, J. White, and D. E. Keyler. 2011. "Venomous" Bites from Non-Venomous Snakes: A Critical Analysis of Risk and Management of "Colubrid" Snake Bites. Elsevier, Amsterdam <Google book>

Werner, D. I. 1983. Reproduction in the iguana Conolophus subcristatus on Fernandina Island, Galapagos: clutch size and migration costs. American Naturalist 121:757-775 <abstract>

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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.

Friday, October 28, 2016

Snakes with feet, anti-goo saliva, and more recent updates

This post will soon be available in Spanish

More of the latest snake news and research (for other recent updates, see posts from March, June, and August)—and, perhaps the most exciting news of all is that I have defended my dissertation and will be returning to writing more in-depth content in the next few months!

Rattlesnake Roundups (I and II)

A Texas conservation licence plate ironically depicting
a Western Diamond-backed Rattlesnake (Crotalus atrox).
Funds from these plates support a variety of valuable
conservation projects in Texas
 under the Texas
Wildlife Action Plan
, although none are specific to snakes.
Advocates for increasing state oversight of rattlesnake roundups in Texas received disappointing news this week when the Texas Parks and Wildlife Commission decided that they would not support a proposed ban on using gasoline fumes to collect rattlesnakes. Rather than reviewing and voting on the issue at their bi-annual meeting next month, the TPW Commission decided to remove it from their agenda entirely, citing "insufficient support from legislative oversight or the potentially regulated community". This decision marks the second time reviewing the ban has been put off, and unfortunately it is likely to be the last until the effort to reform roundups is re-initiated. The announcement included the statement that "TPWD [Texas Parts and Wildlife Department] staff still believe that there are better options for collecting snakes that do not adversely impact non-target species, and we will continue to work with the snake collecting community to develop and implement best practices that reduce potential impacts to these species", although in the absence of specific details it is hard to believe that this issue will remain at the fore of wildlife management in Texas without continued pressure from advocates of scientific rattlesnake management. However, Representative Susan King of Sweetwater's 2015 house bill 763 requires that petitions to state agencies (including TPWD) that are signed by <51% Texas residents are not valid, which means that the ability of non-Texans to influence policy on this issue is now greatly diminished.

If you're not familiar with the issues surrounding the gassing ban, I encourage you to read the 2016 Snake Harvest Working Group report, the same document that was available to the TPW Commission prior to their decision this week. Among other topics, it contains data on the adverse impacts of gassing on non-target endangered species, which is the primary impetus for the ban. It hints at human health impacts of consuming meat from gassed rattlesnakes. The SHWG report also summarizes previously unavailable data on roundup economics, showing that profits are not related to the number of rattlesnakes at an event and did not decline after gassing was banned in Alabama and Oklahoma. Stakeholder survey responses and the vast majority (>90%) of public comments from Texans were in favor of the gassing ban, as are many TWPD employees.

The TPW Commission is solely responsible for this decision. You can let the TPW Commission and Texas State Representative Susan King of Sweetwater (or your own state representative, if you live in Texas) know whether you think they are acting in the best interest of the majority of the public and in accordance with game management principles at the links provided (if you no longer have a fax machine, you can send a fax over the Internet here).

Goo-eating Snakes and the Eggs that Evade Them and Basics of Snake Fangs

Mandibular glands of Dipsas alternans
From Zaher et al. 2014
This discovery is from 2014, but it's newer than either of the past posts to which it's germane and I just found out about it. Perhaps you've seen the incredible rapid hatching behavior that treefrog eggs have evolved to escape from snake predators, including cat-eyed snakes (genus Leptodeira), blunt-headed tree snakes (genus Imantodes), and snail-sucking snakes (genera Sibon and Dipsas). These snakes also eat a variety of other gooey prey, such as earthworms, leeches, snails, slugs, adult frogs, caecilians, and, more rarely, non-gooey prey like lizards and reptile eggs. They have a number of adaptations that help them consume their sticky, viscous prey, including long, slender teeth, skull bones and muscles modified for extreme lower jaw extrusion, and a short-snouted, large-eyed look that resembles a snake embryo. Recently, a team of scientists from Brazil discovered a new one: a protein-secretion delivery system in the lower jaw.

Are the secretions venom? No. Dipsas and its relatives always extract snails using a sudden strike, followed by fast, alternating probing motions of the mandible inside the shell; this behavior could hardly depend on a chemical reaction of any kind. Instead, the gland secretions probably play a role in mucus control and prey transport rather than immobilization or killing of the prey. Although the glands in some species are associated with muscles, they are not connected to any teeth, but rather open onto the floor of the mouth, which in some species is covered with extensively loose, folded skin. Hypertrophied infralabial glands have been known from some dipsadine species since the 1960s, but the new paper describes the muscles and other soft tissues surrounding them and documents their variation among several dozen species of this very speciose group of snakes. On the other side of the world, pareatid snail-eating snakes have independently evolved a similar lifestyle, complete with upper jaw glands of perhaps similar function.

Why snakes are long and Why do snakes have two penises?

Pelvic girdles (dark blue) and hind limbs (red) of lizards,
living snakes, and extinct snakes with fully-developed limbs.
ZRS is the name of the SHH enhancer gene
that has been partially deleted in snakes.
From Leal & Cohn 2016
Many people are familiar with the tiny vestigial legs or "spurs" of boas, pythons, and other henophidian snakes. These structures are sexually dimorphic and are used by male boas and pythons in male-male combat and also to titillate females before and during matingNew data from the University of Florida describes how the spurs are formed: a weak flicker of activity by a gene called Sonic hedgehog (SHH) during the first few hours of embryonic development, in contrast to strong, sustained activity of this gene in lizard embryos throughout their development, forming legs. In snakes, unique genetic deletions from an enhancer of SHH explain its weak activity; transgenic mouse embryos with the same deletions showed similarly weak SHH activity, whereas mouse embryos grown with a lizard enhancer developed normally. Caenophidian snakes, such as vipers, gartersnakes, and cobras, had more extreme deletions and mutations, with the cobra barely retaining any of the SHH enhancer gene.

Amazingly, the researchers also found that HOXD13, the part of the limb-building gene that's responsible for building hands and feet, was unaltered in python embryos, and that python embryos develop not just a pelvic girdle and femur, which form the spur in adulthood, but cartilaginous templates of a tibia, fibula, and foot, which are reabsorbed prior to hatching. Although living snakes appear to follow a gradual pattern of limb shrinkage and loss, some extinct snakes that are thought to have been more similar to boas and pythons than they were to blindsnakes also had fully-developed, albeit small, limbs, complete with feet, as adults. This new discovery helps explain the apparent evolutionary "re-appearance" of these structures; they were never completely lost in the first place. As for the reason why not, snake HOXD genes and their regulators appear to be equally important to the development of their paired hemipenes, structures of obvious importance.


Oliveira, L., A. L. Costa Prudente, and H. Zaher. 2014. Unusual labial glands in snakes of the genus Geophis Wagler, 1830 (Serpentes: Dipsadinae). Journal of Morphology 275:87-99 <link>

Leal, F. & Cohn, M.J. 2016. Loss and re-emergence of legs in snakes by modular evolution of Sonic hedgehog and HOXD enhancers. Current Biology DOI:10.1016/j.cub.2016.09.020 <link>

Leal, F. & Cohn, M.J. 2014. Development of hemipenes in the ball python snake Python regius. Sexual Development, 9, 6-20 <link>

Savitzky, A.H. 1983. Coadapted character complexes among snakes: fossoriality, piscivory, and durophagy. American Zoologist, 23, 397-409 <link>

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

Zaher, H., de Oliveira, L., Grazziotin, F.G., Campagner, M., Jared, C., Antoniazzi, M.M. & Prudente, A.L. 2014. Consuming viscous prey: a novel protein-secreting delivery system in neotropical snail-eating snakes. BMC Evolutionary Biology, 14, 1-28 <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.

Wednesday, September 28, 2016

Xenophidion: The Snake with the Mystery Penis

This post will soon be available in Spanish

Xenophidion schaeferi
From Das 2010, painted by Szabolcs Kókay
For a combination of phylogenetically distinct, taxonomically confusing, and poorly known, you simply cannot beat the spinejaw snakes, genus Xenophidion. Described in 1995, there are two species, each known from a single specimen1. That makes even dwarf pipesnakes (family Anomochilidae), of which we've obtained several new color photos recently, seem relatively well-represented. Putting together this article strained my research powers—xenophidiids don't even have an English language Wikipedia page (yet). Google the name of the family and it asks whether you meant Xenophilius, the first name of a minor character from Harry Potter, who has more than twice as many results. Xenophidion means "small strange snakes" in Greek, and indeed we have barely scratched the surface of how strange these snakes probably are. And, to top it all off, no one has ever seen its penis. Read on to find out why.

Collection locations of the only specimens of
Xenophidion acanthognathus (red)
and Xenophidion schaeferi (green)
The story of Xenophidion begins on the morning of November 20th, 1987. It was 8:15 AM when Chicago Field Museum Herpetologist Robert F. Inger found a snake beneath some moss on rock during field work in a selectively-logged forest near Mendolong, in Sabah's Sipitang District on the island of Borneo. Inger, an expert in the herpetology of southeast Asia who by that time in his life had "made thorough searches of thousands of square meters of forest floor litter with the help of very sharp-sighted local men", had never seen a snake like this before, and he brought it back to Chicago and placed it in the Field Museum collection.

Almost a year later, at 10:00 PM on November 5th, 1988, German amateur herpetologist Christian Schäfer collected and photographed a snake at the edge of a trail near Templer Park, about 12 miles north of Kuala Lumpur in peninsular Malaysia. Schäfer donated his specimen to the Zoological Museum in Berlin in the spring of 1993. Curators Rainer Günther and Ulrich Manthey recognized it as unique and asked esteemed herpetologists Van Wallach and Bob Inger to compare it to specimens at Harvard and Chicago. Inger recognized similarity between Schäfer's specimen and his own, and sent both specimens back to Berlin to be described as new species. The dissimilarity between the two new specimens and all other known snakes was so great that they chose to establish a new genus, which they tentatively placed into the family Colubridae (which at the time was much more inclusive). The genus was elevated into a new family after the dissection of the X. acanthognathus specimen by Wallach and Günther in 1998 failed to reveal an obvious affinity with any existing family.

Drawing and photograph of the jaw spine of X. schaeferi (labeled 'Pp')
From Günther & Manthey 1995
The two specimens share a number of unique features that distinguish them from all other living snakes. Their head scales, especially those along their lips, bear numerous sensory papillae. Their prefrontal scales are much larger than those of other snakes, taking up most of the top of the head in front of the eyes, and the space between their eyes is slightly concave. Their upper jaw bears a long, spiny palatine process, after which X. acanthognathus ("spine jaw" in Greek) is named. Their small eyes, short tail, and wedge-shaped head all suggest a mostly fossorial lifestyle. Like many "henophidian" snakes, their ventral scales are only slightly wider than their dorsal scales. But, unlike so many henophidians, both species of Xenophidion lack any vestiges of a pelvic girdle, left lung, or coronoid bone, suggesting that they are more closely related to caenophidian snakes. Wallach and Günther noted several similarities among the visceral characteristics of Xenophidion and tropidophiids, including a tracheal lung and unlobed kidneys., although we now know that tropidophiids are most closely related to aniliids. They also suggested that Xenophidion and another enigmatic snake family, bolyeriids, might be related.

The only photograph of a living Xenophidion schaeferi (FMNH 235170),
taken by W. Grossmann. From Günther & Manthey 1995

In 2004, the sequence of the cytochrome b gene of X. schaeferi was sequenced. This is still the only gene we have from either species of Xenophidion, and it has suggested a sister relationship between Xenophidion and Bolyeriidae and a distant relationship between Xenophidion and Tropidophiidae in several studies. Evidently, unpublished CT scans of the skull of Xenophidion show that these snakes also have a joint in the maxilla, a characteristic unique to bolyeriids. We know almost nothing about the diet of Xenophidion, but thankfully the stomach of the X. acanthognathus specimen contains a Sphenomorphus skink. Skinks are also eaten by bolyeriids, which use their hinged upper jaws to grasp their hard-bodied,  relatively non-deformable prey. It's not inconceivable that Xenophidion might do this as well. The current geographic distribution of Bolyeriidae is limited to Round Island in the Indian Ocean, which suggests that the common ancestor of these two families was probably ancient and widespread across Gondwanaland.

Ventral view of the sole specimen of
Xenophidion acanthognathus (ZMB 50534)
From Günther & Manthey 1995
There are numerous differences between the two species of Xenophidion. Both have 23 dorsal scale rows at midbody, but the dorsal scales of X. acanthognathus are more heavily keeled than those of X. schaeferi. They have a similar number of ventral scales (181 vs, 178), but X. acanthognathus has 51 subcaudals, 8 more than X. schaeferiXenophidion schaeferi also has more teeth on the palatine (10 vs. 8), pterygoid (16 vs. 13), and especially the dentary bone (19 vs. 12) than X. acanthognathus. Finally, X. acanthognathus has a large yellow-white patch on its neck. Because both of the specimens are females, the hemipenes, which contain many taxonomically useful characters, have not been described. But, conveniently, the oviduct of the X. acanthognathus specimen contains two eggs, so at least we know the reproductive mode of these snakes.

Snake family tree from Figueroa et al. 2016showing
Xenophidiidae + Bolyeriidae as sister to Caenophidia
Click for a larger version
Some phylogenetic studies suggest that Xenophidiidae and Bolyeriidae might be sister to Caenophidia, leading some to call these two families "proto-colubroids". However, other genetic analyses group them with boas, pythons, and other "henophidian" snakes instead. Hopefully further gene sequencing will sort this out, and of course fresh Xenophidion specimens wouldn't hurt. The forestry station where Inger collected X. acanthognathus is still operational and researchers continue to work there—I hope they know to keep their eyes open for small, strange snakes. Unfortunately, the primary forest where X. schaeferi was collected was cleared two years later and is now a banana plantation. Both peninsular Malaysia and Borneo are losing their forests to timber harvesting and oil palm plantations at an alarming rate. People get upset when they learn that deforestation endangers charismatic species such as orangutans, leading to efforts to make palm oil production more sustainable. This is really challenging because palm oil is used in all kinds of delicious things, such as Girl Scout Cookies, and high-profile controversy over its sustainability has been fueled by people's love for orangutans. I'm here to suggest that the many mysteries of Xenophidion—including what its penis looks like—may never be solved if the rain forests of southeast Asia are lost, and that Xenophidion is at least as valuable and interesting as orangutans.

UPDATE: Additional specimens have since been discovered in Malaysia, Sumatra, and Borneo including at least one male, bringing the total number of individuals known to six, potentially representing three species if the Sumatra specimen proves to be distinct. Some photos of one of the new specimens here.

1 The IUCN page for Xenophidion acanthognathus mentions a second specimen from Kinabalu, but I couldn't find any other references to this specimen. Instead, the IUCN references page pointed me, through a couple of intermediates, to a paper (published before the discovery of Xenophidion) that included a reference to the type specimen of Stoliczkia borneensis, which was collected on Mount Kinabalu. Since Stoliczkia borneensis is in the family Xenodermidae, I suspect there may have been some confusion around the somewhat similar family names. VertNet lists only the single Sipitang specimen of X. acanthognathus, as does Wallach et al.'s 2014 edition of Snakes of the World
. Both species of Xenophidion are listed as Data Deficient by the IUCN.


Thanks to Szabolcs Kókay, who painted the only color image of Xenophidion for A Field Guide to the Reptiles of South-east Asia.


Chan-ard, T., Grossmann, W., Gumprecht, A. & Schulz, K.D. 1999. Amphibians and reptiles of Peninsular Malaysia and Thailand: an illustrated checklist. Bushmaster Publishing, Wuerselen, 240 pp. <link>

Das, I. 2010. A field guide to the reptiles of South-East Asia. New Holland Publishers, London, 376 pp. <link>

Das, I. 2012. A naturalist’s guide to the snakes of South-East Asia. John Beaufoy Publishing, Oxford, 176 pp. <excerpt/link>

Figueroa, A., A. D. McKelvy, L. L. Grismer, C. D. Bell, and S. P. Lailvaux. 2016. A species-level phylogeny of extant snakes with description of a new colubrid subfamily and genus. PLoS ONE 11:e0161070 <link>

Günther, R. & U. Manthey. 1995. Xenophidion, a new genus with two new species of snakes from Malaysia (Serpentes, Colubridae). Amphibia-Reptilia 16:229-240 <link>

Lawson, R., J. B. Slowinski & F. T. Burbrink. 2004. A molecular approach to discerning the phylogenetic placement of the enigmatic snake Xenophidion schaeferi among the Alethinophidia. Journal of Zoology 263:285-294 <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 Evolutionary Biology 13:93 <link>

Wallach, V. & R. Günther. 1998. Visceral anatomy of the Malaysian snake genus Xenophidion, including a cladistic analysis and allocation to a new family (Serpentes: Xenophidiidae). Amphibia-Reptilia 19:385-405 <link>

Wallach, V. W., Kenneth J. and J. Boundy. 2014. Snakes of the World: A Catalogue of Living and Extinct Species. CRC Press, Boca Raton, Florida, USA <link/sample>

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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.