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

Wednesday, November 27, 2013

The Truth About Snakebite

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.

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.

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 oreganuson 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.

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.


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.

(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>

Creative Commons License

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

Friday, November 1, 2013

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

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.


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.


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>

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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.