Snakes on the Brain: Essays on Snakes, Science, and Society. Essay 3. Are All Snakes Venomous? The Toxicofera Hypothesis And How We Define Venoms

Snakes on the Brain: Essays on Snakes, Science, and Society. Essay 3. Are All Snakes Venomous? The Toxicofera Hypothesis And How We Define Venoms

Snakes on the Brain: Essays on Snakes, Science and Society #3

I often wonder what exactly about snakes makes them such polarizing figures throughout human history. We tend to react in extremes of when it comes to these often maligned creatures, and both human parties, whether the fascinated or the fearful, view the other as either misguided or perhaps certifiable. To this day it remains surprising to some that fans of snakes, such as me and many other perfectly rational folk, even exist, let alone work with and study these stunning, adaptive, mysterious beasts.

These essays are my menial attempt to bridge some of the gap between the fascinated and the fearful, while exploring some aspect of science or natural history. While I make no claim of authority, I will say that I aim to provide as much reasoning and references for my views wherever necessary throughout. Where I fall short, I apologize and request your unyielding criticism, dear reader, as both punishment for the current and polish for future essays.

The first essay can be found here.

Essay 3. Are All Snakes Venomous? The Toxicofera Hypothesis And How We Define Venom.

One fine spring afternoon, I was bitten on the face by a nearly one meter long snake. Certainly a result of my own stupidity, this animal decided to clamp onto my left cheek. No permanent damage was done, save for my work shirt which suffered minor stains, and after a quick clean up we were back to work. Our job of course is to relocate snakes to more suitable habitats whenever they come across fearful humans, and we knew we were in no real danger. The animal in question was a common tree snake (Dendrelaphis punctulata), an effectively harmless, fangless colubrid snake which had been mistaken by warehouse staff members for a highly venomous eastern brown snake (Pseudonaja textilis). I humbly suggest, had it been P. textilis under the shelves, I would have been much more cautious, perhaps even restraining myself from holding it up near my face, allowing it to strike precisely at the moment I was explaining to watching staff members that these animals aren’t venomous (D. punctulata, that is!) and, while they do have teeth, they only very rarely bite even when handled (they truly are quite docile…most of the time). I was also defecated on and musked, a defensive tactic of releasing a foul smelling, pungent “musk” from glands in the cloaca, by the same individual. Ah, the infinite joys of working with native wildlife.

The common tree snake is a frequent visitor of people’s properties from Australia’s east coast and across the top end, avoiding the more arid inland and colder south (See Wilson & Swan, 2013 (1) for a field guide of Australian reptiles, Cogger, 2014 (1) as a comprehensive at-home companion). These laterally compressed, lightweight, elongate snakes are perfectly adapted for their non-venomous style of prey capture, climbing up to high into forest canopies and vine thickets where tree frogs hide, or slipping stealthily around aquatic and riparian vegetation. A particular fan of frogs, they simply latch on to prey animals small enough to overpower and swallow as soon as possible, often alive. This requires nothing more than small, sharp teeth, a contrast from the highly complicated venom systems of other snakes.

Why are we discussing this rather commonplace, non-venomous, seemingly unexceptional snake? Here we take a tentative step into a yet unsettled and at times especially heated debate in the world of reptile biology (please, nobody hit me!). For according to the captivatingly titled Toxicofera hypothesis, not only D. punctulata but all other snakes, including the various pythons which have also gifted me with unpleasant facial injuries, are venomous or share a venomous ancestor. No recent theory in the world of venomous reptiles has caused as much of a controversy as this idea. Take the non-venomous snakes I was so idly handling my whole life; were they actually in possession of a venom system, even if somewhat deteriorated over evolutionary time through non usage?

The Toxicofera Hypothesis daringly goes even further. The discovery of highly dangerous venom products in the mouths of theoretically non-venomous snakes poses the question; what else out there might be venomous? The venom research team of Fry et al. went looking for venom where there should be none. They found that many lizards, including the largest in the world, the Komodo Dragon (Varanus komodoensis), also possess venom and venom glands in their jaws. The Komodo in particular caught the fascination of the science media, as these gigantic monitor lizards were previously thought to capture their prey by delivering a septic bite full of bacteria, or relying on physical trauma caused by their serrated teeth, like well maintained steak-knives (see (3)). What a phenomenal discovery to further weaponize this already powerful and captivating beast!

Since the expansive work of Fry et al. 2006 (4), the Toxicofera hypothesis had been widely accepted. The Komodo Dragon’s newly acquired venomous status had even been cemented in culture by no lesser natural history orator than the celebrated Sir David Attenborough (stand and salute, or be shot) in the BBC Life documentary series.  Alas, certain aspects of their research and conclusions have come under fire in the following years from other toxinologists around the world, casting some shadow over this idea which had so captured the attention of both public and scientific audiences. What followed was a battle of words and reasoning between the two camps, taking stage on two of the grandest of battlefields known to modern scientists; the “letters to the editor” section of a specialist journal (Toxicon in this case), and a public scientific debate at Oxford. Perhaps we should closely examine the evidence for and against the Toxicofera hypothesis to better arm ourselves before we enter the fray in this venomous war of wits.

Let’s start with the origin of the Toxicofera and the work of Fry and colleagues. How did the authors determine that these animals all belong to the same clade, an evolutionary term for a grouping of related animals with a common ancestor? Traditionally, venom is thought to have arisen twice or even three times in the squamata; in the snakes (perhaps separately in the elapids/vipers and the colubrids) and in the heliodermatid lizards, the Gila Monster and the Beaded Lizard (3). After several years working on the evolution of venom systems in front fanged elapid snakes from the late ‘90s to the early 2000, Fry began investigating the venom systems of some less studied venomous snakes in the Colubridae family, an unresolved, numerous and diverse umbrella family which likely contains several separate families (see Lumsden et al. 2004 (5) for an example of some of their work). Not all colubrid snakes possess fangs or venoms, and where present they’re more sophisticated than in elapids and vipers. While the latter two families deliver a pressurized bolus of venom, passing from the temporally placed gland (venom reservoir inbuilt) through a venom-duct to the font maxilla where it is injected through modified hollow teeth or “fangs “, colubrids rely on simple enlarged teeth halfway down the back of the jaw (thus the term “rear-fanged” vs “front-fanged” for venomous colubrids and elapids/vipers respectively). The rear-fang is grooved at the back, trickling venom proteins down to the puncture site as they flood the mouth at the base of the fangs, extruded from more the simple glands referred to as Duvernoy’s Glands, often with small additional glands along the upper and lower jaw line under the labial scales (see 6, 7).

All this led to an intriguing question; what are the origins of this venom system in snakes? This issue has been debated within the herpetological community for some time, with many suggesting that the elapids and vipers evolved their venom system separately, with certain colubrids later developing their own similar system. This was cited as a case for convergent evolution, the process by which two species evolve similar adaptive traits due to similar selective pressures (7, 8). This is in contrast with shared ancestry, where homology (similarity) between features and species implies a common ancestor in possession of the shared trait, which is then inherited down many generations by separate lineages as they become separate species and groups.

Consider the evolution of flight in vertebrates. Only the birds and the bats are capable of powered flight, but we do not believe they evolved from a recent common ancestor. The birds are essentially a lineage of reptile, closely related to the dinosaurs, the bird line of crown-group Avemetatarsalia evolving within the Archosauria, trading scales for feathered plumage (although both are made of similar keratinized materials), while the mammals evolved much later in the history of life on Earth from smaller, ground-dwelling, lizard or mouse-like reptiles, perhaps similar to shrews or gerbils (9, 10). For these lightweight animals, in a world full of predators, the benefits of being able to glide, or perhaps simply control airtime and corner more precisely while bounding from log to rock to branch, are self evident. Any mutations for increased forearm feathers or perhaps thin membranes between elongate fingers might be quickly swept to ‘fixation’, that is, mutant offspring with these traits are so successful and breed so frequently that within a few generations the whole population is comprised of individuals in possession of their gene variant with the accompanying trait, and the mutant becomes the new norm. In this way, both bird-like and bat-like ancestors successively converged on the same solution independently, as opposed to sharing a flying ancestor like the modern descendents of these early birds or bats do. Flight, incidentally, seems to be a common point of convergence, first evolving in the Pterygota, the winged insects, 170 million years before any other life forms took flight (11).

Back to venom in snakes, did the colubrid and elapid/viper venom systems evolve separately in convergence? Or is the front-fanged condition a derived, more complicated and effective version of the one original rear-fanged condition? Of course, proponents exist for both ideas. Some suggest that it could be naught else but a venom system in infancy, others say that even if so, a venom is defined by its use in prey capture or defense, and can be only defined by its biological function, which has not been demonstrated yet. For example, humans also possess toxic oral compounds in our saliva, known for pre-digestion etc. rather than hunting, but we would hardly be considered venomous (7). Still, whether venom or some other protein product, the shared point of agreement was that the oral products of non-venomous colubrids, even those without fangs or Duvernoy’s Glands, needed more investigation.
Fry was in the process of studying the glands of these less dangerous colubrids when the first suggestions of the Toxicofera came to light. Using LC-MS (Liquid Chromatography–Mass Spectrometry), as well as protein isolation and sequencing from the Asian Ratsnake (12), the team could investigate what proteins were being produced and what genes may be involved. In the mouths of some putatively non-venomous colubrids, highly toxic venomous protein compounds continued to be located, albeit in small quantities. These included three-finger proteins which the team suspects may be the ancestors of the three-finger neurotoxins (3FTXs) of highly venomous elapid snakes. Where does the story of squamate venom end? In following this, Fry and colleagues spend much field time with various Australian lizards. By creating libraries of the DNA expression in oral tissues, they found similar toxic venom products in an agamid and a varanid lizard (3). These proteins closely matched those found in the venom glands of serpents.

Around the same time, another type of study was making waves. Morphological evidence from precise physical measurement of bones and fossils, and the mathematical groupings of these measurements to delineate species and higher orders of taxonomic groups, has a centuries long standing as the standard method for evolutionary research and the study of natural history. It might’ve come as a rude shock to proponents of the traditional schools when a newer style of evolutionary data, molecular evidence from examining sequence variation and expression patterns of DNA, RNA and proteins (the information storage, transcript papers, and physical matter of biology itself), began placing doubts on some of these morphological conclusions. Most notably for us, squamate phylogenies built from multiple gene sequences placed the evolution of iguanid reptiles much later history (13). In fact, they have buddied up right next to the Serpentes lineage, the latest major evolutionary and morphological radiation in the squamate reptile group, a radical difference from the basal placement suggested by morphology.

Both the molecular phylogenetic data and the protein sequence data seemed to agree upon this rearrangement of Iguania. Furthermore, according to the findings of Fry and colleagues, these lizards share the similar venom products in their mouths to all snakes. The blow to our pillar of understanding resounded throughout biology; all snakes are venomous, and so are, most probably, many lizards (excluding several lineages like the geckoes and skinks which diverged before the development of the “venom genes” typifying the Toxicofera). All share a venomous common ancestor some 170 million years ago, lizard-like in form, with some subsequent lineages losing the apparatus through lack of usage. So sayeth the Toxicofera hypothesis. Further support came in the following years with more so called “incipient venom systems” found, including that of the famous Komodo Dragon. Glands and venom proteins were discovered in the mouth tissues of various iguanid and varanid lizards, including several commonly kept commercial pet species, such as the central bearded dragon (Pogona spp). The Toxicofera hypothesis was picking up steam. That’s when someone threw a chair, and a fight broke out.

Not everyone was on board with team Toxicofera. It is easy to view such dissent for new idea as nothing more than the Old Guard complaining that the new kids are being too noisy (with their gosh darn rap music and DNA technologies), but this would dismiss two major tenants of science: that no idea is beyond review, and that an exceptional statement or revision requires exceptional supporting evidence. Scientists complain, particularly about other scientists, and this is how we improve, usually in a polite and courteous manner. While one might publically comment online in various forums, this means little in the world of peer review. What to do? Like any polite but disagreeable reader, one might feel compelled to write a strongly worded, thoroughly research letter to the editor, expressing your discontent and reasoning, for the target audience to see in the next issue of the journal.

Such was the response of Weinstein et al. 2012 (14) to the several years of Toxicofera papers that had been published since 2006. This group of toxinologists and evolutionary biologists had decided to challenge certain aspects of the Toxicofera. Namely they were targeting the definition of these animals as venomous. Team Toxicofera hit back (15), writing a letter to the editor themselves to dispute the points brought up by Weinstein et al., which was naturally replied to in kind (16). If ever the opportunity arises, I thoroughly recommend these letters not just for their scientific content but for entertainment. I truly enjoyed these papers not just for the well formed arguments, but for the salvos of criticism and rebuttal peppered with as much passive aggression as any journal can let past their editorial desk. While Fry and colleagues argued from the functional lab tests on blood pressure, homology of proteins, DNA/RNA sequences and phylogenies, as well as their mere presence in the oral tissues, the detractors maintain it is premature to call this “venom” as its ecological role has yet to be demonstrated.

Fry and colleagues do not accept this and argue instead for a revision in how we define venoms and venom systems. I personally find this hard to agree with as the already-in-use term “oral product/secretion” would suffice for any toxic compound without such a biological function (See 6). The various snakes which have bitten my face were not, in my preferential use of the term, “venomous”, despite perhaps possessing small amounts of toxic compounds in their saliva. The many, many bearded dragon (P. barbata and vitticeps) bites I have sustained on my fingers also never caused excessive bleeding, swelling, or other illness. Even if the toxic compounds in their tissues eventually were recruited to give rise to true venoms through splice variants, gene duplications, and adaptive selection, as may indeed be the case, these pre-venom products still have no natural use in prey capture in bearded dragons, which are primarily vegetarian.

While we of course do not fall within the Toxicofera phylogenetically, humans also possess some of these putative venoms in our buccal cavity (17). These include toxins with similar LD50s (a measure of toxicity) to those being used to attribute venomous status to the Toxicofera. Should we suggest humans are venomous since an intravenous injection of our saliva would disrupt the body’s delicate homeostasis? Does it not seem more consistent to continue using “toxic oral products” or a similar term to refer to any compounds produced in the oral cavity that are toxic, until the ecological function has been ascertained? Or do all snakes, many lizards, and probably a host of other species, including humans, now have the honour of being venomous fauna? In response to the need to ascertain the ecological role of these proteins, Fry has been quoted as saying, somewhat dismissively, “just give us a couple more centuries, an army of research students, unlimited funding, and we will be with you on that” (See this link (18) in the Atlantic), implying perhaps the task of confirming the ecological role of all of these proteins in every species is too arduous, an impossible and preposterous standard to set. This seems like a somewhat facetious argument as a response to genuine criticism. To suggest that every species’ oral products must be tested for function would of course be foolish. However, if it could be demonstrated in but a handful of species across the Toxicofera lineages that these proteins have an envenomation function, the venomous status of these creatures would be gloriously and righteously vindicated. Strong support indeed and hard for anyone to argue against!

Alas, there are no such examples in any taxa thus far. Even the Komodo Dragons venomous status is under debate, with detractors suggesting the amount of trauma generated by the serrated, saw-like teeth seems more than enough to cause bloodloss and shock rather quickly. Ecological function of all of these putative venom compounds thus remains a mystery. Should we perhaps withhold upgrading them from “oral products” to “venoms” until we decide whether ecological function or simple toxicity will be the standard-barer for biology as a whole, us included, from here on? Evolutionary definition can be a bumpy road, but such standards and definitions are set exactly to moderate the kind of confusion currently being generated.

What then are the roles of these oral proteins if they’re not venoms? Many functions have been suggested, for example, lubricating prey to aid swallowing, pre-digestion (just as the enzyme amylase starts digestion of starch compounds in our mouths before food even reaches the stomach), anti-putrefaction since large prey may take days to digest inside a snake, detoxifying/disinfecting prey surfaces, or general oral health and hygiene (Kardong, 1982). However, just as suggestions for venom function in lizards, the above are nothing more than suggestions. Proof requires evidence, which has yet to be gathered, thus judgments should be withheld until we know for sure.

More recent evidence may suggest that these “venom” proteins are not venom proteins at all. A 2014 publication by Hargreaves et al., using emerging technologies, surveyed DNA expression in multiple body tissues of Toxicoferan animals, including the Burmese python and the leopard gecko (19). Comparison of the genes expressed in these tissues with online databanks of gene sequences found matches where one would not expect. These “venom” genes were being expressed throughout multiple parts of the body, and in comparable levels to those found in the oral tissues. How could this happen? The authors suggest that these genes are in fact nothing more than “housekeeping” genes, used for general maintenance purposes rather than venom. Of 74 putative venom genes studies, only two (Laminoacid oxidase b2 and PLA2 IIA-c for those playing at home) showed significant expression of gland-specific splice variants as expected of true venom proteins. This was considered a heavy blow to the Toxicofera hypothesis, although Fry and colleagues maintained that true venom genes had to come from somewhere, and these housekeeping genes were recruited for their toxicity, a by-product of their original function, at some point in the early Toxicofera history. If you suspect that this was far too ambiguous an answer to satisfy everyone, particularly those opposing team Toxicofera, you would be correct.

Obviously the matter needed to be settled before the scientific community came to blows (scientists are after all known for their violent temper and rage issues). The battle in the editorial section had garnered quite an audience, but as yet seemed unresolved. The challenge was set; a public debate at Oxford University. Who wouldn’t appreciate the Schadenfreude of beating one’s opponent in debate at such a renowned establishment, in the grand public arena no less? Both parties naturally agreed and brought their best case to the fore, however the result was an almost unanimous vote against the singular evolution of venom in reptiles (for a brief rundown, albeit from only Hargreaves pen since history is always written by the victorious, see link 20).

Credit must be given here, for as arguing against the establishment is difficult, the house almost always wins, and team Toxicofera obviously stuck to its convictions. The conclusions of Hargreaves and others on the other hand are increasingly hard to deny, as demonstrated by the thoroughly one sided final score. While a convincing victory, the Toxicofera proponents are by no means licked, and not yet abandoning their hypothesis due to the opinion of the masses, be they scientists or otherwise and further interesting research is undeniably in the works from both camps. For example, a recent combined molecular morphological study claims to have resolved the squamate phylogeny once and for all, coming down on the side of the Toxicofera, however this paper’s methods (including removing what they call “rogue” taxa which don’t conform in their phylogeny, a curiously convenient way to prune a tree) have been criticized (21). Why can’t it ever go smoothly?

So, the number of times venom has arisen in the reptiles remains unsettled. But where does this leave us with the unresolved squamate phylogeny? Here I believe, or at least hope that future evidence shows, that we can have our cake and eat it too, while not being venomous people. The Toxicofera clade seems to be consistently recovered by many molecular investigations, and the homology of some oral tissues and protein products seem to suggest shared ancestry. What is the problem with assuming a non-venomous role for these oral products, particularly in light of their low expression in glands and expression throughout the body? Could not such oral products still confer some selective advantage and lead to a successful clade of animals? If, for example, the common oral products of the Toxicofera are simple anti-microbial tools to protect the delicate mouth of the snake, which gave them an immune advantage that was inherited throughout the clade, then while the clade Toxicofera may truly define an evolutionary lineage, then only the implication in the name would be misleading. While a clade of successful reptiles with an improved ability to resists oral infections (I dub this hypothetical clade “Antimicrobifera”) may be a less captivating idea than the highly toxic or bacteria laden bite of the Komodo dragon, weirder things have happened.

But this is all conjecture. For now, we must satisfy ourselves with what we know; current molecular data indicates the Toxicofera may in fact be a real evolutionary grouping sharing a common ancestor, but whether they’re truly “venomous” as the namesake suggests remains to be proven. Until the function of these products is proven, I personally shall simply say they appear to be common in much of the putative Toxicofera group, for some reason or other. Vague I know, for vagueness is our refuge in the face of uncertainty, until sufficient evidence draws us out into the light. Such caution keeps theories alive in the face of predators, for the tastier the theory, the more they shall try to tear it to shreds.

A final thought on complexity. It is tempting to view venom systems as on an upward climb in complexity through the squamata (disregarding examples of degeneration). This view is in agreement with the parsimony principle, referring to finding the simplest solution, the smallest amount of evolutionary branches on the tree, as morphological and evolutionary changes do not happen lightly. However while organs can increase in complexity, it is more important to remember that evolution does not strive for progress, only survival and reproduction. Any “progress” we perceive in biological systems is merely a by-product of life’s branching nature and the great spread of variety and form, rather than a march towards some perfect animal or system. What intricacy life generates is not simply to increase in complexity towards a particular ideal; there is no ideal, or if there is it exists as a broad, fluctuating spectrum built atop a sliding scale, and even we humans can make no claim for superiority or perfection. This, at least, we share with all snakes, lizards, and the rest of life on Earth.

References

1. Wilson, S. & Swan, G. (2013) A Complete Guide to Reptiles of Australia; Fourth Edition. New Holland Publishing.
2. Cogger, H. (2014) Reptiles And Amphibians Of Australia; Seventh Edition. CSIRO Publishing.
3. Fry, B.G., Wroe, S., Teeuwisse, W., van Osch, M.J., Moreno, K., Ingle, J., McHenry, C., Ferrara, T., Clausen, P., Scheib, H., Winter, K.L., Greisman, L., Roelants, K., van der Weerd, L., Clemente, C.J., Giannakis, E., Hodgson, W.C., Luz, S., Martelli, P., Krishnasamy, K., Kochva, E., Kwok, H.F., Scanlon, D., Karas, J., Citron, D.M., Goldstein, E.J.,McNaughtan, J.E., Norman, J.A.. (2009) A central role for venom in predation by Varanus komodoensis (Komodo Dragon) and the extinct giant Varanus(Megalania) priscus. Proc Natl Acad Sci U S A.106:22
4. Fry, B.G., Vidal, N., Norman, J.A., Vonk, F.J., Scheib, H., Ramjan, S.F.R., Kuruppu, S., Fung, K., Hedges, S.B., Richardson, M.K., Hodgson, W.C., Ignjatovic, V., Summerhayes, R., Kochva, E. (2006) Early evolution of the venom system in lizards and snakes. Nature. 439, 7076.
5. Lumsden, N.G., Fry, B.G., Kini, R.M., Hodgson, W.C. (2004) In vitro neuromuscular activity of ‘colubrid’ venoms: clinical and evolutionary implications. Toxicon. 43
6. Kardong, K.V. (1982) The evolution of the venom apparatus in snakes from colubrids to viperids and elapids. Memoirs of the Institute of Butanan 46: 105–118.
7. Kardong, K.V. (1996) Snake toxins and venom evolution. Herpetologica 52.1
8. Jackson, K (2003) Evolution Of Venom-Delivery Systems in Snakes. Zoological Journal of the Linnean Society The Linnean Society of London, 137
9. Brusatte, S.L. , Benton, M.J. , Desojo, J.B., Langer, M.C. (2010) The higher-level phylogeny of Archosauria (Tetrapoda: Diapsida), Journal of Systematic Palaeontology, 8:1
10. Close, R.A., Friedman, M., Lloyd, G.T., Benson, R.B.J  (2015) Evidence for a Mid-Jurassic Adaptive Radiation in Mammals. Current Biology Volume 25, Issue 16.
11. Engel, M. (2015) Insect Evolution. Current Biololgy. 25
12. Fry, B.G., Wuster, W., Ramjan, S.F.R., Jackson, T., Martelli, P.,  Kini, R.M. (2003). Analysis of Colubroidea snake venoms by liquid chromatography with mass spectrometry: evolutionaryand toxinological implications. Rapid Commun. Mass Spectrom. 17: 2047–2062
13. Vidal, N., Hedges, S.B. (2005). The phylogeny of squamate reptiles (lizards, snakes, and amphisbaenians) inferred from nine nuclear protein-coding genes. C R Biol. 328(10-11)
14. Weinstein, S., Keyler, D.E., White, J. (2012) Replies to Fry et al. (Toxicon 2012, 60/4, 434–448). Part A. Analyses of squamate reptile oral glands and their products: A call for caution in formal assignment of terminology designating biological function. Toxicon. 60(4)
15. Jackson, T.N.W., Caswell, N.R., Fry, B.G. (2012) Response to “Replies to Fry et al. (Toxicon 2012, 60/4, 434–448). Part A. Analyses of squamate reptile oral glands and their products: A call for caution in formal assignment of terminology designating biological function”. Toxicon 2012. 1-17
16. Weinstein, S.A., White, J., Keyler, D.E. (2013) Response to Jackson et al. (2012), Toxicon. 64.
17. Bonilla, C.A., Fiero, M.K., Seifert, W. (1971) Comparative biochemistry and pharmacology of salivary gland secretions. I. Electrophoretic analysis of the proteins in the secretions from human parotid and reptilian parotid (Duvernoy’s) glands. J Chromatogr.  56(2)
18. http://www.theatlantic.com/science/archive/2015/11/reptile-scientists-bear-their-fangs-in-debate-over-venom/413485/
19. Hargreaves, A.D., Swain M.T., Logan, D.W., Mulley, J.F. (2014) Testing the Toxicofera: Comparative transcriptomics casts doubt on the single, early evolution of the reptile venom system. Toxicon. 92
20. http://www.ox.ac.uk/news/science-blog/amicable-venomous-debate
21. Reeder, T.W., Townsend, T.M., Mulcahy, D.G., Noonan, B.P., Wood, P.L. Jr, Sites, J.W. Jr, Wiens, J.J. (2015) Integrated analyses resolve conflicts over squamate reptile phylogeny and reveal unexpected placements for fossil taxa.PLoS One.10(3)

Cheers,
Janne
www.snakeoutbrisbane.com.au

Your SnakeOut.