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Evolving a Venom or Two
By filthy
Posted on: 3/17/2007
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Please do pass the Tabasco, so long as it isn't neurotoxic.
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Question: Which came first, the chicken or the egg?
Answer: Who cares; pass the Tabasco… A similar and equally idiotic question has been snidely put forth about serpent venoms, and it’s rather like the “half-wing” strawman hoo-hah we hear from various Creationist groups, “Of what use is venom with no injection system, and vise-versa?”
The answer they give is, “Obviously none, therefore it all had to be created at the same time, proving the Theory of Evolution is a crock, and ha-ha all over you!”
In this screed, I shall pass the Tabasco.
First, what exactly is a venom? What makes an animal “hot,” and how is this any different from a poison? We’ll quickly straighten the latter out first. Basically, venoms are injected while poisons are ingested or absorbed.
A venom is any substance produced and used by an animal to subdue prey and/or defend itself. The list of venomous animals is an extensive one and it goes far beyond snakes. There are hot insects and arachnids, as we all know, and even a few mammals such as shrews. A stab from the venomous spurs of a male platypus and its fellow monotreme, the echidna, can be a quite painful experience. There are even poisonous birds. A great many sea creatures are venomous, including but not restricted to jellyfish, anemones, corals, octopi, and some fish such as stingrays, stonefish and the gafftops’l catfish have venomous spines. The fish and octopi, incidentally, fit nicely in the skillet, venom notwithstanding.
I once met a man who claimed to have been stung by a cone shell, species unknown (the mollusc, that is, not the man). He found it in shallow water, and as it was pretty, he took it to his hotel room and began digging at it with a jack knife — to kill it, one presumes. The gastropod reacted predictably and stabbed him in the finger with one of its “harpoons.” He told me that he has never experienced such pain, before or since.
I can personally attest to the effectiveness of sea wasp and Portuguese man-o’-war venoms. They are hot!
But back to the question of the evolution of venoms and their injection systems in serpents. As I have stated elsewhere, venom evolved first, and exactly how it happened can be easily seen in living Colubrid species today. All snakes have Duvernoy’s organs, the venom glands, although they are degenerate in constrictors such as the Boids (boas and pythons), that don’t rely as much on biting to subdue prey. These are modified salivary glands that are used to produce a saliva of varying degrees of toxicity. It shows up in that the bites of many non-venomous snakes are messy far past the severity of the bite, which seldom amounts to more than a few pinpricks. It is considered an anticoagulant, but it is a lot more than that in that it contains toxins that will slow down a frog, or in the case of small snakes such as ringnecks and DeKay’s, large earthworms and slugs, and render them more manageable. None of these are medically significant to humans and, having no fangs, these snakes must chew vigorously to inject the, well, saliva (can’t quite bring myself to call it a venom).
But as we proceed, it starts getting more interesting and quite a bit warmer, because now we come to the rear-fanged Colubrids. Most of these too, are not medically significant, but some few certainly are. Most have rather stubby, grooved teeth set far to the rear of the mouth; only a little larger than the others. The venom (it really is, this time), is applied through the grooves in these teeth and cannot be effective until the prey is pretty much on its way into the snake’s gullet. The American hognosed snakes are good examples of this. The venom of these animals is so mild that some have questioned if it’s really a venom at all. Me, I call it a venom.
The rear-fanged list further includes such lovely and innocuous creatures as the vine, lyre, and parrot snakes among many others. But some, such as the mangrove and cat snakes can deal a very painful and incapacitating bite, if not a fatal one.
But time and evolution march on and so do fangs, and suddenly we come to the boomslang, a beautiful, rear fanged, arboreal serpent with fangs that are not all that far to the rear, are relatively large for a Colubrid and capable of injecting a pretty decent amount of a venom so specialized that it requires its own antivenin. Herpetologist Karl P. Schmidt was killed by one some 28 days after what he thought was a minor bite. Further, this delicate-appearing snake can gape its mouth to an appalling 180 degrees, as much as any Viperid. Like all of the rear-fangers and the Elapids, the fangs do not fold, and the ’slang’s are still no more than grooved teeth, albeit deeply grooved.
The author’s arms two-and-a-half days after
a bite from a sub-adult Southern Copperhead
(Agkistrodon c. contortrix) |
The real fun begins with a visit to Elapidae. These include the coral snakes, sea snakes, cobras, mambas, tiapans, kraits, death adders, and numerous others.
Again, we have fixed-fanged animals, but with a major difference. The less-than-efficient, grooved fangs of the Colubrids have closed up and become hollow needles no less effective in delivering their product than a modern hypodermic. However, due to the fixed setup, the fangs are still relatively short, but they inject some venoms that are extremely potent. And again, we see the progression of the fangs toward the front of the mouth, especially in Dendroaspis species (mambas). In D. polylepis, the fangs are nearly under the nostrils.
Some Elapids such as the rinkhals (Hemachatus haemachatus) are spitters. These have orifices in the front of the fang’s tip, aiming straight out. Spitting venom is one of their major, defense postures. And if a drop gets in the eye, it will quickly discourage just about anything. I’ve had the misfortune to learn this first hand…
Arguably, some of the most amazing reptiles on the planet, and my personal favorites, are the Viperids. I like ’em a hell of a lot better than I do most people, so let’s have a look at the next advance in evolutionary complexity.
Decent snake fossils are hard to come by. Most ancient species are known only from vertebra, skull fragments, the odd ribs here and there, and teeth. It is as yet impossible to say when the Viperids split off from their more conventional ancestors. But split off they did and in doing so adopted the most efficient venom delivery system the planet has ever seen. The fangs are very long and fold into the roof of the mouth, and the Duvernoy’s glands are much larger in proportion than those of Colubrids and Elapids.
Elapids and venomous Colubrids are active hunters. They are fast-moving snakes, yet have a relatively slow strike. The exact opposite is true with the Viperids, who tend to be ambush predators. They are somewhat sedentary, but strike like lightning.
The strike is amazing. As the head begins to move, the jaws open to full gape. The fangs are implanted and in the minute fraction of a second before being withdrawn, a measured amount of venom is injected — just enough to subdue the prey, and no more.
The fangs are attached to a short, muscular, highly movable maxillary bone that acts like a hinge, allowing them to fold out and back in, rather like high-speed jack knives. This allows for a longer fang and therefore much deeper penetration of the venom. Shed fangs from Gaboon Vipers (Bitis gabonica) have been measured in excess of two inches, and I have found some close to that in the feces of specimens that I have kept (all snakes shed their teeth as well as their skins, and most of them are simply ingested. To collect teeth and fangs, simply dissolve the feces in alcohol and strain them out).
It is difficult to imagine where Viperid evolution might go from here. They are just so damned good at what they do…
The slightly toxic spit of the non-venomous Colubrids to the thick, almost ropey venoms of the large Bitis, all have something in common: they are composed of a large variety of compounds. Mostly, venoms are classified as either neurotoxic and haemotoxic — nerve and blood agents, but like anything else, it ain’t all that simple. All of these venoms are a virtual witch’s brew of haemo-, neuro-, myo- and cardiotoxins, among many other enzymes and proteins. The main difference is in the mix, and this varies a great deal from species to species, and even within a species.
However, Elapid venoms mainly affect the nervous system, ultimately causing death from asphyxiation. Many Viperids go heavy on haemo-, myo- and cardiotoxins causing the circulatory system to shut down. And remember our pretty boomslang? This one has a highly haemotoxic venom; a powerful anticoagulant is the main ingredient. And thus, it requires its own antivenin, effective on no other species.
Venoms also account for as much as 30% of the digestion process.
All pretty straightforward so far, yes? Well, we can’t have that, now can we? Let’s drop an evolutionary wrench in the gearbox: venoms change! For example, the timber rattlesnake (Crotalus horridus) and its southern phase, the canebrake rattlesnake (same Latin) have somewhat different venoms. The timber is strictly a predator of small rodents and the occasional young bunny. Its venom suits this diet with a predominantly haemotoxic chemistry. But pocket populations of canebrakes have some frogs in their diet. These are a lot harder to kill than a mouse, and their venom is adapting neurotoxins to cope with them.
The tiger rattlesnake’s (Crotalus tigeris) diet is predominantly lizards. It has mostly neurotoxic venom.
A Central American rattlesnake, Crotalus durissus terrificus, is the hottest of the rattlers with a very high percentage of neurotoxins along with all the rest. I kept a terrificus for some years, and “Gomez” would stop an adult lab rat virtually in its tracks if he got any kind of a good hit. A very impressive animal and my respect for him was vast!
I have not gone into the chemical analysis of these venoms for the simple reason that I’m not all that well versed in it beyond the basics — my experience is mostly in the field, and this is getting a little too long, anyway. But, we can’t leave it without a look at venomous lizards.
The Heloderma, two species, are the only venomous lizards in the world. While there has never been a recorded fatality from a bite, they are still medically significant, as the venom is primarily neurotoxic, extraordinarily painful and at least temporarily incapacitating.
These have no fangs, neither fore nor aft, merely a multitude of rather lightly grooved teeth, and unlike snakes, the Duvernoy’s gland is in the lower jaw. Their venom is not used to subdue prey, but as a defense against misguided predators that might think a short, fat lizard would make a tasty snack. A little more on these fascinating animals can be found at the Arizona-Sonora Desert Museum.
For no better reason than to be a fractious bastard on a herpetology forum, and to enrage as many educated people as possible in as short a time as possible, I once argued the position that the giant komodo monitor was venomous. My very excellent reasoning went thus:
The mouths of these lizards are a festering charnel-pit of bacteria-caused swollen and bleeding gums and loose teeth; truly foul! They salivate constantly, bacteria-infested skeins of thick saliva often hanging from their jaws. These bacteria cause such raging infections in the lizard’s prey that a bite from a komodo is virtually a death sentence. Thus, as they often prey upon large animals that they can’t immediately subdue — deer, water buffalo, and the like — they rely upon the bacteria, which turns the wounds septic in short order, ultimately and horribly finishing the unfortunate beast off. And like some dread creature from any low-budget horror flick, the dragon patiently and relentlessly stalks it down by the scent put forth by the sick animal.
These virulent bacteria do not occur naturally in komodos, but a young lizard will get its share when feeding on a kill made by its elders, to the bacteria’s reproductive advantage. Therefore, what we have here is a rather ghastly, symbiotic relationship that has been in place for a very long time. So, I ask: how is this not a venom? It certainly works like one, and like many venoms, it is being studied for its medical properties.
Venoms is as venoms does, and evolution is a lively, syncopated melody — and more than a little strange. I find it wonderful that an evolutionary progression such as venoms and their injection systems can be observed (carefully) right in my snake-, cat- and opossum-infested home…
(More photos of envenomations can be found on the website of the Southeastern Hot Herp Society.)
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