Snakebite, antivenom research, and basic science

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In the past few weeks, a peculiar congruence of several seemingly-unrelated events took place. (At least) two new scientific papers about snake biology were published, a new video series was announced, some scientists entered contests, and the U.S. executive branch announced a budget proposal with deep cuts to science funding. However, these events aren't as unrelated at they might seem at first glance, and they have something to tell us about where snake biology, and science in general, are going in the future.

The science: part I (puff adders)

A puff adder (Bitis arietans)

Puff Adders (Bitis arietans) are among Africa's most widespread vipers. They are heavy-bodied snakes that are found in savannas and open woodlands. Like most vipers, they eat mostly rodents as adults, which they ambush from carefully-selected sites, which they sometimes occupy for weeks at a time. Recently, Xavier Glaudas and Graham Alexander published a new study showing that, even though Puff Adder strikes last less than two seconds, they can choose to either hold onto or let go of the prey depending on its size. Specifically, they hold onto small mice, shrews, birds, toads, and lizards, but strike & release larger rodents and rabbits, because retaliatory rat bites are dangerous to them. After they let go of these larger prey, which usually run off a short distance before the venom kills them, they track them down again using stereotypic strike-induced chemosensory searching behavior to locate the scent of non-toxic components of their own venom. This is really similar to findings by Bree Putman and Rulon Clark that Southern Pacific Rattlesnakes (Crotalus oreganus) were more likely to hold onto smaller rodents than to larger ground squirrels. You can watch 26 awesome videos selected from an archive of thousands of hours of video taken in the wild over more than two years.¹

This research matters because venomous snakes and their prey are in constant evolutionary arms races, leading to:

1. a mosaic of new biochemical compounds that are often useful in treating disease
2. a mosaic of new biochemical compounds that can make venomous snakebite really hard to treat

We'll come back to the second one in a minute. The obvious importance of human medicine and venomous snakebite treatment overshadow a third important reason to study snakes and what they eat. Although the beneficial role of snakes in rodent control is taken as gospel by many advocates of snake conservation, the amount of data that we actually have on what snakes eat in the wild is surprisingly small. For many species, we don't even have a general idea of what kinds of prey they like to eat. Given recent estimates that spiders eat about as much meat as people do worldwide, and the potential for snakes to reach very high population densities in certain habitats, it's likely that the top-down effects of snakes as predators are significant ecosystem services that most humans aren't aware of and thus undervalue. Indirect effects on other aspects of the ecology of snake prey species, such as predation release and disease transmission, link snake predation even more strongly to human health. This is particularly timely in light of recent predictions that 2017 will be a big year for white-footed mice and thus for Lyme disease in the northeastern USA, controversy over the reintroduction of Timber Rattlesnakes, one of the white-footed mouse's top predators, to Quabbin Island in Massachusetts², and the continuation of both the infamous Sweetwater Rattlesnake Roundup³ and the reformed Claxton Wildlife Festival and Lone Star Rattlesnake Days earlier this month.

The science: part II (how cobras got their flesh-eating venoms)

A Mozambique spitting cobra (Naja mossambica) spitting its venom

Spitting cobras are even more well-known than puff adders because of their defensive venom spitting abilities, showcased on the BBC's Life in Cold Blood. They are found in Africa and Asia and are thought to have evolved two or three times from non-spitting cobras. A new paper from the lab of Bryan Fry at the University of Queensland sheds some light on when and why venom spitting evolved. Elapid snakes, including cobras, have venoms rich in neurotoxins, which are highly potent toxins that are very effective at paralyzing their prey. Cobras also have less potent cytotoxins that kill cells directly, which is a bit weird. What is the function of these toxins?

Toxicity of snake venom to human cells grown in culture.
Warm colors indicate higher toxicity.
From Panagides et al. 2017

The hypothesis put forth here is that the first step towards venom spitting was the evolution of hooding behavior and morphology, which happened twice in elapids: once in "regular" cobras and once in King Cobras, which are more closely related to mambas. Only once a conspicuous visual display was present was there selective pressure for cytotoxic venom components delivered to the eyes of potential predators via spitting. Although the venom of both groups is cytotoxic, Hemachatus (rinkhals) and Naja cobras use three-finger toxins, whereas King Cobras use L-amino acid oxidase enzymes, consistent with the undirected, opportunistic nature of our current model of venom evolution by gene duplication and mutation. The authors suggest that further elevations in cytotoxicity are linked to bright bands and other aposematic colors or hood markings, although their paper did not attempt to quantify these attributes of cobra displays, which can be quite diverse even within species. Further evidence in support of the hypothesis is that Naja naja and Naja oxiana seem, based on their nested position, to have lost spitting but to have retained cytotoxicity, and their close relatives Naja atra and Naja kaouthia might represent steps down this evolutionary path, being capable of spitting only in some populations and with less accuracy than the African and southeast Asian clades of true spitting cobras.

This is an extremely cool and popular topic. It was covered by IFLS, The Wire, Gizmodo, and the Washington Post. It goes to show that people worldwide are fascinated by venomous snakes, and the Fry lab has done a great job capitalizing on that interest (among other accolades, Fry's graduate student Jordan Debono recently won the Queensland Women in Science Peoples' Choice Award [a contest that was decided by an online popular vote; more on this later] for her research on global snakebite treatments). One reason for this fascination has to do with the question of who, exactly, these cobras are defending themselves from? The most reasonable hypothesis, given the timing and geography of the diversification of spitting cobras and the precision with which they can target forward-facing eyes and hominoid faces, is primates. Us, and our ancestors, who have eaten and been eaten by snakes for millions of years. Studying spitting cobras is a window into our own evolutionary past, a way for us to learn about ourselves. But, let us not be misled into thinking that interactions between humans and cobras are a thing of the past.

The upshot: the truth about snakebite

You can follow the ASV @Venimologie

If you haven't read the blog by medical toxinologist Leslie Boyer, you really should. Earlier this month she wrote about the vicious circle of antivenom shortage in sub-Saharan Africa, where millions of people are bitten by venomous snakes every year, many of which die or suffer awful injuries because they lack access to good antivenom. This crisis has prompted the creation of the African Society of Venimology and a new series of snakebite training videos in English, French, and Spanish. The politics and economics of antivenom are complicated and reflect larger issues in medicine, education, quality control, supply and demand, and how global economics and corporations have failed to respond to the needs of local communities and consumers. In a nutshell, the issue is that antivenom manufacturers don't make enough good antivenom, because not enough people buy it. People don't buy it because it's expensive, and it's expensive because not that much is made. This is despite a huge need for it—but not everybody with a snakebite goes to a hospital and gets antivenom in Africa, partially because it's not certain there will be any and partially because a lot of patients and doctors don't know about antivenom, because it's not in widespread use (which is mostly because of the reasons above). Other exacerbating problems include that it's often not certified, fake products can price the real antivenom out of the market, and the infrastructure for distributing antivenom and information in Africa is sub-optimal (but improving). Fixing any one or even most of these problems won't fix the whole system—if any one of them break down, supply and demand will be out of balance and people won't get the care they need.

A lot of the same issues used to be present in Mexico, but product improvements, government outreach, and massive education efforts in the 1980s and 1990s dramatically reduced mortality from venomous snakebite and led Mexico to become a major producer and consumer of high-quality, affordable antivenom, so much so that the USA now imports some of these drugs from Mexico. The Mexican government enabled the Mexican antivenom industry to be competitive and reach its market, which is much larger than the domestic market for American antivenom manufacturers—medically-serious venomous snakebites (and scorpion stings) in the USA are mostly confined to the southwest, and the per-capita risk of snakebite is the lowest in the world. This creates its own unique problems. You may have heard about the controversy surrounding the discontinued coralsnake antivenom made by Wyeth, and there are compelling arguments that the Mexican polyvalent antivenoms Anavip (made by Bioclon for humans) and ViperSTAT (made by Veteria Labs for cats and dogs) are more effective and much less expensive (although this is due almost exclusively to the idiosyncrasies of the US healthcare finance system) than the only FDA-approved viper antivenom, CroFab (although BTG, the maker of CroFab, filed a complaint asserting that these Mexican products infringe on its patent).

Finally, the global importance of the availability of high-quality, affordable antivenom for Latin American, African, and other exotic snakes is only going to increase as venomous snakes become more popular as pets and in zoos. This is particularly true in parts of the world completely lacking venomous snakes or with only very benign, non-life-threatening species, such as northern Europe, Scandinavia and northern North America, where doctors may be totally unprepared for a snakebite emergency and may not have appropriate antivenom on hand. This is exactly the kind of situation where government funding, in the form of orphan disease R&D grants, could play a role in making it affordable for researchers and doctors to save lives.

For a great introduction to and more in-depth coverage of these issues, you should watch The Venom Interviews or read their coverage of the recent video series.

The future: sequence the Temple Pitviper genome

Temple or Wagler's Pitvipers (Tropidolaemus wagleri)
at the famous Temple of the Azure Cloud in Penang, Malaysia
You can vote to sequence their genome here!

Genomics of snakes is taking off in a big way, and we stand to learn a lot more about the evolution and function of snake venoms and the treatment of their effects. But, funding for basic science isn't a priority for many people, and more and more scientists are turning to crowd-funding their research or relying on limited funding from private foundations, which often decide which projects to fund through a crowd-sourced voting process. This isn't necessarily a bad thing; in fact, I think it's a great thing in many cases. But, it's important to realize that government funding for science is different from private funding in two crucial ways: 1) there is a lot more of it (at least for now), and 2) it's not driven by specific, private interests. A great example is the Orianne Society, a non-profit reptile conservation organization whose founding purpose was preventing the extinction of Eastern Indigo Snakes (Drymarchon couperi). Thanks to generous donations from private funding sources, the Society succeeded in purchasing large areas of critical habitat for this endangered snake and protecting them in perpetuity, probably the most effective and laudable conservation goal in existence. Another good example is the work of the Durrell Wildlife Conservation Trust, who have essentially saved a globally-rare snake, Casarea dussumieri, from extinction in the wild. I wish the quality conservation work that these organizations have become well-known for were more common, but to date their donors are some of the only large private backers of reptile research and conservation in the world.

Snakes are part of human economics, albeit to a lesser extent than many insects, fishes, birds, and mammals—they are hunted for food (although there are many issues surrounding better management of unsustainable harvests), kept as pets, their skins made into leather, and their venom harvested to make antivenom and other drugs. But, in their current form, these industries place very little emphasis on finding out more about snake biology in the wild; it just isn't necessary for them to make a profit, even though the information is important for what they do. Antivenom manufacturers are accountable to their shareholders, but trying to block FDA approval of Mexican antivenom is certainly not going to result in better treatment for snakebite victims in the USA, and American companies aren't investing in any research to create new, better products themselves, since drug development is expensive and risky, and they already have a monopoly on antivenom in the USA.

It's no secret that snakes and snake research have a PR problem: even scientific journals are less likely to publish research articles about snakes than about mammals and birds (although the bias is likely subliminal). Many people prefer cute fuzzy animals that are similar to humans, but research into the biology of un-fuzzy animals is equally important. There's a parallel to the divide between funding for basic and applied science. Basic science isn't usually as sexy as the exciting, fun applications that come later, like saving lives, curing diseases, or discovering new complex biological phenomena. However, important applied science like antivenom creation cannot happen without basic science, in particular basic science on snakes. Private companies can't afford to invest in basic science the way they once did. Which leaves government funding and that from a limited number of interested, private backers.

We should support public funding for science and elect politicians who will do the same; better treatment for snakebite should be the least partisan and most universally-agreed-upon goal in the world. I think the path between basic (snake ecology, venomics, and genomics) and applied (antivenom manufacturing and public health) science is shorter and clearer in this context than in many, but the same principles apply—you cannot have medicine, conservation, and the other good parts of civilization without science.

You can vote now through April 5th 2017 for a project sequencing the entire genome of the Temple Pitviper (Tropidolaemus wagleri) co-led by Ryan McCleary.

Stay tuned for more about the role of snake venom proteins in treating human diseases, and the role of snakes as predators in ecosystems.

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1 Naturally, I wanted to link to the full-text of the paper so that anyone interested in learning more could read it, but the publisher (Wiley) has a 12-month embargo on posting the PDF anywhere online. They actually expect you to pay between $6 and $38 to read the article. Now, I think it's great research, and it probably cost Glaudas, Alexander, and their university thousands of dollars and thousands of hours to do it. But, if you pay Wiley to read their paper, none of that money will go to them, nor to the scientists who peer-reviewed their work for free. It will go to Wiley, who Xav paid (maybe) to publish. They could have paid $3,000 to make it open access, but you can understand why they didn't. No wonder most most science is read by fewer than 10 people. It's an outdated model that can't go away fast enough. In contrast, the spitting cobra paper is open access, which cost its authors over $1,500. This is typical; academic authors almost always lose money on a publication.^↩

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2 Recent update here; you can write the governor of Massachusetts here.^↩

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3 Reports suggest that this year, like last year, a much larger number of live rattlesnakes were collected than markets could support, and at least one person died from a snakebite sustained while trying to capture a rattlesnake for a roundup.^↩

ACKNOWLEDGMENTS

Thanks to Bryan Fry for alerting me in advance of his publication, and to Colin Donahue, Markus Oulehla, and Ian Glover for the use of their photos.

REFERENCES

Bonnet, X., R. Shine, and O. Lourdais. 2002. Taxonomic chauvinism. Trends in Ecology & Evolution 17:1-3 <link>

Boyer, L. V. 2016. On 1000-Fold Pharmaceutical Price Markups and Why Drugs Cost More in the United States than in Mexico. The American Journal of Medicine 128:1265-1267 <full-text>

Boyer, L. V. and A.-M. Ruha. 2016. Pitviper Envenomation Guidelines Should Address Choice Between FDA-approved Treatments for Cases at Risk of Late Coagulopathy. Wilderness and Environmental Medicine. 27:341–342 <full-text>

Boyer, L. V., P. B. Chase, J. A. Degan, G. Figge, A. Buelna-Romero, C. Luchetti, and A. Alagón. 2013. Subacute coagulopathy in a randomized, comparative trial of Fab and F (ab′) 2 antivenoms. Toxicon 74:101-108 <full-text>

Cao, N. V., N. T. Tao, A. Moore, A. Montoya, A. Rasmussen, K. Broad, H. Voris, and Z. Takacs. 2014. Sea snake harvest in the Gulf of Thailand. Conservation Biology 28:1677-1687 <full-text>

Chew, M., A. Guttormsen, C. Metzsch, and J. Jahr. 2003. Exotic snake bite: a challenge for the Scandinavian anesthesiologist? Acta Anaesthesiologica Scandinavica 47:226-229 <full-text>

Chippaux, J.-P. 2012. Epidemiology of snakebites in Europe: a systematic review of the literature. Toxicon 59:86-99 <full-text>

Glaudas, X., T. C. Kearney, and G. J. Alexander. 2017. To hold or not to hold? The effects of prey type and size on the predatory strategy of a venomous snake. Journal of Zoology 10.1111/jzo.12450 <abstract>

Glaudas, X. and G. Alexander. 2017. Food supplementation affects the foraging ecology of a low-energy, ambush-foraging snake. Behavioral Ecology and Sociobiology 71:5 <link>

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 <full-text>

McCleary, R. J. and R. M. Kini. 2013. Non-enzymatic proteins from snake venoms: a gold mine of pharmacological tools and drug leads. Toxicon 62:56-74 <full-text>

Natusch, D. J. D., J. A. Lyons, Mumpuni, A. Riyanto, S. Khadiejah, N. Mustapha, Badiah, and S. Ratnaningsih. 2016. Sustainable Management of the Trade in Reticulated Python Skins in Indonesia and Malaysia. IUCN, Gland, Switzerland <full-text>

Nyffeler, M. and K. Birkhofer. 2017. An estimated 400–800 million tons of prey are annually killed by the global spider community. The Science of Nature 104:30 <full-text>

Panagides, N., Timothy N. Jackson, R. Pretzler, M. P. Ikonomopoulou, Kevin Arbuckle, D. C. Yang, S. A. Ali, I. Koludarov, J. Dobson, B. Sanker, A. Asselin, R. C. Santana, I. Hendrikx, Harold van der Ploeg, J. Tai-A-Pin, R. v. d. Bergh, H. M. I. Kerkkamp, F. J. Vonk, A. Naude, M. Strydom, L. Jacobsz, N. Dunstan, M. Jaeger, W. C. Hodgson, J. Miles, and Bryan G. Fry. 2017. How the cobra got its flesh-eating venom: cytotoxicity as a defensive innovation and its co-evolution with hooding and spitting. Toxins 9 <full-text>

Putman, B. J., M. A. Barbour, and R. W. Clark. 2016. The foraging behavior of free-ranging Rattlesnakes (Crotalus oreganus) in California Ground Squirrel (Otospermophilus beecheyi) colonies. Herpetologica 72:55-63 <full-text>

Stock, R. P., A. Massougbodji, A. Alagon, and J.-P. Chippaux. 2007. Bringing antivenoms to Sub-Saharan Africa. Nature Biotechnology 25:173-177 <full-text>

Wade, L. 2014. For Mexican antivenom maker, US market is a snake pit. Science 343:16-17 <full-text>

Willson, J. D. 2016. Indirect effects of invasive Burmese pythons on ecosystems in southern Florida. Journal of Applied Ecology 10.1111/1365-2664.12844 <full-text>

Willson, J. D. and C. T. Winne. 2016. Evaluating the functional importance of secretive species: A case study of aquatic snake predators in isolated wetlands. Journal of Zoology 298:266-273 <full-text>

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

Spitting cobras

Click here to read this post in Spanish!

Haga clic aquí para leer este artículo en español!

Spitting cobras have been known for centuries,
as you can see from this report published in the
Journal of the Bombay Natural History Society in 1900¹

A clever comic from birdandmoon
highlighting the fact that king cobras
are not true cobras

Cobras are some of the most iconic snakes in the world, instantly recognizable by their hoods even to those who have never seen one. They are also among the most dangerous snakes—fast-moving, with potent neurotoxic venom, cobra bites cause injury or death to many people in Asia and Africa. Cobras are elapids, together with coralsnakes, mambas, kraits, seasnakes, and numerous terrestrial Australian snakes both well-known and obscure. What unites these ~350 species of snakes is their short, immovable, and hollow ("proteroglyphous") fangs. Elapids probably evolved in Asia between 25 and 30 million years ago. By 16 million years ago, cobras were found in Europe, where they no longer live, and in Asia and Africa, where they are still found today. The core cobra clade consists of three small genera (Hemachatus, Aspidelaps, and Walterinnesia) and one large one, Naja. Other hooded snakes that are usually called "cobras" include tree cobras (genus Pseudohaje), whose placement remains uncertain, and the king cobra (Ophiophagus hannah), which is probably more closely related to mambas than it is to true cobras. Ironically, most people, if asked for a species of cobra, would almost certainly come up with the king first. But, probably they would think of a spitting cobra second, and with good reason from an evolutionary perspective, as we shall see.

Mozambique Spitting Cobra (Naja mossambica)

Almost all spitting cobras belong to the genus Naja, a large genus that comes from the Sanskrit word for snake, nāga. Literature buffs will recognize the name of the cobras in Kipling's Rikki Tikki Tavi, which led to the name of the snake Nagini in the Harry Potter books. Over the past 50 years, the number of species within the genus Naja has risen from six to 29², and more will probably become recognized in the future. At least 15 of these species can spit their venom through the air. The best of them are capable of aiming at targets the size of a human face with >90% accuracy up to 8 feet away. This adaptation represents the only purely defensive use of venom by any snake. Vipers and other venomous snakes occasionally eject venom from their fangs into the air, particularly when being handled, but these snakes are not aiming at anything, so they are not really using their venom defensively. Spitting in cobras is an adaptation that involves changes to the morphology of the fangs, their head musculature, and the chemistry of their venom.

Fangs of  cobras progressively adapted for spitting. Dotted lines show the venom canal, dark arrows indicate the flow of water injected into the top of the fang. From Bogert 1943

Left: "normal" non-spitting cobra fang (Naja kaouthia) Right: spitting cobra fang (Naja pallida) The sutures are visible above the exit orifices. From Young et al. 2004

All snake fangs are modified teeth provisioned with grooves that vary in depth and degree of closure. In vipers and elapids, the grooves are completely closed, forming hollow tubes, along the front edge of which a narrow suture can still be seen where the ridges forming the tube have come together in the developing embryo. In spitting cobras, the inside of this tube contains ridges, which act like rifling in a gun barrel to impart spin on the venom. The discharge orifice, located near but not at the point of the tooth (like a hypodermic needle), is large and elliptical in non-spitting cobras but small and round in spitting cobras, which has the same velocity-increasing effect as putting your thumb most of the way over the end of a garden hose. A sharp 90° bend at the distal end directs the jet of venom forward or slightly upward, instead of downward as in most snakes, and venom stream spins towards the exit orifice, which prevents the flow from slowing down as it goes through the sharp bend at the exit (similar strategies are used in pressure washers). These adaptations of the fang enable a cobra to spit venom in defense but do not prevent venom injection when biting, which is used both defensively and for killing prey. In fact, spitting cobras can meter the duration of their venom pulse, which is normally about five times longer during biting (1/4th of a second) than during spitting (1/20th of a second). This affects the quantity of venom ejected, which varies considerably from bite to bite and may consist of up to 100 times more venom than the fairly consistent 1.9-3.7 milligrams (~1/10th of a milliliter) of venom per spit. Most estimates suggest that a single cobra has enough venom to spit about 40-50 times consecutively. The fluid dynamics of such tiny volumes over relatively long distances are complex, and spitting cobra venom has shear-reducing properties, such as high surface tension and viscosity, which hold the droplets together as they fly through the air. Some species of spitting cobra eject their venom as a spray, whereas others eject two pressurized parallel streams. Reports of the maximum distance achievable by a spitting cobra vary from surely exaggerated distances of 12 feet or more to more believable (though still impressive) distances of five to eight feet.

Venom spray patterns of Red Spitting Cobras (Naja pallida) From Westhoff et al. 2005

Middle: Examples of head movement patterns of  Black-necked Spitting Cobras (Naja nigricollis). Black dots represent the positions of the upper and lower jaws,  red dots indicate the period of venom spitting. From Westhoff et al. 2005

Bottom: Congruence between target (back; blue) and cobra’s head (red; front plot) motion during spitting. Data are offset 180 ms to reflect the cobra's reaction time. From Westhoff et al 2010

Unlike vipers, cobras cannot move their fangs, so in order to accurately hit their targets, they move their heads instead. When a spitting cobra spits, it opens its mouth slightly and contracts the muscles around the venom glands so that a small amount of venom is forced out of the glands and down the venom canal of the fangs. At the same time, the upper lip scales and the fang sheaths are levered up out of the way and the maxilla levered down, removing soft tissue barriers between the venom glands and the fangs as well as between the exit orifices of the fangs and the air around them³. Most often, the spit is accompanied by slight movements of the head in response to change in direction of the target, which disperse the venom over an area about the size of a human face. Measurements indicate that more head rotation corresponds to a larger area covered by the venom stream, allowing cobras to adjust for target size and distance. Splattering of the venom when it hits the target and partial disintegration of the venom stream as it travels through the air increase the chance that at least some of the venom will hit the target's eye. Consequently, cobras only need to aim at the center of the face, rather than precisely at the eyes, in order to hit the eyes 90-100% of the time. They adjust for target movement by using a strategy familiar to any Space Invaders or Galaga player: firing not at where you are but at where you're going to be. Chameleons, archer fish and spitting spiders do the same kind of thing. In some species venom spitting is often accompanied by an audible hiss as the cobra exhales, but in contrast to early reports that spitting cobras propelled their venom with their breath, this is not an essential part of the spitting process. In one experiment, spitting cobras restrained in tubes did not seem to suffer from reduced spitting ability or range. How do they choose their targets? Cobras have good vision and moving human faces are the stimuli that normally elicit spitting, although in lab experiments they will also spit at masks, photos of human faces, and even plain ovals without eyes, as long as they are moving, but not at moving triangles. Adult cobras will not spit at stationary human faces or moving human hands, although newly hatched cobras will spit at nearly anything, even if it is beyond their maximum target distance, including human hands, unhatched eggs, other baby cobras, and even their own reflection. Hatchling cobras also spit more of their venom, proportionally, and rotate their heads in a more pronounced fashion; their spitting performance improves following their first shed. Like many stereotypical snake defensive behaviors, most spitting cobras apparently habituate to humans when in captivity and are disinclined to spit after a while, although some spit without hesitation and willingness to express defensive behavior is very variable from individual to individual.

Sumatran Spitting Cobra (Naja sumatrana)

Although the color and consistency of spat venom does not change noticeably with repeated spitting, the venom chemistry of at least one species, Red Spitting Cobras (Naja pallida), changed over 10 minutes of repeated spitting. The quantity of venom remained the same and the toxin concentration rose over the first 20 spits, but both decreased afterward. The first five spits contained a protein that was not found in later spits, which might be involved in venom storage. Although this protein is non-toxic, most of the other molecules in spitting cobra venom are not. African spitting cobra venom is rich in cytotoxins and PLA₂s, which cause tissue damage; spitting cobra cytotoxins lack certain acidic proteins, which frees them to damage tissues in the eyes. If even a small quantity of venom contacts the eye it causes instant, intense pain and damage to the cornea and mucous membranes. If left untreated, it can lead to blindness. Treating spitting cobra venom in your eyes involves flushing it out with water for 15-20 minutes. Anti-inflammatory eye drops are sometimes prescribed.

Rinkhals (Hemachatus haemachatus)

The 29 living species of Naja fall into four groups: a basal Asian clade of eleven species (subgenus Naja, including six accomplished spitting members, two non-spitters, and three species of intermediate spitting ability), an African spitting group of eight species (subgenus Afronaja), and two African non-spitting groups of six and four species, respectively (subgenus Uraeus, found mostly in open areas, and subgenus Boulengerina, found mostly in forests). This pattern of species relationships suggests that spitting evolved more than once! In Asia, the six spitting cobras (Naja siamensis, N. sumatrana, N. sputatrix, N. mandalayensis, N. samarensis, and N. philippinensis⁴) are probably one another's closest relatives, and their closest cousins are a group of three cobra species (Naja atra, N. kaouthia, and N. sagittifera) with somewhat modified fangs and intermediate spitting ability. They can spit their venom, but they do so rarely and with less accuracy than the "true" spitters. The remaining Asian cobras, Naja naja and Naja oxiana, do not spit their venom but nevertheless are more closely related to Asian spitting cobras than to other cobras. This means that venom spitting arose independently in the common ancestor of the seven species of African spitting cobras (N. pallida, N. nubiae, N. katiensis, N. nigricollis, N. ashei, N. mossambica, and N. nigricincta), which form a monophyletic group sometimes referred to as Afronaja. Their cousins, the other African Naja (i.e., subgenera Uraeus and Boulengerina), do not spit. Finally, a member of one of those small genera, a very interesting cobra known as the rinkhals (Hemachatus haemachatus) also spits its venom, indicating that venom spitting has evolved three times in cobras (or, alternatively, been lost twice, in Naja naja/N. oxiana and in the common ancestor of Uraeus and Boulengerina, with a third partial loss in N. atra & kin). Because the details of spitting behavior and morphology differ slightly among the three groups of spitting cobras, the former hypothesis is more likely.

The largest Giant Spitting Cobras (Naja ashei) can top 9 feet.
This species was described in 2007.
From Wüster & Broadley 2007

Why do some cobras spit their venom? Herpetologist Thomas Barbour, who published one of the first studies on spitting cobras, thought that spitting cobras evolved venom spitting for much the same reason that rattlesnakes were thought to have evolved their rattles—to alert large ungulates to their presence and avoid getting stepped on. He was speculating in the absence of any direct evidence when he wrote in 1922 that "The African veldt is the only other region in the world where snakes abound and where hoofed animals grazed in numbers comparable with those of the western American plains. Snakes probably found the heavy antelopes equally dangerous though unwitting foes and many antelopes probably suffered from snake bite. No rattle was evolved, however but some of the common veldt-ranging snakes secured protection in another way. Several common cobras and cobra-allies learned to expel their poison in a fine spray for very considerable distances, and with a fairly shrewd aim at the eye."

A rinkhals (Hemachatus haemachatus) spits its venom

Nearly 100 years after Barbour, we have just as little direct evidence—published field observations of spitting cobras interacting with their non-human predators are non-existent. The main reason we now think that the evolutionary cause of these adaptations isn't so simple is that spitting is too old. Molecular dating methods suggest that African spitting cobras evolved about 15 million years ago, whereas the spread of open grasslands and their characteristic megafauna (elephants, etc.) didn't happen until about 5 million years ago. Asian spitting cobras don't inhabit open grasslands, so this hypothesis seems unlikely to explain their evolution either. African spitting cobras are eaten by birds and other snakes, against which spitting venom would be a relatively ineffective weapon, and in captive experiments cobras do not spit at mounted bird specimens. Given what we know about face targeting, it's possible that spitting may represent a defense that is specifically adapted for use against primates [Edit: Harry Greene hinted at this idea in his recent book, Tracks and Shadows]. Barbour's comment that "...[venom spitting] must antedate man's coming, for contact between man and the snakes can hardly be conceived as sufficiently frequent to account for the modification" may be technically correct, but the evolution of spitting cobras coincides roughly with the evolution of apes in Asia and Africa, which (as we all know) are diurnal primates with forward-facing eyes, some of which are omnivorous and many of which (ourselves included) habitually kill snakes either for food or in defense. Could it be that spitting cobras evolved their venom spitting capacity to deal with threats from our own ancestors? Only further research into the co-evolution of apes and snakes can tell us. Perhaps this is why, although certain toads, salamanders, insects, and scorpions can also eject their toxin defensively, spitting cobras are by far the longest- and best-known organisms to do so. Clearly, much remains to learn about them and their fascinating habits.

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1 The cobra in this account was undoubtedly Naja mandalayensis, which was described by Joe Slowinski & Wolfgang Wüster 100 years later. Before 2000, no spitting cobras were known from Burma. Cobra specimens with fangs highly modified for spitting from northeastern India may represent a seventh species of undescribed Asian spitting cobra.^↩

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2 This number includes species of cobras formerly placed in the genera Boulengerina and Paranaja, both of which have been synonymized with Naja in the last 15 years. In part, the reason for this change is that, when scientists realized that some species of Naja were more closely related to Boulengerina and Paranaja than they were to other Naja (i.e., that Naja was paraphyletic), they were reluctant to split up the genus Naja because they didn't want to change the name of medically-important snakes and create potential confusion. However, a few sources use Afronaja and other other subgenera as full genera anyway.^↩

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3 The fang sheath is soft tissue that completely surrounds the fang at rest, including at the top, which keeps the venom from dribbling out. In other venomous snakes, physical contact with a target is required for displacement of the fang sheath and release of venom, but spitting cobras have co-opted the movements normally used for jaw-walking over a prey item (the ‘pterygoid walk’) to free their fangs for spitting in the absence of any external physical contact. This has been termed the "buccal buckle" (pronounced "buckle buckle") by the research group of Bruce Young, of Kirksville College, which has studied several aspects of the functional morphology of spitting in cobras.^↩

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4 Naja philippinensis is the only spitting cobra species with pronounced sexual dimorphism in discharge orifice size—females have longer orifices less well-adapted for spitting, whereas males have small round orifices. The evolutionary causes and consequences of this dimorphism are not understood.^↩

This post is part of a Reptile and Amphibian Blogging Network (RAmBlN) online event called #CrawliesConverge. We are writing about convergent evolution in reptiles and amphibians. Find our event schedule here, or follow on Twitter or Facebook.

ACKNOWLEDGMENTS

Thanks to Stu Porter, Dan Rosenberg, and Ray Hamilton for allowing me to use their photos.

REFERENCES

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Bogert, C.M. 1943. Dentitional phenomena in cobras and other elapids, with notes on adaptive modifications of fangs. Bulletin of the American Museum of Natural History 81:285-360 <link>

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Wüster, W. and D.G. Broadley. 2007. Get an eyeful of this: a new species of giant spitting cobra from eastern and north-eastern Africa (Squamata: Serpentes: Elapidae: Naja). Zootaxa 1532:51-68 <link>

Wüster, W., S. Crookes, I. Ineich, Y. Mané, C.E. Pook, J.F. Trape, and D.G. Broadley. 2007. The phylogeny of cobras inferred from mitochondrial DNA sequences: Evolution of venom spitting and the phylogeography of the African spitting cobras (Serpentes: Elapidae: Naja nigricollis complex). Molecular Phylogenetics and Evolution 45:437-453 <link>

Wüster, W. and R.S. Thorpe. 1992. Dentitional phenomena in cobras revisited: spitting and fang structure in the Asiatic species of Naja (Serpentes: Elapidae). Herpetologica:424-434 <link>

Young, B.A., M. Boetig, and G. Westhoff. 2009. Functional bases of the spatial dispersal of venom during cobra “spitting”. Physiological and Biochemical Zoology 82:80-89 <link>

Young, B.A., M. Boetig, and G. Westhoff. 2009. Spitting behaviour of hatchling red spitting cobras (Naja pallida). The Herpetological Journal 19:185-191 <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.