Tuesday, April 21, 2015

Spitting cobras

This post will soon be available in Spanish!

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

A clever comic from birdandmoon
highlighting the fact that king cobras
are not true cobras
Cobras are some of the most iconic snakes in the world, instantly recognizable by their hoods even to those who have never seen one. They are also among the most dangerous snakes—fast-moving, with potent neurotoxic venom, cobra bites cause injury or death to many people in Asia and Africa. Cobras are elapids, together with coralsnakes, mambas, kraits, seasnakes, and numerous terrestrial Australian snakes both well-known and obscure. What unites these ~350 species of snakes is their short, immovable, and hollow ("proteroglyphous") fangs. Elapids probably evolved in Asia between 25 and 30 million years ago. By 16 million years ago, cobras were found in Europe, where they no longer live, and in Asia and Africa, where they are still found today. The core cobra clade consists of three small genera (Hemachatus, Aspidelaps, and Walterinnesia) and one large one, Naja. Other hooded snakes that are usually called "cobras" include tree cobras (genus Pseudohaje), whose placement remains uncertain, and the king cobra (Ophiophagus hannah), which is probably more closely related to mambas than it is to true cobras. Ironically, most people, if asked for a species of cobra, would almost certainly come up with the king first. But, probably they would think of a spitting cobra second, and with good reason from an evolutionary perspective, as we shall see.

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

Fangs of  cobras progressively adapted for spitting.
Dotted lines show the venom canal, dark arrows indicate
the flow of water injected into the top of the fang.
Left: "normal" non-spitting cobra fang (Naja kaouthia)
Right: spitting cobra fang (Naja pallida)
The sutures are visible above the exit orifices.

From Young et al. 2004
All snake fangs are modified teeth provisioned with grooves that vary in depth and degree of closure. In vipers and elapids, the grooves are completely closed, forming hollow tubes, along the front edge of which a narrow suture can still be seen where the ridges forming the tube have come together in the developing embryo. In spitting cobras, the inside of this tube contains ridges, which act like rifling in a gun barrel to impart spin on the venom. The discharge orifice, located near but not at the point of the tooth (like a hypodermic needle), is large and elliptical in non-spitting cobras but small and round in spitting cobras, which has the same velocity-increasing effect as putting your thumb most of the way over the end of a garden hose. A sharp 90° bend at the distal end directs the jet of venom forward or slightly upward, instead of downward as in most snakes, and venom stream spins towards the exit orifice, which prevents the flow from slowing down as it goes through the sharp bend at the exit (similar strategies are used in pressure washers). These adaptations of the fang enable a cobra to spit venom in defense but do not prevent venom injection when biting, which is used both defensively and for killing prey. In fact, spitting cobras can meter the duration of their venom pulse, which is normally about five times longer during biting (1/4th of a second) than during spitting (1/20th of a second). This affects the quantity of venom ejected, which varies considerably from bite to bite and may consist of up to 100 times more venom than the fairly consistent 1.9-3.7 milligrams (~1/10th of a milliliter) of venom per spit. Most estimates suggest that a single cobra has enough venom to spit about 40-50 times consecutively. The fluid dynamics of such tiny volumes over relatively long distances are complex, and spitting cobra venom has shear-reducing properties, such as high surface tension and viscosity, which hold the droplets together as they fly through the air. Some species of spitting cobra eject their venom as a spray, whereas others eject two pressurized parallel streams. Reports of the maximum distance achievable by a spitting cobra vary from surely exaggerated distances of 12 feet or more to more believable (though still impressive) distances of five to eight feet.

Venom spray patterns of Red Spitting Cobras (Naja pallida)
From Westhoff et al. 2005
Middle: Examples of head movement patterns of  Black-necked
Spitting Cobras (Naja nigricollis). Black dots represent the
positions of the upper and lower jaws,  red dots indicate the
period of venom spitting.
From Westhoff et al. 2005
Bottom: Congruence between target (back; blue)
and cobra’s head (red; front plot) motion during spitting.
Data are offset 180 ms to reflect the cobra's reaction time.
From Westhoff et al 2010
Unlike vipers, cobras cannot move their fangs, so in order to accurately hit their targets, they move their heads instead. When a spitting cobra spits, it opens its mouth slightly and contracts the muscles around the venom glands so that a small amount of venom is forced out of the glands and down the venom canal of the fangs. At the same time, the upper lip scales and the fang sheaths are levered up out of the way and the maxilla levered down, removing soft tissue barriers between the venom glands and the fangs as well as between the exit orifices of the fangs and the air around them3. Most often, the spit is accompanied by slight movements of the head in response to change in direction of the target, which disperse the venom over an area about the size of a human face. Measurements indicate that more head rotation corresponds to a larger area covered by the venom stream, allowing cobras to adjust for target size and distance. Splattering of the venom when it hits the target and partial disintegration of the venom stream as it travels through the air increase the chance that at least some of the venom will hit the target's eye. Consequently, cobras only need to aim at the center of the face, rather than precisely at the eyes, in order to hit the eyes 90-100% of the time. They adjust for target movement by using a strategy familiar to any Space Invaders or Galaga player: firing not at where you are but at where you're going to beChameleonsarcher fish and spitting spiders do the same kind of thing. In some species venom spitting is often accompanied by an audible hiss as the cobra exhales, but in contrast to early reports that spitting cobras propelled their venom with their breath, this is not an essential part of the spitting process. In one experiment, spitting cobras restrained in tubes did not seem to suffer from reduced spitting ability or range. How do they choose their targets? Cobras have good vision and moving human faces are the stimuli that normally elicit spitting, although in lab experiments they will also spit at masks, photos of human faces, and even plain ovals without eyes, as long as they are moving, but not at moving triangles. Adult cobras will not spit at stationary human faces or moving human hands, although newly hatched cobras will spit at nearly anything, even if it is beyond their maximum target distance, including human hands, unhatched eggs, other baby cobras, and even their own reflection. Hatchling cobras also spit more of their venom, proportionally, and rotate their heads in a more pronounced fashion; their spitting performance improves following their first shed. Like many stereotypical snake defensive behaviors, most spitting cobras apparently habituate to humans when in captivity and are disinclined to spit after a while, although some spit without hesitation and willingness to express defensive behavior is very variable from individual to individual.

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

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

The largest Giant Spitting Cobras (Naja ashei) can top 9 feet.
This species was described in 2007.
From Wüster & Broadley 2007
Why do some cobras spit their venom? Herpetologist Thomas Barbour, who published one of the first studies on spitting cobras, thought that spitting cobras evolved venom spitting for much the same reason that rattlesnakes were thought to have evolved their rattles—to alert large ungulates to their presence and avoid getting stepped on. He was speculating in the absence of any direct evidence when he wrote in 1922 that "The African veldt is the only other region in the world where snakes abound and where hoofed animals grazed in numbers comparable with those of the western American plains. Snakes probably found the heavy antelopes equally dangerous though unwitting foes and many antelopes probably suffered from snake bite. No rattle was evolved, however but some of the common veldt-ranging snakes secured protection in another way. Several common cobras and cobra-allies learned to expel their poison in a fine spray for very considerable distances, and with a fairly shrewd aim at the eye."

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

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

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

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

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

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


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


Barbour, T. 1922. Rattlesnakes and spitting snakes. Copeia 105:36-38 <link>

Berthé, R., S. de Pury, H. Bleckmann, and G. Westhoff. 2009. Spitting cobras adjust their venom distribution to target distance. Journal of Comparative Physiology A 195:753–757 <link>

Berthé, R.A., G. Westhoff, and H. Bleckmann. 2013. Potential targets aimed at by spitting cobras when deterring predators from attacking. Journal of Comparative Physiology A 199:335-340 <link>

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

Cascardi, J., B.A. Young, H.D. Husic, and J. Sherma. 1999. Protein variation in the venom spat by the red spitting cobra, Naja pallida (Reptilia: Serpentes). Toxicon 37:1271-1279 <link>

Chu ER, Weinstein SA, White J, Warrell DA (2010) Venom ophthalmia caused by venoms of spitting elapid and other snakes: Report of ten cases with review of epidemiology, clinical features, pathophysiology and management. Toxicon 56:259-272 <link>

Goring Jones, M.D. 1900. Can a cobra eject its poison? Journal of the Bombay Natural History Society 8:376 <link>

Hayes, W., S. Herbert, J. Harrison, and K. Wiley. 2008. Spitting versus biting: differential venom gland contraction regulates venom expenditure in the Black-Necked Spitting Cobra, Naja nigricollis nigricollis. Journal of Herpetology 42:453-460 <link>

Keogh, J.S. 1998. Molecular phylogeny of elapid snakes and a consideration of their biogeographic history. Biological Journal of the Linnean Society 63:177-203 <link>

Petras, D., L. Sanz, Á. Segura, M. Herrera, M. Villalta, D. Solano, M. Vargas, G. León, D.A. Warrell, and R.D.G. Theakston. 2011. Snake venomics of African spitting cobras: toxin composition and assessment of congeneric cross-reactivity of the pan-African EchiTAb-Plus-ICP antivenom by antivenomics and neutralization approaches. Journal of Proteome Research 10:1266-1280 <link>

Rasmussen, S., B. Young, and H. Krimm. 1995. On the ‘spitting’ behaviour in cobras (Serpentes: Elapidae). Journal of Zoology 237:27-35 <link>

Slowinski, J.B. and W. Wüster. 2000. A new cobra (Elapidae: Naja) from Myanmar (Burma). Herpetologica 2000:257-270 <link>

Szyndlar, Z. and J.C. Rage. 1990. West Palearctic cobras of the genus Naja (Serpentes: Elapidae): interrelationships among extinct and extant species. Amphibia-Reptilia 11:385–400 <link>

Triep, M., D. Hess, H. Chaves, C. Brücker, A. Balmert, G. Westhoff, and H. Bleckmann. 2013. 3D Flow in the Venom Channel of a Spitting Cobra: Do the Ridges in the Fangs Act as Fluid Guide Vanes? PLoS ONE 8:e61548 <link>

Wallach, V., W. Wüster, and D.G. Broadley. 2009. In praise of subgenera: taxonomic status of cobras of the genus Naja Laurenti (Serpentes: Elapidae). Zootaxa 2236:26-36 <link>

Westhoff, G., K. Tzschätzsch, and H. Bleckmann. 2005. The spitting behavior of two species of spitting cobras. Journal of Comparative Physiology A 191:873-881 <link>

Westhoff, G., M. Boetig, H. Bleckmann, and B.A. Young. 2010. Target tracking during venom ‘spitting’by cobras. Journal of Experimental Biology 213:1797-1802 <link>

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

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

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

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

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

Young, B.A., K. Dunlap, K. Koenig, and M. Singer. 2004. The buccal buckle: the functional morphology of venom spitting in cobras. Journal of Experimental Biology 207:3483-3494 <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.

Monday, March 16, 2015

Rattlesnake Roundups

This post will soon become available in Spanish

Eastern Diamondback Rattlesnake (Crotalus adamanteus),
the world's largest species of rattlesnake (maximum 8'3")
Rattlesnakes are one of North America's most iconic symbols. I think of them as herpetological Bald Eagles, only more diverse. Our continent boasts all 41 species, from huge diamondbacks to tiny pygmies, all of which diversified from a common ancestor 10-20 million years ago. Both American Indians and America's founding fathers viewed rattlesnakes as symbols of independence and strength, and new research is revealing that they are among the most social and behaviorally complex of snakes, caring for their young and displaying signs of spatial awareness and self-identity. Large species may take as long as ten years to become sexually mature, reproduce only once every three years in the northern part of their range, and live up to 30 years. Although many people fear rattlesnakes, comparatively speaking you are more likely to be bitten by a dog, struck by lightning, killed by office supplies, by your pajamas, or by just about anything other than a venomous snake in the USA. However, numerous communities in Alabama, Georgia, Kansas, New Mexico, Oklahoma, and Texas [Edit: Melissa Amarello of Advocates for Snake Preservation tells me that the Kansas & New Mexico roundups have been discontinued since 2007 for economic reasons.] carry out annual rattlesnake roundups, events with the purpose of exterminating wild rattlesnakes from the landscape.

Western Diamondback Rattlesnake (Crotalus atrox),
the species targeted by most roundups these days
Wholesale slaughter of rattlesnakes and other venomous snakes is nothing new. Already in 1750 the Swedish naturalist Pehr Kalm observed that "Formerly there were large numbers of these snakes in New Sweden as well as in other parts of North America now occupied by Europeans; however, they have nearly been exterminated." The first recorded bounties were paid to rattlesnake hunters in the 1680s in Masssachusetts. At first, communities and informal groups organized roundups in an attempt to improve public safety—although whether rounding up and killing rattlesnakes actually accomplishes this goal is debatable. Financial gain was not the purpose of early roundups, because the rattlesnakes themselves were considered worthless. In the 1950s, civic organizations such as fire departments, Jaycees, Kiwanis, and the Lions Club took on the role of organizing roundups, which became larger and began to gain more commercial potential; people would pay to attend and would support vendors by buying rattlesnake products. Modern roundup organizers are primarily motivated by raising money for their local community or for charity, and rattlesnake roundups are now more similar in nature to other public events, such as county fairs or rodeos. Most include other events, including occasional educational programs and/or daredevil shows, as well as music, dances, beauty pageants, and carnival rides (some to the point where the rattlesnakes are more of a sideshow, such as the case of the roundup in Freer, Texas). The population of the small communities where these events occur can increase tenfold during roundups, and millions of dollars can enter the local economy, only a small percentage of which come directly from the sale of the rattlesnakes. As a result of geography, competition among one another for visitors, and declining demand and prices for dead rattlesnakes, 36 of the 47 rattlesnake roundups in Texas closed their doors between 1991 and 2006. Texas state laws have also increased the requirements for hunting rattlesnakes, requiring a costly non-game permit and prohibiting collecting snake on roads, and high gas prices have made the costs of hunting snakes over wide areas prohibitive, as many herpers know.

Western Rattlesnake (Crotalus oreganus), the species
by which most Americans are bitten—about 1,500 a year1
In the past, rattlesnakes gathered for roundups were shot, stomped, buried, or otherwise wasted. Now, at the Texas roundups that remain, all parts of the rattlesnake are used: the venom is ostensibly sold for antivenom production and medical research2, the meat cooked and eaten, often right there at the roundup, the rattles, heads, and skins made into curios and souveniers, the gall bladders are sold to a growing Asian-American market, and the remaining guts are used for fish bait. It's likely that the incentive to amass live, healthy rattlesnakes of commercial value has reduced the amount of cruel and inhumane treatment that the snakes suffer, although snakes subjected to the exploitative and sensational daredevil contests or otherwise manhandled for the amusement of the public are certainly not treated ethically, and I doubt such behavior would be tolerated if its target were any other kind of animal.

Dead snakes, mostly homalopsids, for sale at a market in
Indonesia. One cylindrophiid is visible in the upper right.
Photo by Nurcholis Anhari Lubis, National Geographic.
At a broader scale, the economic incentives associated with rattlesnake roundups might also provide incentives for communities to "manage" their local rattlesnake populations and prevent their extinction. Is it possible that rattlesnakes might one day be regulated as a game species and managed, as we manage deer, turkey, quail, and so many other species? Probably not, unfortunately—it is extremely difficult to know how many snakes are in an area, because mark-recapture techniques used for other wildlife are hampered by the low detection probably of individual snakes. As a result, state DWRs aren't very likely to try to manage snakes as game species, even though western diamondbacks in Texas effectively are one, because are traded and have a market value, at roundups and also outside of them. New techniques for monitoring snakes and programs to enhance management efforts for non-game wildlife, such as State Wildlife Action Plans, could help bring about this change. It's an approach that has worked for crocodilians, which are harvested for their meat and skins, and it might be needed to help regulate the billion-dollar global snake trade for food, skins, and pets, particularly in light of emerging markets in southeast Asia. Even some wildlife biologists are reluctant to view venomous snakes as wildlife rather than as pests, and as a result the responsible management of venomous snakes is lacking. For instance, in Georgia there are essentially no restrictions on the harvest of non-threatened "poisonous" snakes, whereas non-venomous snakes and most other non-game wildlife are protected. It might be beneficial if we started managing more herps as game rather than non-game, if only because more people would care if they disappeared. If state wildlife agencies mandated that rattlesnake hunters mark and release a certain portion of their catch, and those hunters hunted the same areas every year and at the same time of year (which already happens), and the same effort were put forth in control areas where no snakes were removed, then a real monitoring program could be built. A modeling exercise showed that a minimum size limit could protect most females, improve hunter profits, and has the potential to result in a sustainable harvest, particularly in the southern part of Texas where western diamondbacks and their populations likely grow rapidly.

Timber Rattlesnake (Crotalus horridus), the species whose
former range overlaps with the most densely-populated areas
of the USA. Even so, most people will never see one.
Evidence from roundup reports suggests that rattlesnake roundups in Alabama & Georgia are indeed negatively affecting populations of eastern diamondbacks, whereas limited evidence suggests that those in Texas and New Mexico [Edit: The New Mexico roundup is now defunct.] might not be affecting western diamondback populations quite so much—the average number of western diamondbacks brought to the Sweetwater roundup (about 2,900; range 800-9,700) did not decrease between 1959 and 20063. It's likely that Timber Rattlesnake roundups in Pennsylvania were once quite harmful, considering the extent of habitat development throughout the range of this species and its reliance on a limited number of communal dens, but a Pennsylvania state law has prohibited the killing of native venomous snakes since the 1970s4. Certainly different species of rattlesnakes respond differently to harvest; some are more fecund than others, and differences in lifespan, age at maturity, and biological interactions also play a role. A survey showed that many roundup organizers and rattlesnake hunters believe that roundups do not harm rattlesnake populations, but they also paradoxically think that removing rattlesnakes from land does protect humans, pets, and livestock from rattlesnake bites. In reality, the ecological effects of removing predators are as unknown and controversial as ever. Ecological research has shown that predator control does not always accomplish what people think it does. The ecological effects of pumping gasoline fumes into rattlesnake burrows and dens in order to evacuate the residents (which is how the majority of rattlesnakes brought to roundups are collected) are also unclear, although it's hard to imagine that they aren't negative. As for the claim that rattlesnake roundups prevent snakebite, there is little to no data to support or refute this claim, but I find it very hard to conclude that this is true. Snakebite in the USA is already so exceedingly rare that any reduction in its incidence would be almost impossible to detect, and fine-scale data to assess the rate of snakebite in the areas hunted for rattlesnake roundups do not exist. Bill Ransberger, a rattlesnake handler from Sweetwater, says he has been bitten 42 times by rattlesnakes since 1958, a number that represents about one-twentieth of one percent of all the rattlesnake bites in the USA during that time period. There really is no way to evaluate the number of snakebites caused or prevented by rattlesnake roundups.

Active since 1971, in 2012 the Evans County Wildlife Club
decided to discontinue their annual rounding-up of wild
rattlesnakes and now hosts the Claxton Rattlesnake Festival,
which features live captive rattlesnakes which are provided by the
Georgia DNR and displayed but not killed. I took this photo
along Interstate 16 in Georgia in 2009.
All told, habitat destruction and fragmentation are probably worse for rattlesnakes than roundups, although actual estimates of the effects of either on rattlesnake populations are scarce and fraught with uncertainty. The destruction of rattlesnakes at roundups or by other means has probably never benefited livestock or grazing lands or human safety or "the balance of nature". The educational messages at roundups, if they exist, are mostly ones of "bad environmental science and senseless risk-taking". However, it's hard to deny that the roundups, particularly Sweetwater, have become symbols of community identity, publicity extravaganzas, and boons to struggling local economies. Today, between 17 and 25 roundups exist in towns in seven states [Edit: four states: Texas (10), Oklahoma (5), Georgia (1), and Alabama (1); five states if you count the 8 catch-and-release events in Pennsylvania]. Whether these events transform into more positive, respectful events, or wither and die, probably has more bearing on the future of the communities that host them than on the future of rattlesnakes. But, in keeping with the theme that wildlife-human interactions ought to be more respectful than they are, foresightful roundup organizers might want to imitate those in Georgia and Pennsylvania by beginning to shift the focus of their events towards conserving and learning more about native wildlife, perhaps by focusing on finding rattlesnakes in order to contribute data about them to citizen science programs. It's time we start treating rattlesnakes with the poise and dignity with which they treat us.

If you'd like to encourage the remaining rattlesnake roundups to reform, sign this petition, join Rise Against Rattlesnake Roundups, and attend one of these events: 
If you're aware of other reformed rattlesnake roundups or events that portray venomous snakes in a positive way, please let me know in the comments!

1 It's tough to estimate this number because not all snakebites are reported and the species is not reported or may be incorrectly identified in all reported snakebites. To get 1,500, I used data from southern California suggesting that 80-90% of snakebites in that region are from C. oreganus, and extrapolated to the figures reported in the most recent review that ~4,700 human exposures to native venomous snakes occur each year, about half of which are to rattlesnakes. I assumed that half of the 48% of bites from unidentified venomous snakes were also from rattlesnakes. Although the actual figure might be anywhere from 1,000 to 2,000, I'm fairly confident that C. oreganus is the species of rattlesnake by which most Americans are bitten every year, because it's among the most common and widespread. Probably slightly more people are bitten by Copperheads (Agkistrodon contortrix) each year.

2 Herpetologists and physicians claim that venom collected at roundups is unsuitable for use in the manufacture of antivenin, because it is not sterile. Both venom dealers and antivenom producers are quite guarded about the sources that they use, so it is difficult to evaluate this claim or that made by the organizers of rattlesnake roundups that the venom that they collect is put to some useful purpose.

Data from Adams & Thomas 2008 (p.69)

3 Interviews conducted by the same authors found that claims that area hunted has increased or that roundups are importing snakes from far away to sustain themselves are apparently unfounded (except, see the Pennsylvania comment below). At least, snake hunters at Sweetwater and other Texas roundups reported hunting the same dens year after year, and the lower prices paid per pound of snake (see graph) suggest that importing snakes or hunting them over a wider range is not a viable economic strategy. In 1991, 83 of 111 Texas counties within the range of the western diamondback were hunted for roundups, with much of the effort clumped around the communities holding the roundups and at dens adjacent to roads, because the equipment used for pumping gasoline fumes into dens is heavy. It's likely that much less of this land is hunted today, given the number of roundups that have shut down, new TX state laws prohibiting the collection of any snakes from roads, the increased price of gas, the decreasing price of rattlesnake meat & skins, and liability concerns of landowners.

4 It seems that most Pennsylvania roundups have converted to catch-and-release events as per Pennsylvania state law, while a minority import (and kill, and eat) a limited number western diamondbacks from the southwest each year. The state legislature is reluctant to ban the events completely, as they are mainstays of firehouse fund-raisers in almost a dozen rural communities, but they have instituted bag and size limits and a two-day season, restricted collection to male snakes, and mandated that all snakes be marked and released where they were captured (although enforcement is understandably quite challenging). [Edit: Melissa Amarello helped me confirm the truth of this.]


Thanks to Dave Irving, Rich, Augustus Rentfro, and Nurcholis Anhari Lubis for the use of their photographs.


Adams, C.E. and J.K. Thomas. 2008. Texas Rattlesnake Roundups. Texas A&M University Press, College Station, Texas <link>

Adams, C.E., J.K. Thomas, K.J. Strnadel, and S.L. Jester. 1994. Texas rattlesnake roundups: Implications of unregulated commercial use of wildlife. Wildlife Society Bulletin 22:324-330 <link

Campbell, J.A., D.R. Formanowicz Jr, and E.D. Brodie Jr. 1989. Potential impact of rattlesnake roundups on natural populations. Texas Journal of Science 41:301-317. 

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

Clark, R.W., W.S. Brown, R. Stechert, and H.W. Greene. 2012. Cryptic sociality in rattlesnakes (Crotalus horridus) detected by kinship analysis. Biology Letters 8:523-525 <link

Douglas, M.E., M.R. Douglas, G.W. Schuett, and L.W. Porras. 2006. Evolution of rattlesnakes (Viperidae; Crotalus) in the warm deserts of western North America shaped by Neogene vicariance and Quaternary climate change. Molecular Ecology 15:3353-3374 <link

Fitch, H.S. 1998. The Sharon Springs Roundup and prairie rattlesnake demography. Transactions of the Kansas Academy of Science 101:101-113 <link

Fitzgerald, L.A. and C.W. Painter. 2000. Rattlesnake commercialization: Long-term trends, issues, and implications for conservation. Wildlife Society Bulletin 28:235-253 <link/full-text

Larsen, E.L. 1957. Pehr Kalm's Account of the North American Rattlesnake and the Medicines Used in the Treatment of its Sting. American Midland Naturalist 57:502-511 <link

Means, D.B. 2009. Effects of rattlesnake roundups on the Eastern Diamondback Rattlesnake (Crotalus adamanteus). Herpetological Conservation and Biology 4:132-141 <link

Mushinsky, H.R. and A.H. Savitzky. Position of The American Society of Ichthyologists and Herpetologists Concerning Rattlesnake Conservation and Roundups <link

Reber, D.L. and A.S. Reber. 1994. Kansas Herpetological Society position paper regarding rattlesnake roundups <link

Seifert, S.A., L.V. Boyer, B.E. Benson, and J.J. Rogers. 2009. AAPCC database characterization of native U.S. venomous snake exposures, 2001-2005. Clinical Toxicology 47:327-335 <link

Speake, D.W. and R.H. Mount. 1973. Some possible ecological effects of "rattlesnake roundups" in the southeastern coastal plain. Pp. 267-277 27th Annual Conference of the Southeastern Association of Game and Fish Commissioners <link

Thomas, J.K. and C.E. Adams. 1993. The social organization of rattlesnake roundups in rural communities. Sociological Spectrum 13:433-449 <link

Weir, J. 1992. The Sweetwater Rattlesnake Round‐Up: A Case Study in Environmental Ethics. Conservation Biology 6:116-127 <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.

Saturday, February 28, 2015

Anilius: The Pipesnake that Wasn't

Anilius scytale, the only living member of the family Aniliidae,
one of only two snake families containing just a single species
Deep in the Amazon rain forest there lives a fairly small, fairly obscure, red and black snake called Anilius scytale. It is banded, like many red and black snakes, but it has no venom, so it may be a coralsnake mimic. It spends most of its time under ground or in the water. Morphologically, it has a mixture of characteristics that place it somewhere in the no-man's-land we call "henophidia"—it has pelvic vestiges like many boas and pythons, but it has a small gape and is not capable of eating large bulky prey. It mostly feeds on elongate vertebrates, including other snakes, amphisbaenians, caecilians, and eels, and like other snake-eating snakes individuals can eat prey approaching their own total length. Its ventral scales are only barely wider than its dorsal scales, and it has just a few enlarged head scales, including one large hexagonal scale covering the eye and the surrounding skin. Males are smaller than females, which are viviparous, capable of giving birth to as many as 24 live young at a time. In 1946, the great naturalist William Beebe wrote "This is a strange snake", meaning that it's not quite like any other snakes. It is alone in its family, Aniliidae.

Head of Anilius showing the large scale covering both
the eye and the surrounding skin, like blindsnakes but
unlike most heno- and caenophidians
Snake biologists have used the term "pipesnake" to refer to any of three different lineages of snakes: the cylindrophiids (10 species of "Asian pipesnakes"), anomochilids (3 species of "dwarf pipesnakes"), and aniliids (1 species of "red pipesnake"; i.e., Anilius scytale). I'd like to propose that we begin to think of Anilius as "the pipesnake that wasn't", because (as I alluded to last month), it is now thought to be most closely related to tropidophiids (aka "the boas that weren't), superficially boa-like snakes found mostly in the Caribbean. Molecular data and some morphological data, especially that of the soft anatomy of the lungs and reproductive system, suggests that these two groups are each others' closest relatives, and they are now placed together in the Amerophidia (aka Anilioidea), the basal-most lineage of alethinophidia, which was apparently isolated in South America during the split-up of west Gondwana. Details of the skull anatomy cast some doubt on this classification, suggesting a closer relationship between aniliids and other non-macrostomatan pipesnakes, although even if this is true there are undoubtedly deep splits between Anilius and any other living snakes. Like the tuatara and the coelacanth, Anilius has not had close living relatives for tens of millions of years. Only it knows if it's lonely out there on such a long branch of the snake family tree.

Top: The plate of Anilius and a caiman as it appeared in
the 1719 printing of Merian's Metamorphosis
Insectorum Surinamensium

Bottom: A later version of the plate,
recolored and with the eggs removed
If Anilius is lonely, it can take some solace from having been noticed and beautifully illustrated by one of the first ecologists, Maria Sibylla Merian. Merian was a remarkable artist and scientist who lived from 1647 to 1717. She was one of the first trained artists to conduct detailed, long-term studies of living organisms, and the first published female naturalist. Most of her drawings, which she sketched from life on vellum and later engraved herself on copper plates, depict the life cycles of insects and their plant hosts, which she raised in captivity. She was the first to document that caterpillars turned into butterflies, and she described the life cycles of hundreds of insects, amassing evidence that contradicted the then-widespread notion that insects were "born of mud" by spontaneous generation (although others were credited with this discovery for a long time because her work was largely ignored, because it was written in Dutch rather than Latin). In 1699, Merian and her fifteen year-old daughter traveled to Surinam, where they spent the next two years studying and drawing the indigenous animals and plants, including several snakes. Her most famous work, Metamorphosis Insectorum Surinamensium, contains plates of many of these snakes, including one of an Anilius eating the egg of a caiman and being simultaneously attacked by the adult crocodilian. Like most of her drawings, it shows aspects of the natural history and ecology of the organisms in it, and helped establish a style of scientific illustration that later inspired naturalists from Catesby to Audubon. She depicted most of her insects life-sized, from various angles, in all stages of their life cycles, and most importantly, interacting with their host plants and predators. Her observations of animal behavior and plant-animal interactions are so detailed that many consider her the first ecologist. Considering that she died when Linnaeus was only 10 years old, it is all the more remarkable that her writings and drawings emphasize where organisms live and what they do rather than how they should be classified. Her works became very popular among Europe's upper class, and Czar Peter the Great in particular purchased many of her original watercolors and recruited her daughter as an art advisor and teacher at the newly-founded Academy of Arts in St. Petersburg. The Argentine Black and White Tegu, Tupinambis merianae, is named after her. Merian's text has not been translated into English, but I have taken a stab at translating her paragraph about snakes here:

Like crocodiles, some snakes hatch from eggs. They lay many small ones. The head and the tail of this snake, the Amphisbona, are the same shape and size, but you can tell which is the head because it has a mouth and small eyes, whereas the tail does not. Of all snakes, this one is the cleanest in color, being black, red, and yellow; others are grayish white, yellow, and brown with bodies that are more flattened.


Thanks to Patrick Campbell and Andrew Snyder for allowing me to use their images.


Anilius from d'Orbigny's 1849 Dictionnaire
Universel d'Histoire Naturelle
Beebe, W. 1946. Field notes on the snakes of Karatabo, British Guiana, and Caripito, Venezuela. Zoologica 31:11-52.

Duellman, W.E. 1978. The biology of an equatorial herpetofauna in Amazonian Ecuador. Miscellaneous Publications, Museum of Natural History, University of Kansas 65:1-352 <link>

Etheridge, K. 2011. Maria Sibylla Merian: The First Ecologist. in V. Molinari and D. Andreolle, editors. Women and Science: Figures and Representations – 17th century to present. Cambridge Scholars Publishing, Newcastle upon Tyne <link>

Marques, O. A. V. and I. Sazima. 2008. Winding to and fro: constriction in the snake Anilius scytale. Herpetological Bulletin 103:29-31 <link>

Martins, M. and E. M. Oliveira. 1998. Natural history of snakes in forests of the Manaus region, Central Amazonia, Brazil. Herpetological Natural History 6:78-150 <link>

Maschio, G. F., A. L. da Costa Prudente, A. C. de Lima, and D. T. Feitosa. 2007. Reproductive biology of Anilius scytale (Linnaeus, 1758) (Serpentes, Aniliidae) from eastern Amazonia, Brazil. South American Journal of Herpetology 2:179-183 <link>

Maschio, G. F., A. L. C. Prudente, F. S. Rodrigues, and M. S. Hoogmoed. 2010. Food habits of Anilius scytale (Serpentes: Aniliidae) in the Brazilian Amazonia. Zoologia (Curitiba, Impresso) 27:184-190 <link>

Merian, M.S. 1719. Metamorphosis Insectorum Surinamensium. Joannes Oosterwyk, Amsterdam <link>

Pieters, F. F. J. M. and D. Winthagen. 1999. Maria Sibylla Merian, naturalist and artist (1647-1717): a commemoration on the occasion of the 350th anniversary of her birth Archives of Natural History 26:1-18 <link>

Sawaya, R. J. 2010. The defensive tail display of Anilius scytale (Serpentes: Aniliidae). Herpetology Notes 3:249-250 <link>

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

Wednesday, January 28, 2015

Dwarf Boas

Ambergris Cay Dwarf Boa (Tropidophis g. greenwayi)
Now that the USA and Cuba are finally warming up to one another after a chilly fifty years, we might be poised to learn a lot more about a really interesting group of snakes that reach their highest diversity on Cuba. These are the tropidophiids, or "dwarf boas". Their name is a little misleading—like the splitjaw snakes, they were once thought to be related to the true boas, and the name sticks even now that we now know better. At least the dwarf part is accurate: most tropidophiids are only 1–2 feet long. But this unassuming group of drab, nocturnal, live-bearing snakes holds more surprises and lessons about snake evolution that one would expect at first glance, with no shortage of interesting natural history to boot.

Top: Tropidophis melanurus constricts an anole
From Torres et al. 2014
Bottom: Madagascar Ground Boa
(Acrantophis madagascarensis)
constricts an oplurid lizard
Tropidophiids eat mostly frogs and lizards, and they constrict their prey in the same way as true boas: by winding the anterior part of their body neatly around their prey like a rope around a windlass, usually with an initial twist in the first loop, so that the snake's belly faces its head. This behavior, along with their relatively large gape size, seemed to suggest that they were related to the true boas (family Boidaesensu stricto), including well-known tree boas, boa constrictors, and anacondas. All true boas are neotropical and there are quite a few in the West Indies, so unlike many of the other boid "hangers-on" (such as the Malagasy Sanzinia & Acrantophis, African Calabaria, North American rosy and rubber boas, Pacific Candoia, and Old World sand boas), a close relationship between tropidophiids and boids was easy to accept in terms of the biogeography of the living species. A comparative analysis of constriction behavior in extant alethinophidian snakes done by Harry Greene and Gordon Burghardt showed that this pattern of constriction is shared by essentially all "henophidian" snakes, including booids, pythonoids, and some uropeltoids, notwithstanding a few fossorial species that have apparently secondarily lost constriction behavior alltogether, because it doesn't work in tight spaces.1

Top: Panamanian Dwarf Boa (Ungaliophis panamensis),
a member of the group to which tropidophiids were
once thought to be most closely related.
Bottom: Red Pipesnakes (Anilius scytale)
don't resemble tropidophiids very closely,
but we now think that they are each others'
closest living relatives.
In particular, what we now call tropidophiids were thought to be particularly closely related to two other small genera of neotropical boids, Exiliboa and Ungaliophis, which they superficially resemble both morphologically2 and ecologially. These still share their common name of "dwarf boa", but about 15 years ago a new picture began to emerge. While DNA from Exiliboa and Ungaliophis suggested that they were indeed related to true boas, evidence from both mitochondrial and nuclear DNA and immunological proteins of Tropidophis and Trachyboa, along with details of their muscular, circulatory, and reproductive anatomy, suggested that they were most closely related to the monotypic family Aniliidae, which contains a single South American species known as the Red Pipesnake (Anilius scytale). As far as we know, Anilius doesn't normally constrict its prey3, because it mostly forages underground on elongate vertebrates such as eels, caecilians, amphisbaenians, and other snakes, similar to various Asian pipesnakes to which it was once thought to be closely related. But, we are now fairly certain that these Asian pipesnakes are convergent with Anilius, that tropidophiids and aniliids are each others' closest relatives, and that the similarity between the gape size and constriction behavior of tropidophiids and that of boas and pythons probably still represents the shared retention of a paired morphology/action pattern used by their common ancestor, it's just a common ancestor that is much older than we originally thought. Estimates suggest that tropidophiids and aniliids diverged from one another 60-110 mya in South America4, after their common ancestors were isolated from those of all other modern alethinophidian snakes, which radiated in Africa following the mid-Cretaceous split-up of west Gondwana 70-120 mya. This was the split that formed South America and Africa, and we are now getting used to diving the alethinophidians into two major lineages, Amerophidia (tropidophiids and aniliids) and Afrophidia (everybody else), instead of into a monophyletic "crown-group" Macrostomata containing boas, pythons, and caenophidians, and a basal group of non-macrostomatan pipesnakes more similar in ecology to scolecophidians. My snake taxonomy article from 2013 is actually out-of-date with respect to this major shift in snake taxonomy, because at the time it was still unclear to me (and there are still some strong arguments from paleontologists that the molecular data may be misleading).

The Greater Antilles, Bahamas, and Turks & Caicos
The "new" family Tropidophiidae consists of two species of "eyelash dwarf boas" in the mainland genus Trachyboa (there we go with the boa thing again), and the diverse genus Tropidophis, which contains 32 species in total: 5 from mainland South America, and a West Indian radiation consisting of 17 Cuban species (one of which is shared with Jamaica and one with both Jamaica and Hispaniola), 1 on Hispaniola (shared with Cuba), 5 on Jamaica (two shared with Cuba), 2 in the Bahamas, one from the Turks & Caicos Islands, one each on the three Cayman Islands (Grand Cayman, Little Cayman, and Cayman Brac), and one endemic to Navassa Island, a small, uninhabited, disputed island in the Caribbean Sea between Cuba, Jamaica, and Hispaniola (which is known from four specimens and has not been seen in over 100 years). The West Indian species, particularly the Cuban ones, represent a radiation which rivals and parallels that of Darwin's finches. Morphological and molecular data suggest that the 17 species on Cuba are descended from a single colonization event, and that the island species appear to be more distantly related to the mainland ones than they are to Trachyboa, although four-fifths of the species of Tropidophis have no published sequence data yet so both of those conclusions could change.

Tropidophis xanthogaster bleeding from the mouth,
with blood behind the spectacle making the eyes appear red.
From Torres et al. 2013
As early explorers and biologists collected these snakes from bromeliads, within stone walls, and underneath rocks, they noted that species of Tropidophis made no effort to escape their collection, but rather coiled up into tight balls when captured. Another peculiar defensive behavior was soon noted—autohemorrhage of the nose and mouth. In other words, these snakes spontaneously bleed from these orifices and smear the blood all over themselves when handled. Creepily, the space between their spectacle and their eyes fills with blood momentarily beforehand, so that their eyes appear to flash red. Blood collected from their mouths doesn't clot for over half an hour, whereas blood collected simultaneously from their tails has clotted after 10 minutes, and the mouth blood is more acidic and has fewer red blood cells, presumably because it is mixed with saliva. However, it is not harmful to frogs or lizards, so it is not a substitute for venom. The exact function is unclear, but it appears to be to freak out would-be predators. Like many snakes, Tropidophis habituates to captivity and eventually does not exhibit this behavior.

Tropidophis melanurus, the largest species of Tropidophis
and the first described, from Cocteau & Bibron's 1843
volume on reptiles
in de la Sagra's Histoire physique, politique,
et naturelle de l’Ile de Cuba
Just when you thought things couldn't get any more interesting, brace yourself, because most Tropidophis can change color! They are light silver-white at night, when they are active, and dark grayish-brown during the day, when they are not. It takes a Tropidophis 1-2 hours to go from completely light to completely dark, which they accomplish via mobilization of melanosomes (organelles containing the light-absorbing pigment melanin) from the core of a melanophore cell deep within their skin into finger-like extensions of the melanophore that are closer to the surface of the skin, partially blocking stationary xanthophores and iridiophores, which contain yellow, blue, or green pigments. Both adults and juveniles undergo diel color change, and it does not seem to be affected by age, sex, pregnancy, or feeding, although prior to shedding snakes remain dark and inactive for several days. The change is probably predominantly triggered by photoperiod, but exposure to cool temperatures (<63°F) can elicit a partial change from dark to light even in the middle of the day. When captive snakes were transported from Cuba to Czechoslovakia, they became jet-lagged—it took them several days to synchronize their rhythm to the new photoperiod, and keeping them in complete darkness for several days desynchronized their rhythm from that of the sun. The proposed function of this color change is to help nocturnally-active snakes retain their body heat, as light-colored objects lose heat more slowly than dark-colored ones. This is probably similar to the reason that Round Island Splitjaw Snakes, Pacific Keel-scaled Boas (Candoia carinata), and the Hogg Island race of Boa constrictor also become lighter-colored at night.

Tropidophis pardalis on a Cuban stamp
There's much more to learn about tropidophiids, the Cuban radiation of Tropidophis in particular. To date, little ecological information has been collected on most species, owing in part to their rarity and in part to the difficulty of working in the region. How do five or six sympatric species partition resources and coexist in various parts of Cuba? What was the order of speciation and colonization of the islands, and when did it happen? Hopefully tropidophiids will be around long enough for us to find out. They are faced with numerous threats. As in many places, local people not especially fond of them, despite the fact that no Greater Antillean snakes are dangerous to people. Collection for the pet trade may also be a concern, particularly since one former government official in the Turks & Caicos Islands apparently granted a permit to reptile dealers to remove thousands of Tropidophis greenwayi from North Caicos for the pet trade, allegedly implying that it would be preferred if they removed all of the snakes! Throughout the West Indies, most native ecosystems have been absent for centuries, and increasingly rapid development, especially due to tourism, threatens what little remains. And introduction of non-native predators, particularly the Small Indian Mongoose (Herpestes javanicus), may be their biggest threat. As early as 1919, herpetologist Thomas Barbour wrote "In Jamaica [Tropidophis maculatus] is almost extinct owing to the appetite of the introduced mongoose". Ironically, Operation Mongoose was the codename for the Kennedy administration's attempt to create Cuban diplomatic, political, and economic isolation in hopes of weakening Castro’s regime. Cats, dogs, rats, goats, pigs, cane toads, and even other introduced snakes also threaten not just tropidophiids, but all 120+ snake species endemic to the West Indies as well as the rest of the native fauna. Improved PR and conservation programs have benefited several lizard species, and could help snakes too.

Tropidophis haetianus
I'm going to go ahead and wager that we'll discover a few new species of Tropidophis in the not-too-distant future, and that possibly the mainland species will get moved into a new genus. I also think that we need a more creative common name for them than "dwarf boa", preferably one that doesn't include the word "boa" at all. One existing option is "wood snakes", which is mediocre at best. They are also called "rock pythons" in the Caicos Islands, an equally misleading name as "dwarf boa", "culebras bobas" (dumb snakes) in Cuba, and "shame snakes" on Andros Island in the Bahamas, both of which may refer to their head-hiding defensive behavior. However, my favorite is the name they are known by in many parts of the West Indies: "thunder-snakes", because they are more frequently seen after severe rainstorms. Caribbean Thunder-snakes has a nice ring to it, and it could help improve their image.

1 1: Constriction behavior has become a lot more variable within the Colubroidea, where it has also been lost in several venomous lineages. Venom and constriction can be thought of as two different solutions to the same problem—how to kill large prey without exposing yourself to undue risk. Also, the contention that constriction and large gape size were lost in fossorial henophidians (aka "regressed" macrostomatans, including uropeltids, anomochilids, and aniliids) is seemingly contradicted by the complex multipinnate morphology of their jaw adductor muscles, which is sufficiently similar to that of their lizard ancestors that it is unlikely to have re-evolved in the exact same way multiple times. This problem might also be an issue for scolecophidians, given that they have similar jaw muscle morphology to pipesnakes but appear to be more closely related to other living snakes than they are to some basal fossil macrostomate snakes with limbs (symoliophiids). Stay tuned for more on the unresolved relationships at the base of the snake family tree, including a look at what fossil snakes can tell us.

2 2: All four genera (Exiliboa, Ungaliophis, Tropidophis, and Trachyboa) either completely lack a left lung or have a greatly reduced one, a characteristic they share with anomochilids and some caenophidians, but not with most other henophidians, which have a somewhat reduced but functional left lung. In addition, all four genera also have a "lung" on the dorsal wall of the trachea: the tracheal cartilages do not form closed rings but remain open on the top, where a greatly expanded ligament forms the tracheal lung. It has alveoli just like a regular lung, which are especially deep near the head, and is contiguous with the true lung in the vicinity of the heart. But, although this might seem like very strong evidence that these four genera are closely related, tracheal lungs of diverse structure are widespread among snakes, being found in certain scolecophidians, xenophidiids, acrochordids, vipers, atractaspidids, sea snakes, and many colubroid snakes.

3 3: A tantalizing bit of evidence emerged in 2008—biologists in Brazil videotaped the prey subjugation behavior of a captive Anilius scytale, which essentially constricted an amphisbaenian that they tried to feed it. In general its constriction behavior agreed with that of other henophidia, although it was more variable in the particulars, which could have been due to the difficulty of holding onto the elongate, "vigorous and constantly twisting prey". But, data from a single observation do not a generalization make, and more studies are needed.

4 4: Fossils of t
en extinct species in five genera from the Paleocene, Eocene, and Oligocene of Europe, Africa, & North and South America have been assigned to the Tropidophiidae, although all of them are probably actually either ungaliophiines or stem afrophidians. Two genera, Falseryx and Rottophis, both from the Oligocene of western Europe, have some similarities with living tropidophiids as well as with ungaliophiines, but for the most part their skulls are poorly preserved, leaving paleontologists to work on just their vertebrae. Paleogene erycines dominated the snake fauna of North America prior to the Miocene explosion of colubroids, but as far as we know all of these species were much more closely related to modern rosy and rubber boas than they were to tropidophiids. The only unequivocal tropidophiid fossils are from the Pleistocene of Florida and the Bahamas.


Thanks to Kenny Wray, Nick Garbutt, Alex Figueroa, Patrick Campbell, Pedro Bernardo, and Carlos De Soto Molinari for the use of their photographs.


Battersby, J. 1938. LXIV.—Some snakes of the genus Tropidophis. The Annals and Magazine of Natural History 1:557-560 <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.