The Truth About Snakebite

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Many people live in fear of snakes, especially of venomous species that can inflict a lethal bite. There is evidence that our fear of snakes is innate, because our ancestors have been preyed upon by them for millions of years, even before we were primates. Other evidence suggests a significant learned component to ophidiophobia. Either way, few people today are at risk of being eaten by snakes, but bites from venomous snakes are still fairly common. However, in my experience fear of snakes is way out of proportion to the actual risk they pose, especially among my fellow North Americans. It's surprisingly hard to find good information on the prevalence of venomous snakebite (hereafter, just 'snakebite'), but it's getting easier, and I was able to gather almost 100 papers that include data on the subject, which I've synthesized here. As a result, this article has many footnotes, and because I used so many references to prepare this article I've provided a selected list at the end of this post, with a link to the full list.

Map of snake envenomings per year, from Wikimedia Commons

So how dangerous is a snake bite? If you're bitten by the wrong kind of snake and you're far from help, it's pretty dangerous. But the truth about snakebite is that it's a lot less likely to endanger your life than people think. First of all, you're pretty unlikely to ever get bitten. Worldwide, estimates range from 1.2 million to 5.5 million snakebites annually. Remember, there are several billion people out there, so although those numbers are large, each year over 99.92% of people are not bitten by a venomous snake. These bites result in 420,000-1.8 million envenomings leading to 20,000-94,000 deaths. This probably seems really low, until you realize that unlike when they are biting their prey, snakes that are biting in defense don't inject venom every time (i.e., the bite is "dry"). Depending on the species of snake and the context of the bite, estimates for dry bites range from 8% to more than 80%, with North American rattlesnakes, one of the best studied groups, injecting venom only 20-25% [edit 10/23/2015: I made a mistake here. The source cites two other sources that say that rattlensakes inject venom 75-80% of the time (i.e., 20-25% dry bites), not the other way around as I originally wrote. But, Hayes goes on to say that neither of these sources appear to be based on empirical data, and then he gives some other sources that do. These list rattlesnake and other viper dry bite percentages between 7 and 43% (i.e., injecting venom 57-93% of the time). So, indeed, much higher than the 20-25% I originally listed, but still less than for predatory strikes. I apologize for the error.] of the time when biting in defense, compared to more than 99% of the time for predatory strikes.¹ This behavior is partly because the strike itself may startle attacker sufficiently and wasting expensive venom needed to eat is useless, and partly because even injecting venom into an attacker is unlikely to immediately incapacitate it. Most snake venom is fast-acting, but it's not that fast. As a result of these dry bites, a lot of snakebites go untreated and unreported because they fail to produce symptoms, leading the bitten person to assume (correctly) that they are safe or (incorrectly) that the snake was not venomous. This is one major cause of the wide range of numbers given above for the prevalence of snakebite.

Copperheads (Agkistrodon contortrix) bite
a few hundred people a year in my home state of
North Carolina, more than in any other state.
Fatalities are exceedingly uncommon.

Worldwide, about 1 out of every 20 people envenomated by venomous snakes dies from the bite, according to the best available estimates for the prevalence of bites and resulting deaths between 1985 and 2008. Depending on where you live, your chances of surviving a venomous snakebite are really good, although in a few places they're pretty bad. I'm going to focus on the USA because I live here and because we have some of the best data. In the USA, only 1 out of every 500 people bitten by a venomous snake dies as a result, which includes deaths from bites that take place under several special circumstances that we'll discuss later. You're actually safer from venomous snakebite in the USA than in any other country on Earth where venomous snakes kill people, thanks to our excellent medical care, relatively benign venomous snake fauna, and large proportion of the population that live in urban areas where venomous snakes are scarce. There are some countries, such as Canada² and Norway, where venomous snakebites occur but nobody has apparently been killed by one in recent history, except for people who have been killed by their exotic, captive snakes (more on this later).

Western Diamondback Rattlesnakes (Crotalus atrox)
are large and widespread in the southwestern USA.
Contrary to the popular myth, a recent study showed that
larger rattlesnakes cause more serious bites than smaller ones,
which makes sense because they have more venom to inject
(see also unpublished data from the Hayes lab at
Loma Linda University showing the same trend and also
that smaller bite victims have more serious bites).

How about all the people who are bitten and survive? Being bitten by a venomous snake isn't exactly a pleasant experience. It's been described as feeling like “hitting your thumb with a hammer”, “stepping on a bare electrical wire”, or “being repeatedly stabbed with a knife”. This alone is a good enough reason to avoid snakebite. However, not every venomous snakebite is a recipe for a nightmare. In the USA, most people are bitten by pit vipers (copperheads, cottonmouths, and rattlesnakes). Very few people are bitten by coralsnakes, and I'd be surprised if anyone has ever been bitten by a coralsnake that they didn't first pick up. Pit vipers are generally pretty retiring snakes, a fact observed most poignantly by both the herpetologist Clifford Pope, who called them first cowards, then bluffers, then warriors, and also by Ben Franklin, who wrote of a rattlesnake: "She never begins an attack, nor, when once engaged, ever surrenders...she never wounds 'till she has generously given notice, even to her enemy, and cautioned him against the danger of treading on her."

Figure from Gibbons & Dorcas (2002)

In a field test of these famous anecdotes, Whit Gibbons and Mike Dorcas molested 45 wild cottonmouths (Agkistrodon piscivorus) in South Carolina swamps and found that only 2 in 5 bit their fake hand when picked up, only 1 in 10 bit a fake foot when it stepped on them, and none bit a false leg that stood beside them. In a similar test, Xav Glaudas and colleagues picked up over 335 pigmy rattlesnakes (Sistrurus miliarius) in Florida and found that only 8% bit the thick glove they were wearing. Further evidence to support the fact that vipers are reluctant to bite potential predators comes from anecdotes from snake biologists radio-tracking snakes to study their spatial ecology, in which the biologist has accidentally stood on Timber and Eastern Diamondback Rattlesnakes and Puff Adders without provoking any responses. This makes sense because striking is a last resort for these snakes, which have a lot to lose and very little to gain by it. Although this isn't a perfect simulation of a typical snake-human interaction (these researchers weren't trying to kill the snakes in their experiments, after all), these findings are a good argument in the snakes' defense - if they bite you, they probably had a good reason.

Russell's Vipers (Daboia russelii) are probably
one of the world's most dangerous snakes,
combining a relatively aggressive demeanor
and relatively potent venom with a habitat
and geographic range that overlaps areas of
very dense, rural human population in south Asia.

Although the above news is hopeful, it is of course impossible to predict whether an individual snakebite will end in tragedy, so it is prudent to avoid snakebite at all costs. Each year in the USA, between 2,400 and 4,700 (edit: some sources say up to 8,000) bites occur, putting your chances of being bitten by a venomous snake in the USA at about 1 in 100,000 (1 in 40,000 with higher bite estimate).³ If you live in southern or southeastern Asia, you're more justified in having a fear of snakes. In India, at least 80,000 and possibly as many as 165,000 people are bitten by snakes each year (1 in 7,000-14,000). India's venomous snake fauna isn't that much more diverse than the USA's, but medical care isn't as good, and it has about 4 times as many people, many of whom live in rural areas and work in agricultural or pastoral professions, both of which really increase your chances of being bitten. Even in India, "only" about 10,000-15,000 people a year die from snakebite (edit: a more recent study that estimated snakebite mortality in India using household surveys instead of hospital records came up with a figure of ~46,000 deaths in 2005, which is probably more accurate because many victims elect to use traditional therapy in their village and most do not die in government hospitals, where the data are collected; for a more thoughtful discourse on snakebite in India, click here), meaning that about 4 out of 5 (edit: using the newer data, between 1 in 4 and 1 in 2) snakebite victims survive. Taking into account your chances of being bitten and your chances of dying from the bite, many countries in sub-Saharan Africa, Asia, and Latin America are risky places to live. Snakebite in these places is a legitimate public health concern. The USA is the least risky country in terms of snakebite. The only safer countries are places like Ireland, New Zealand, Madagascar, and oceanic islands in the Pacific & Caribbean, where no venomous snakes occur. Snakebite risk in the USA is thousands of times lower than it is in many parts of the world, and it would be even lower if people modified their behavior in a few key ways, starting with not attempting to kill every snake they see.

The USA (bottom left) is the safest country in the world in terms of snakebite risk.
Countries without any venomous snakes not shown.
Data from Kasturiratne et al. 2008
Click for larger version

You might be surprised to hear that attempting to kill venomous snakes actually increases your risk of snakebite. This masterful post written by David Steen at Living Alongside Wildlife is a good argument for why this is the case. Specifically, the reason is that up to 2/3rds of snakebites in the USA are a direct result of intentional exposure to the snake and could be avoided if the people involved had made different decisions [Edit 16 May 2018: although recently, more well-replicated studies have shown that this figure is actually closer to 20% to 30%. Even so, I think it's safe to say that trying to catch a snake for any reason increases the chances that it will try to bite you. Killing a snake from a distance, e.g. by shooting it, is of course not nearly as risky from a snakebite perspective, but there are other associated risks and plenty of good reasons not to do that.]. These bites resulted from people who were trying to kill snakes or molest them, or who chose to interact with them for some other reason (ranging from snake handling churches to collection for rattlesnake roundups). Although snakebite is an occupational hazard for some, such as zookeepers and herpetologists, the vast majority of Americans are at extremely low risk of snakebite.

Black Mambas (Dendroaspis polylepis) are among
Africa's most dangerous snakes, but they still kill fewer
people than hippos or mosquitos

Let's take a closer look at those 5 people a year who die from venomous snakebite in the USA. Not all of these people are hikers, fishermen, and gardeners who fall victim to 'legitimate' bites, as you might assume. This number includes deaths that result from a pair of special cases that deserve special attention. The first is people who keep exotic venomous snakes in captivity in their homes. Although this can be done safely, it isn't always, and it is a little unfair to group these cases in with 'legitimate' bites, envenomations, and deaths from native, wild venomous snakes. It inflates USA snakebite statistics because the risk is not evenly distributed among the entire population and it inflates death statistics because antivenom may not be available for these exotic snakes. About 1 of the 5 deaths each year in the USA can be attributed to these circumstances. The second special case, people who refuse or do not seek treatment after they are bitten, includes some of the bites that also fall under the first case, because some snake owners that keep snakes illegally may not seek treatment out of fear that they will be arrested, fined, or have their animals confiscated. This case also covers religious snake handlers proving their faith, which in many cases entails foregoing treatment. It's harder to put a finger on how many people die in the USA each year from untreated snakebites, but I think it's probably fair to say that most of those people got what was coming to them. Let's not overlook the role of alcohol in people's decisions to interact with venomous snakes: studies show that around 40% of snakebite victims have been drinking. Data on intentionality of exposure to snakes in developing countries is sparse, but I would be willing to bet that exposure in these places is much less intentional, as it once was in the USA.

CroFab antivenom used to
treat most snakebites in the USA

Today in the USA, medical treatment for snakebite is so good (thanks to synthetic antivenoms with few side-effects), and research on snake venom has come so far (with much left to learn!), that there is little justification for the overblown fear bordering on hatred people have of snakes. Progress toward this same goal is being made by some really smart people researching the venom of snakes in developing countries in Africa, south Asia, and Latin America, and figuring out better ways to make antivenom available outside of a hospital setting.

Yet more than 1 in 20 people in the USA have a pathological fear of snakes, as defined by criteria including uncontrollable, greater than justified, and significantly interferes with a person’s routine, occupational or academic functioning, or social activities or relationships. Leading to situations like this recent news story and this bizarre interaction between a man, a gun, and a snake. Risk perception is influenced by many things, including the rarity of the event, how much control people think they have, the adverseness of the outcomes, and whether the risk is voluntarily or not. For example, people in the United States underestimate the risks associated with having a handgun at home by 100-fold, and overestimate the risks of living close to a nuclear reactor by 10-fold. Ironically, evidence suggests that two of these things (how much control you have and how voluntary the risk is) are actually quite high for snakebite, despite popular perception that they are low.

Eastern Brown Snakes (Pseudonaja textilis) are one of
Australia's more dangerous snakes, but even they won't
chase, bite, or attack people without trying to escape
or bluff first. Australia's low population density
also contributes to their low prevalence of snakebite.

Data on fear of snakes in developing countries is lacking, and it is difficult to generalize, but based on the impressions of several people I know who have lived and worked there, most inhabitants of rural areas in developing countries are terrified of snakes. One notable exception is Madagascar, where no venomous snakes occur and it is fady to kill any snake (edit: although apparently superstitions still abound). In contrast, in Australia people seem to have a relatively high level of respect for snakes and don't seem to mess with them solely out of machismo the way they do in the USA. Venomous snakebites are relatively rare, which is remarkable considering that the majority of snakes in Australia are venomous. I heard a story recently about a newly-hired Australian CEO of an American mining company. When the new boss asked about the snake policy, the employees jokingly replied that it was "a No. 2 shovel". The Australian CEO was not amused, because at his previous company Down Under routinely relocated much more dangerous snakes at their job sites. He instituted a company-wide training program to teach safe venomous snake practices. These classes are also available to the general public in some areas, especially in southern Africa.

As people and wildlife come to share more and more space, snake-human interactions are inevitable. The future of conservation will probably be in maximizing compatibility between humans and wildlife rather than preserving pristine areas, we will need to get a lot better about behaving ourselves to keep ourselves safe from the defense mechanisms of wildlife, starting with educating ourselves about the real risks that underlie our fears. Everyone should read these guidelines for snakebite prevention and first aid. I would add to this: don't kill snakes! It only puts you at risk. Don't try to kill them, don't let your friends kill them, don't let your family members kill them. They won't try to kill you. I promise.

For more about snakebite research and treatment, check out Dr. Leslie Boyer's blog and Bill Hayes's snakebite research page.

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1 Venomous snakes that are striking at their prey practically always inject venom, and some evidence suggests that they can precisely meter their venom so that they inject exactly the right amount needed to kill each particular prey item, based on its mass. Fortunately for humans, there are no venomous snakes large enough to consider us prey. Dry bites to humans may result from the snake's deliberate decision to withhold venom or from kinematic constraints that reduce the duration and coordination of fang contact when striking a large, vertical object.^↩

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2 Although global snakebite statistics frequently list 0 fatalities out of 200-300 snakebites for Canada, this seems not to be quite accurate. In Ontario, at least two people have been killed by Timber Rattlesnakes (Crotalus horridus), a soldier who was bitten at the battle of Lundy's Lane near Niagara Falls in 1814, and an American Indian chief prior to 1850. Two or three people have been killed by bites from Massasaugas (Sistrurus catenatus) in Ontario, all before 1962, and between 0 and 10 people were bitten annually from 1971-2007, mostly men aged 10-29. In 1981, a man who was "quite intoxicated" was killed by a bite from a Northern Pacific Rattlesnake (Crotalus oreganus) on the Nk’meep reserve near the town of Osoyoos in British Columbia's Okanagan Valley. He was the first person to be bitten by a native venomous snake in BC in over 50 years. The only other Canadian provinces that are home to venomous snakes are the Prairie Provinces of Alberta and Saskatchewan, where no recorded deaths have occurred from Prairie Rattlesnake (Crotalus viridis) bites. So we can conclude that native snakebites in modern Canada are even more infrequent than but follow the same basic pattern as those in the USA.^↩

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3 In the US, relative to dying from heart disease (1 in 5), cancer (1 in 7), in a motor vehicle accident (1 in 80), in a fall (1 in 185), from a gunshot (1 in 300), by drowning (1 in 1100), by choking (1 in 4400), from drinking too much alcohol (1 in 10,900), by a sting from a wasp, bee, or hornet (1 in 63,000), from being struck by lightning (1 in 80,000), from a dog bite (1 in 120,000), or in an earthquake (1 in 150,000), you are very unlikely to be killed by a snake (1 in 480,000). The only less-likely causes of death are being trapped in a low-oxygen environment (1 in 548,000), being killed by ignition or melting of nightwear (1 in 767,000), and being bitten by a spider (1 in 960,000). These odds are for your entire lifetime; your annual chance of being killed by a venomous snake is more like 1 in 50 million. Worldwide, they're more like 1 in 200,000, which is a lot higher but still pretty low overall. ^↩

ACKNOWLEDGMENTS

Thanks to Julia Riley and James Baxter-Gilbert for providing me with information on deaths from snakebite in Canada, to Wes Anderson, James Van Dyke, and Xav Glaudas for sharing with me with their impressions of people's fear of snakes outside of North America, and to Matt Clancy, John Worthington-Hill, Larsa D., Todd Pierson, and Pierson Hill for the use of their photography. If you're so inclined, check out David Steen's post on why it doesn't make sense to kill venomous snakes in your yard here and Jessica Tingle's historical view of the subject here.

SELECTED REFERENCES
(click here for a longer list of references pertaining to snakebite [last updated February 2017])

Scientific illustrator Liz Nixon made this infographic

featuring facts in this post!

Click here for a larger version.

Bellman, L., B. Hoffman, N. Levick, and K. Winkel. 2008. US snakebite mortality, 1979-2005. Journal of Medical Toxicology 4:43 <link>

Gibbons, J. W. and M. E. Dorcas. 2002. Defensive behavior of Cottonmouths (Agkistrodon piscivorus) toward humans. Copeia 2002:195-198 <link>

Glaudas, X., T. M. Farrell, and P. G. May. 2005. The defensive behavior of free–ranging pygmy rattlesnakes (Sistrurus miliarius). Copeia 2005:196-200 <link>

Hayes, W. K., S. S. Herbert, G. C. Rehling, and J. F. Gennaro. 2002. Factors that influence venom expenditure in viperids and other snake species during predator and defensive contexts. Pages 207-234 in G. W. Schuett, M. Höggren, M. E. Douglas, and H. W. Greene, editors. Biology of the Vipers. Eagle Mountain Publishers, Eagle Mountain, UT <link>

Isbell, L. A. 2006. Snakes as agents of evolutionary change in primate brains. Journal of Human Evolution 51:1-35 <link>

Janes Jr, D. N., S. P. Bush, and G. R. Kolluru. 2010. Large snake size suggests increased snakebite severity in patients bitten by rattlesnakes in southern California. Wilderness and Environmental Medicine 21:120-126 <link>

Juckett, G. and J. G. Hancox. 2002. Venomous snakebites in the United States: management review and update. America Family Physician 65:1367-1375 <link>

Kasturiratne, A., A. R. Wickremasinghe, N. de Silva, N. K. Gunawardena, A. Pathmeswaran, R. Premaratna, L. Savioli, D. G. Lalloo, and H. J. de Silva. 2008. The global burden of snakebite: a literature analysis and modelling based on regional estimates of envenoming and deaths. PLoS Medicine 5:e218 <link>

Morandi, N. and J. Williams. 1997. Snakebite injuries: contributing factors and intentionality of exposure. Wilderness and Environmental Medicine 8:152-155 <link>

Parrish, H. M. 1966. Incidence of treated snakebites in the United States. Public Health Reports 81:269-276 <link>

Ruha, A.-M., K. C. Kleinschmidt, S. Greene, M. B. Spyres, J. Brent, P. Wax, A. Padilla-Jones, and S. Campleman. 2017. The epidemiology, clinical course, and management of snakebites in the North American Snakebite Registry. Journal of Medical Toxicology 13:309-320. <link>

Swaroop, S. and B. Grab. 1954. Snakebite Mortality in the World. Bulletin of the World Health Organization 10:35-76 <link>

Tierney, K. J. and M. K. Connolly. 2013. A review of the evidence for a biological basis for snake fears in humans. The Psychological Record 63:919-928 <link>

Van Le, Q., L. A. Isbell, J. Matsumoto, M. Nguyen, E. Hori, R. S. Maior, C. Tomaz, A. H. Tran, T. Ono, and H. Nishijo. 2013. Pulvinar neurons reveal neurobiological evidence of past selection for rapid detection of snakes. Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1312648110 <link>

Walker, J. P. and R. L. Morrison. 2011. Current management of copperhead snakebite. Journal of the American College of Surgeons 212:470-474 <link>

Wasko, D. K. and S. G. Bullard. 2016. An Analysis of Media-Reported Venomous Snakebites in the United States, 2011-2013. Wilderness and Environmental Medicine 27:219-226. <link>

How snakes see through closed eyes

Early American symbols depicting rattlesnakes:

(top) Rattlesnake on the $20 bill issued in 1778 by Georgia.
The Latin motto (Nemo me impune lacesset) means,
"No one will provoke me with impunity."

(middle) Benjamin Franklin's "Join or Die" cartoon,
first published in the Pennsylvania Gazette in 1754.
Franklin advocated a rattlesnake as the national symbol
by writing: "...her eye excelled in brightness...she has no
eye-lids. She may therefore be esteemed
an emblem of vigilance."

(bottom) The Gadsden flag, used during
the American Revolution

Click here to read this post in Spanish!

Haga clic aquí para leer este blog en español!

Normally when someone asks me how to tell the difference between a snake and a legless lizard, I tell them to look at the eyes. Lizards have eyelids whereas snakes do not. Whenever I say this I am lying, although really I am just oversimplifying for the sake of clarity. Most lizards have obvious movable eyelids and so can blink like we do. Snakes, by contrast, seem not to have eyelids. They are ever-staring, ever-vigilant. Ben Franklin esteemed the rattlesnake as a symbol of vigilance because its eyes were always open.

Snakes' eyes are closed all the time. Rather than having movable eyelids, snakes have a single, fused, clear layer of skin over their eye, called a spectacle or brille (German for "glasses"), which protects the eye. A snake's skin is covered in scales, and the outer part of the spectacle is indeed a scale. The deeper layers of the spectacle are formed, during development, from the same embryonic tissue that in other animals forms the eyelid. The spectacle is not attached to the snake's eye in any way, so the eye can move freely behind it, although its movement is limited. This limited movement is because snakes are probably descended from fossorial lizard ancestors that lived underground and had degenerate eyes, much like today's amphisbaenians, although fossil evidence for this hypothesis is scant (as are snake fossils in general).

Eye of an Eastern Ribbonsnake (Thamnophis sauritus)
during the phase prior to shedding when fluid
has built up between the old and new spectacles

Unlike other animals' eyelids, snakes' spectacles are transparent, like a window in their skin, allowing them to see out through their always-closed eyelids. Just before a snake sheds its skin, a layer of fluid builds up between the new inner skin and the old outer layer, clouding the spectacle and causing the other scales to have a faded, milky appearance. This period usually lasts a few days, during which snakes have difficulty seeing and usually will not eat. People who keep snakes as pets have observed that they may become particularly ornery during this period, perhaps as a result of not being able to see clearly.

The horizontal, key-shaped pupil
of Ahaetulla prasina

Another obstacle to snake vision that has been long known but little studied is that snakes' spectacles are vascularized, meaning that they have blood vessels running through them. It is very unusual for tetrapods to have blood vessels in a place that might interrupt their field of vision. First noticed in 1852, these vessels are small but symmetrically distributed across the optically transmissive region of the eye in most species, although the arrangement is radial in basal snakes, acrochordids, and vipers but vertical in colubrids and elapids. In one visually-oriented species, the Asian vine snake (Ahaetulla nasuta), these blood vessels are less dense in the region of the field of vision known as the fovea, where the maximum sharpness is achieved. Most snakes don't have foveas, suggesting that the unusual arrangement of blood vessels in the eyes of Ahaetulla is an adaptation to minimize visual disturbance in this region of highest visual acuity.

Until recently, no one had considered whether movement of blood into and out of the spectacle blood vessels might aid snakes in being able to see. In an article published this week in The Journal of Experimental Biology, Kevin van Doorn and Jacob Sivak of the University of Waterloo in Ontario presented the first evidence that snakes are able to do this. When van Doorn was investigating the mechanisms snakes' eyes use to focus, he noticed the blood vessels in the spectacle, which led him to look more closely at their function. He found that coachwhips, another highly visual species, were able to reduce blood flow to the spectacle in the presence of a potential threat. At rest and undisturbed, newly oxygenated blood flowed into the spectacle blood vessels of the coachwhips for about a minute at a time, interspersed with approximately two minute periods during which no flow took place. When an experimenter walked into the room to perform some routine tasks, spectacle blood flow was almost completely cut off. What little flow there was during this period occurred in short spurts of around 30 seconds each, about half the length of the flow period in undisturbed coachwhips. When the experimenter left the room, the pattern of blood flow in the snakes' eyes returned to normal almost immediately.

Figure from van Doorn & Sivak 2013 showing blood vessels in the spectacle of a Coachwhip (Masticophis flagellum). (A) Image taken during the renewal phase of the integument when the spectacle becomes cloudy. The vessels are most apparent in the region that overlays the iris–pupil boundary because of their higher contrast with the background in this region. (B) The spectacle under retro-illumination, showing the vessels in the illuminated anterior portion of the pupil on the right side. The vessels are dorso-ventrally arranged as is typical for colubrid snakes. Debris and scratches are visible on the spectacle scale (particularly the left side), attesting to its protective role.

Shed skin of a Cornsnake (Pantherophis guttatus)
showing the shed spectacles

Furthermore, van Doorn & Sivak found that when snakes were handled they cut off blood flow to the spectacle completely, probably as part of a sympathetic nervous response. In contrast, blood flow to the eye was continuous and uninterrupted, even during handing, in shedding snakes. You can see a video of blood flow in the spectacle of a shedding corn snake here. Although no experimental evidence has been gathered that filled blood vessels in the spectacle reduce a snake's ability to see, it seems likely given that the blood vessels themselves are quite difficult to see when they are not filled with blood. Snakes actually have remarkably good color vision, better than that of rats and on par with the visual acuity of a cat. Because they move their eyes so little compared to humans, they might be less likely to notice the interruption to their visual field by these blood vessels.

Geckos and some other lizards also have spectacles. A few other species of tetrapods have blood vessels in their optical path, including manatees, armadillos, and some blind salamanders, none of which are renowned for their visual prowess. Little is known about the images these vessels might project onto the vision of these animals, but because they are part of the cornea and so move about with the eye rather than remain stationary relative to it, their area of occlusion would appear to remain stationary to the animal. This is not true for animals with nictitating membranes (diving animals such as penguins or crocodilians) or those with spectacles, both of which have the potential to interrupt the animal's vision. We don't know yet how crocodilians and geckos deal with this issue, but as with so many other features of their lives, snakes have evolved an ingenious and potentially unique solution to a vexing problem, allowing them to remain vigilant as well as keep their eyes protected. Snakes have guarded the Golden Fleece in the Greek tale of the hero Jason and his band of Argonauts, a treasure chamber beneath an ancient city in Rudyard Kipling’s The Jungle Book, and various other treasures in Hindu, Inca, and Basque mythology, all with their eyes closed.

ACKNOWLEDGMENTS

Thanks to Hans Breuer and Kwok Wai for their photographs of Ahaetulla prasina.

Correction: I originally said that the fovea work was done on Ahaetulla prasina, but it was actually Ahaetulla nasuta. Both species have horizontal pupils so it is likely that the reduction in blood vessels is found in both.

REFERENCES

Baker, RA, TJ Gawne, MS Loop, and S Pullman (2007) Visual acuity of the midland banded water snake estimated from evoked telencephalic potentials. J. Comp. Physiol. A 193, 865-870 <link>

Crump, M. Expected 2015. Eye of Newt and Toe of Frog, Adder's Fork and Lizard's Leg. University of Chicago Press, Chicago, Illinois.

Foureaux, G, MI Egami, C Jared, MM Antoniazzi, RC Gutierre, and RL Smith. (2010) Rudimentary eyes of squamate fossorial reptiles (Amphisbaenia and Serpentes). Anat. Rec. (Hoboken) 293, 351-357 <link>

Franklin, B (1775) The rattlesnake as a symbol of America. Pennsylvania Journal. <link>

Lüdicke M, 1969. Die kapillarnetze der brille, der iris, des glaskörpers und der chorioidea des auges vom baumschnüffler Ahaetulla nasuta Lacepede 1789 (Serpentes, Colubridae). Zoomorphology 64:373-390.

Mead, AW (1976) Vascularity in the reptilian spectacle. Invest. Opthalmol. Vis. Sci.15, 587-591 <link>

Neher, EM (1935) The origin of the brille in Crotalus confluentus lutosus (Great Basin rattlesnake). Trans. Am. Ophthalmol. Soc. 33, 533-545 <link>

Quekett, J. (1852). Observations on the vascularity of the capsule of the crystalline lens, especially that of certain reptilia. Trans. Microsc. Soc. Lond. 3, 9-13. doi:10.1111/j.1365-2818.1852.tb06020.x

van Doorn, K. and Sivak, J. G. (2013). Blood flow dynamics in the snake spectacle. J. Exp. Biol. 216, 4190-4195 <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.

50,000 Hits & Snakes from Florida

Click here to read this post in Spanish
Haga clic aquí para leer este blog en español

Brown Anole (Anolis sagrei)
The purpose of my trip

This week I am in northeast Florida collecting lizards for my PhD research (don't tell anybody who still thinks I only work on snakes). This is a special place for me because it is where I started writing this blog a year and a half ago. Since that time Life is Short but Snakes are Long has received thousands of visitors: almost 100,000 if you go by the stats included with Blogger, but probably closer to 48,000 using stats from the more conservative Google Analytics, which doesn't count bots and other non-human visitors. The true number is probably somewhere in the middle. With many thanks to Alvaro Pemartin and Estefania Carrillo, all posts are available in English and Spanish (the links to the Spanish versions are at the top of each post), and I am working on converting the format of the Spanish pages from PDF to HTML to more closely resemble the English pages. Readers from the USA make up the majority of visitors, but the UK, Canada, Australia, and India are also well-represented, and readers from 177 countries or territories have visited. 

Map of visits to this blog

I am proud to have been able to disseminate knowledge about snakes to so many people. The first post on snake sheds is still the most popular, garnering between 44 and 100 hits a day and appearing in the top 10 hits for Google searches for 'snake shed' and 'snake sheds'. Its popularity prompted me to write another article that was less storytelling and more detail about the processes used in snake shed identification. As proof positive that it works, last week I received images of a snake shed from Jean in Lawrence, Kansas, who wrote:

The first photo

I happened across your blog while searching for a way to identify a snake species by it's shedded skin.

We found this [snake shed] in our barn near Lawrence Kansas. I had this extreme fear of snakes so I became proficient in identifying them, if I see them. We have only seen 3 types of venomous snakes in our area, the timber rattler, the western massasauga, and the osage copperhead. Unfortunately, I find that I am truly inept at identifying them by their skins.

We have seen more poisonous snakes this year than usual and we found this skin inside our barn. It very easily could have been trapped inside as we close it up every other evening. We primarily use the barn for storage and workshop. Hopefully, we have allowed plenty of opportunity for the snake to escape.

I mainly want to know if you can help me to identify whether this is a poisonous snake. After reading your blog I am concerned is that it is possibly a copperhead and that it could be hiding. There are numerous places for a creature to stay hidden in our 70 ft barn and I fear that I will open a bin or cabinet and find it, dead or alive.

We love our wildlife and try to be protective and careful, but it seems we have failed at this lately as we recently had to scare an endangered skink out of the barn.

I would appreciate your assistance in possibly identifying this snake. I don't think we have the tail end of the skin. We do have the fairly intact head portion of the skin and can send more pics if needed.

Your blog is very informative and I learned a great deal from it. I thank you in advance for your assistance.

Although the first photo wasn't detailed enough, she was able to find the tail and I was able to help her identify it as a harmless ratsnake, after which she wrote:

The second photo,
showing divided subcaudals

Thank you so much! I checked your blog to take a double look at your pics there and was still unsure, so thank you so much! We did see a few rat snakes earlier this year so my guess is you are spot on!

It is still scary that we didn't see it! We live in a rural area very near to public hunting and fishing but don't have a lot of traffic. It makes my blood boil at the number of snakes we see dead on the SIDES of the roads!

Please keep up the great work! Yours was the first site when I googled snake skin id and by far the most informative i found! I learned so much by reading your blog and I really feel that people need more education about snakes!

Identifying snake sheds has been a new challenge for me. I probably wouldn't have gotten so much practice at it if I hadn't started this blog. I am working on a lot of new content, but I particularly want to develop content that people will find useful and interesting. With that in mind, here are a couple upcoming articles that I've planned:

• Basics of snakebite
• Venomous bites from "non-venomous" snakes
• Common urban snakes
• Snake predators
• Invasive snakes
• Some personal stories about how I became interested in herpetology
• Several taxon-specific posts

I'm open to suggestions about how to prioritize these and I'm especially open to ideas from readers about new posts that aren't on this list. Some of the best ones I've written so far are ones people have suggested to me. I'm also open to hosting guest posts if there are any interested guest authors out there. Feel free to leave a comment or to contact me by email.

Cornsnake from the island

I also wanted to share a couple of stories from this week. Yesterday we found a young Cornsnake on one of our islands when one of us chased a lizard into the tree hollow where it was hiding. That snake had eaten one of the Brown Anoles in our study, a large male that we marked back in 2011. Young cornsnakes are particularly fond of lizards and ambush them from hiding spots under bark and within decaying trees. My former student David Delaney, now in the Warner lab at UAB, will be conducting research on the effects of cornsnake predation on anole sleeping site selection. I thought this would be the coolest find of the whole trip, but today some folks alerted us to the presence of an Eastern Diamondback Rattlesnake on a public beach right near where we were collecting mainland lizards for David & Dan's lab experiments. This snake was in the surf, which was really foamy due to the wind. The lady who found it said she almost stepped on it. Usually when someone tells you they saw a rattlesnake nearby it's either not a rattlesnake or not there or both, but this time it was for real! I have read about EDBs entering the ocean occasionally, but apparently it is fairly rare. My friend Kerry Nelson, who worked as a naturalist on Little St. Simons Island in Georgia for almost two years and saw diamondbacks in the sand dunes daily, said to me that he never saw one in the surf.

Me with Diamondback

ACKNOWLEDGMENTS

Thanks to Hans Hillewaert, Dan Warner, and Jean Ostrander for their photos and to Jean Ostrander for allowing me to reprint her email.

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

Basics of Snake Fangs

Click here to read this post in Spanish!
Haga clic aquí para leer este blog en español!

Solenoglyphous fangs of a Gaboon Viper

Snake fangs are specialized, elegantly modified teeth. Some are like hypodermic needles, others are more like water slides. But all serve essentially the same purpose: to inject venom into the snake's prey. Occasionally, the fangs are also used in defense, but studies show that snakes striking in defense are far less likely to inject venom than when they're striking at a prey item, a fact that has assuaged the fears of many an ophidiophobic. I wanted to write a brief review of snake fang types, because their anatomy is very interesting and also because of their important role in classifying snakes and understanding how different groups of snakes are related to each other.

Cross-sections of fangs:
F is an aglyphous tooth.
G is an opisthoglyphous fang.
H is a proteroglyphous fang.
I is a hollow solenoglyphous fang.
From Bauchot (2006)

Many snakes produce venom, which is essentially very strong saliva, in glands in their heads (which is where you produce your saliva, too). We call these glands venom glands if they are well-developed, complete with an interior cavity, a duct connecting to a hollow fang, and compressor muscles that generate high pressures when the jaws are rapidly closed. If they lack these features, we usually call them Duvernoy's glands instead. Because there is a lot of variation among snake species in the structure of these glands and their associated teeth, there is some debate about whether or not venom glands and Duvernoy's glands are really two forms of the same thing. Either way, three groups of snakes (atractaspidines, elapids, and viperids) have independently evolved an advanced apparatus to deliver large quantities of venom during a brief strike, and many other snakes (and a few lizards) have evolved less sophisticated, but still relatively effective, modifications to their teeth in order to deliver venom after they have grabbed their prey and are "chewing" on it. The teeth of modern snakes are classically divided into four types, three of which are typically called fangs. The four tooth types have fancy names, all of which involve the Greek word glyph, one of the meanings of which is "groove". They are as follows:

Solenoglyphous

Folding of solenoglyphous fangs.
Fang is in red, maxilla green,
prefrontal orange, pterygoid yellow,
ectopterygoid purple. Vipers lack
premaxillary and palatine teeth.
From Bauchot (2006)

This most sophisticated fang type evolved once, in the ancestor to all modern vipers, which lived in Asia about 40 million years ago. Fossils suggest that solenoglyphous fangs have changed little since that time, even though vipers have undergone an incredibly successful radiation into 320 extant species found on all continents except for Australia and Antarctica. Solenoglyphous fangs are long and tubular and are attached to the snake's maxillary bone. Most snakes have several tooth-bearing bones, including four (the premaxilla, maxilla, pterygoid, and palatine) in the upper jaw, and one (the dentary) in the lower. In humans, three of these bones (the premaxilla, maxilla, and dentary) also bear teeth - your premaxilla holds your top incisors, while your maxilla holds your upper canines and molars and your dentary all your lower teeth - while the others form part of the roof of the mouth. In vipers, the maxilla bears only a single tooth (the fang) and is hinged so that the fangs can be folded back parallel to the jaws when the mouth is closed, or erected perpendicular to the jaws, the position when striking. The teeth in the pterygoids and dentaries work together to manipulate food once it gets into the mouth. Solenoglyphous fangs are strikingly similar to hypodermic needles. They have a hollow core that receives venom from the venom gland at the entrance orifice near the base and injects it from a slit-like exit orifice on the front of the fang near the tip. If the opening were at the very tip of the fang, its strength would be compromised and it would lack the sharp point needed to penetrate the target. Even under normal use, vipers shed their fangs every two months.

Modified solenoglyphous fang of
African Burrowing Asp (Atractaspis engaddensis)

A similar fang type evolved a second time about 29 million years ago in a group of African snakes, currently placed in the family Lamprophiidae, subfamily Atractaspidinae. Two genera, Atractaspis (mole vipers, burrowing asps, or stiletto snakes) and Homoroselaps (African dwarf garter or harlequin snakes), possess elongate anterior fangs, although only those of the stiletto snakes are movable. Stiletto snake fangs pivot on a socket-like joint that is more flexible than those of vipers, allowing these snakes to strike beside and behind them with their mouth closed. This is an adaptation to living underground and envenomating small mammals and other reptiles in narrow subterranean burrows. The fang morphology of atractaspidines and viperids is remarkably similar, considering that these two snake lineages last shared a common ancestor over 40 million years ago.

Proteroglyphous

Proteroglyphous fangs of an Eastern Green Mamba
(Dendroaspis angusticeps). Don't try this.
From Bauchot (2006)

This fang type also evolved only once, in the ancestor to all modern elapids, which lived 25-40 mya in Asia or Africa. Proteroglyphous fangs are in the front of the mouth and are about three times shorter than solenoglyphous fangs. This is because they are not hinged. Snakes with proteroglyphous fangs typically strike their prey and hang on until the venom has taken effect, as opposed to releasing they prey and then tracking it down. Some elapids constrict their prey at the same time as envenomating it. Over 350 species of elapids exist today, including well-known groups such as cobras, mambas, death adders, taipans, coralsnakes, and sea snakes, and less-well-known species, mainly found in Australia, of which a good number are small, secretive, and not considered dangerous to humans.

Maxilla of a proteroglyphous snake showing the almost
completely closed groove along the anterior edge connecting
the two orifices, as well as the aglyphous tooth at the
rear of the maxilla. This line may be obscured in longer fangs.
From Shea et al. 1993

Unlike solenoglyphs, some proteroglyphs have other teeth on the maxilla behind the fang. However, the fang is always separated from the other teeth by a gap, called a diastema. Some elapids have more than one functional fang on each side. In both vipers and elapids, there are usually at least two fangs on each maxilla at any one time, one that is in use and one that is a reserve fang. Both fangs are draped in a layer of connective tissue and skin called the fang sheath. Some proteroglyphs have partially movable fangs, including many of the most dangerous species such as mambas, taipans, and death adders. A few, such as spitting cobras, have modified exit orifices to their fangs that are smaller and rounder than in other cobras, a modification that increases the velocity with which venom is ejected. Modifications to the muscles and the fang sheath also facilitate spitting in these cobras. A few elapids, such as sea snakes that eat only fish eggs, have lost their fangs and their venom glands, which suggests that the primary role of venom, at least among elapids, is in feeding rather than in defense.

Opisthoglyphous

Opisthoglyphous fang of Eastern Hog-nosed Snake

These are commonly known as "rear-fanged" snakes. Opisthoglyphous fangs are grooved rather than hollow and are found near the back of the maxilla, behind the normal teeth. Typically, snakes with rear fangs must chew on their prey to bring their fangs into a biting position. There is considerable variation in the size, shape, and number of opisthoglyphous fangs from species to species, and sometimes even within a species. Most opisthoglyphous fangs are connected to Duvernoy's glands, which differ from true venom glands in several important ways, most notably in that they lack associated muscles to generate the pressure needed to evacuate venom, as in solenoglyphous and proteroglyphous snakes. The pressure on the venom glands of biting solenoglyphs and proteroglyphs can exceed 30 psi, the pressure of a car tire, whereas the pressure inside the Duvernoy's glands of opisthoglyphs is generally less than 5 psi. Because Duvernoy's glands also lack a chamber for storing venom, the idea is emerging that opisthoglyphous snakes probably secrete their venom only during chewing, which explains why prolonged bites by opisthoglyphs generally have more severe effects.

Opisthoglyphous fangs of Boomslang (Dispholidus typus)
Don't do this either.

Most of these snakes are not harmful to humans, with a few notable exceptions. Boomslangs and Twigsnakes are arboreal, diurnal African colubrines that prey on lizards and birds. They have short heads, rear fangs situated comparatively close to the front of the mouth, and partially muscled Duvernoy's glands. They also have potent venoms and their bites have killed several people, including two prominent snake biologists, Karl Schmidt and Robert Mertens. Bites from other rear-fanged snakes are known to cause relatively mild, transient, and local symptoms, but clinical documentation of these bites and their effects is scattered, incomplete, and frequently anecdotal. Many are written by the victim themselves! The above notwithstanding, bites from opisthoglyphs are generally less medically important than those from proteroglyphs and solenoglyphs. As a result, snake venom research has not focused on them, so there is still much that we do not know about this group of snakes, some of which are becoming increasingly common in the pet trade. Based on what little we do know, the composition of opisthoglyph venom/Duvernoy's secretion is fairly similar to that of viperids, elapids and atractaspidines, which makes sense given that each of these groups is more closely related to certain opisthoglyphs than they are to one another.

A: python, B: viper, C: rear-fanged colubroid, D: cobra
The f  marks the portion of the maxilla where the fang develops.
E shows the elongation of the posterior part of the
maxilla pushing forward the developing fang of a
night adder (d.a.o. = days after oviposition)
From Vonk et al. 2008

Unlike the first two groups, opisthoglyphous fangs appear to have evolved more than once, in snakes as diverse as Quill-snouted Snakes, Neck-banded Snakes, and Boomslangs. At least, that's what we used to think. Actually, it is likely that both solenoglyphous and proteroglyphous fangs evolved from opisthoglyphous fangs, as revealed by an ingenious study that used evidence from embryology and genetics to reveal the evolutionary origins of the three types of snake fangs. In a snake embryo, tubular fangs are formed by the infolding of ridges on the front and back sides of the fang, such as those that form the groove of opisthoglyphous fangs. Furthermore, front fangs develop from the rear part of the upper jaw, and are strikingly similar in their formation to rear fangs. They are pushed into the front of the mouth by disproportionate growth of the initially small part of the maxilla that is behind them. Finally, in the anterior part of the maxilla of front-fanged snakes, expression of a gene called sonic hedgehog, which is responsible among other things for the formation of teeth, is suppressed.

Relative size of the venom gland (VG) in
A: rear-fanged colubrid (Helicops leopardinus),
B: boomslang, C: homalopsid,
D: cornsnake, E: African egg-eater
SG = supralabial salivary gland
From Fry et al. 2008

Although developmental similarity is not conclusive proof of structural homology (similarity due to inheritance rather than due to other factors), these findings are consistent with the hypothesis that solenoglyphous, proteroglyphous, and at least some opisthoglyphous fangs are homologous structures. The hypothetical evolutionary trajectory was thus: some snakes evolved grooved fangs in the rear of their mouth. In a few cases (viperids, elapids, and atractaspidines), they subsequently lost the preceding teeth as what was formerly a rear fang became a tubular front fang. Other snakes retained their anterior teeth (at least some non-front-fanged colubroids), and still others developed fangs but then lost them (aglyphs such as ratsnakes). Evidence for this surprising final part comes from the formation of the maxilla and its teeth, which takes place in a single piece in pythons, but from two pieces in all fanged snakes as well as in ratsnakes, a pattern which supports a single evolutionary origin and subsequent loss of fangs. Additionally, vestigial Duvernoy's glands have been found in ratsnakes, egg-eaters, pareatid slug-eaters, and other nonvenomous aglyphs, a discovery that has led to the misleading generalization that all snakes are venomous and much subsequent misunderstanding among the non-scientific community. Toxic saliva does not a venomous animal make, as evidenced by the fact that even human saliva injected subcutaneously will produce pain and swelling.

Aglyphous

Both boas and pythons have only
aglyphous teeth, which is about
the only thing this film got right.

This word is used to describe unmodified teeth, essentially non-fangs. All snakes, even those that possess fangs of the first three types, have aglyphous teeth which they use for gripping their prey as they manipulate it during swallowing. As I just mentioned, many advanced snakes that today have only aglyphous teeth probably evolved from fanged ancestors. Several of these snakes, such as North American kingsnakes, ratsnakes, and bullsnakes, have atrophied Duvernoy's glands that lack toxin-producing serous cells. These snakes employ other sophisticated techniques, such as constriction, which is also used by more primitive snakes like boas and pythons (which did not evolve from fanged ancestors).

There are very few dangerous species of aglyphs, but one, Rhabdophis tigrinus, is becoming well-known as one of the only snakes capable of sequestering toxins from its prey for use in its own defense. This species has enlarged posterior maxillary teeth that lack grooves, so they are by definition aglyphous. However, it has relatively potent venom and has caused the deaths of several people. Among colubroids, the distinction between opisthoglyphs and aglyphs has never been entirely clear, but I'm distinguishing between them here because they are two of the four traditionally recognized types of snake teeth. Although the four types of snake teeth in this article are commonly discussed, a more accurate classification for snake teeth might be to divide them into tubular (the fangs of viperids, elapids, and atractaspidines), grooved (the rear fangs of non-front-fanged colubroids), and ungrooved (all other snake teeth).

Aglyphous (ungrooved) teeth and rear fangs of
Rhabdophis tigrinus
From Mittleman & Goris 1974

Happily for snake biologists like myself, the evolution of fangs opened the door for a massive evolutionary radiation of advanced snakes (>2800 species, or >80% of all living snake species). Although sophisticated venom delivery systems, of which fangs are just one of many integral parts, were clearly evolutionary advantageous, they have obviously also been costly at times, leading to their loss in ratsnakes, egg-eaters, and other lineages of advanced snakes. Also worth noting is that many lineages of basal snakes have got along just fine without venom, so there is not an inherent superiority of being venomous as the word "advanced" seems to imply. Rather, some have suggested that during periods of transition from forest to grassland, such as that which took place simultaneous to the dramatic colubroid radiation during the Miocene, snake taxa that were characterized by slow locomotion and constriction (boas & pythons) were supplanted by those characterized by rapid locomotion (many aglyphous colubrids) or passive immobilization (tubular- and grooved-fanged vipers, elapids, and atractaspidines that could use venom to catch their prey). Of course, both slow locomotion and constriction have subsequently been re-evolved among the colubroids, but there has been a lot of time since the Miocene.

ACKNOWLEDGMENTS

Thanks to Daniel Rosenberg (boomslang fang) and Nick Kiriazis (hognose fang) for use of their photographs.

REFERENCES

Bauchot R, editor. 2006. Snakes: A Natural History. New York, New York: Sterling Publishers. <link>

Cundall, D., (2002) Envenomation strategies, head form, and feeding ecology in vipers. In: Biology of the Vipers: 149-162. G. W. Schuett, M. Höggren, M. E. Douglas & H. W. Greene (Eds.). Eagle Mountain Publishers, Eagle Mountain, UT <link>

Greene, H. W. (1997) Snakes: The Evolution of Mystery in Nature. Berkeley: University of California Press <link>

Fry BG, Scheib H, van der Weerd L, Young B, McNaughtan J, Ramjan SR, Vidal N, Poelmann RE, Norman JA, 2008. Evolution of an arsenal: structural and functional diversification of the venom system in the advanced snakes (Caenophidia). Mol Cell Proteomics 7:215-246 <link>

Hayes, W. K., S. S. Herbert, G. C. Rehling & J. F. Gennaro, (2002) Factors that influence venom expenditure in viperids and other snake species during predator and defensive contexts. In: Biology of the Vipers: 207-234. G. W. Schuett, M. Höggren, M. E. Douglas & H. W. Greene (Eds.). Eagle Mountain Publishers, Eagle Mountain, UT <link>

Jackson K, 2002. How tubular venom‐conducting fangs are formed. J Morphol 252:291-297 <link>

Kardong, K. V. & T. L. Smith, (2002) Proximate factors involved in rattlesnake predatory behavior: a review. In: Biology of the Vipers: 253-266. G. W. Schuett, M. Höggren, M. E. Douglas & H. W. Greene (Eds.). Eagle Mountain Publishers, Eagle Mountain, UT <link>

Kardong KV, 1996. Snake toxins and venoms: an evolutionary perspective. Herpetologica 52:36-46 <link>

Kuch, U., J. Müller, C. Mödden & D. Mebs (2006). Snake fangs from the Lower Miocene of Germany: evolutionary stability of perfect weapons. Naturwissenschaften 93, 84-87

LaDuc, T. J., (2002) Does a quick offense equal a quick defense? Kinematic comparisons of predatory and defensive strikes in the Western Diamond-backed Rattlesnake (Crotalus atrox). In: Biology of the Vipers: 267-278. G. W. Schuett, M. Höggren, M. E. Douglas & H. W. Greene (Eds.). Eagle Mountain Publishers, Eagle Mountain, UT <link>

Mittleman M, Goris R, 1974. Envenomation from the bite of the Japanese colubrid snake Rhabdophis tigrinus (Boie). Herpetologica 30:113-119 <link>

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

Savitzky AH, 1980. The role of venom delivery strategies in snake evolution. Evolution 34:1194-1204 <link>

Shea G, Shine R, Covacevich JC, 1993. Elapidae. In: Glasby C, Ross G, Beesley P, editors. Fauna of Australia. Canberra: AGPS <link>

Vonk FJ, Admiraal JF, Jackson K, Reshef R, de Bakker MA, Vanderschoot K, van den Berge I, van Atten M, Burgerhout E, Beck A, 2008. Evolutionary origin and development of snake fangs. Nature 454:630-633 <link>

Weinstein SA, Warrell DA, White J, Keyler DE, 2011. "Venomous" Bites from Non-Venomous Snakes: A Critical Analysis of Risk and Management of "Colubrid" Snake Bites. Amsterdam: Elsevier <link>

Weinstein SA, White J, Keyler DE, Warrell DA, 2013. Non-front-fanged colubroid snakes: A current evidence-based analysis of medical significance. Toxicon. 69, 103-113 <link>

Weinstein S, White J, Westerström A, Warrell DA, 2013. Anecdote vs. substantiated fact: the problem of unverified reports in the toxinological and herpetological literature describing non-front-fanged colubroid (“colubrid”) snakebites. Herpetological Review 44:23-29.

Wüster, W., L. Peppin, C. Pook & D. Walker (2008). A nesting of vipers: Phylogeny and historical biogeography of the Viperidae (Squamata: Serpentes). Mol. Phylogenet. Evol. 49, 445-459 <link>

Young BA, Dunlap K, Koenig K, Singer M, 2004. The buccal buckle: the functional morphology of venom spitting in cobras. J Exp Biol 207:3483-3494 <link>

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