Monday, March 14, 2005

Got Venom?

As I’ve mentioned previously, I’ve long been fascinated by snakes. Opisthoglyphous or “rear-fanged” snakes are of particular interest to me. The mangrove snake (Boiga dendrophila), the boomslang (Dispholidus typus), and the eastern hognose snake (Heterodon platyrhinos) are all opisthoglyphous snakes, so described because of their venom delivery system, consisting of two long, grooved teeth at the back of their upper jaw. When prey is bitten, venom pours down the grooves into the wounds made by the snake’s teeth.

Though I often talk of these species’ beauty, I think a great deal of my affection for these reptiles has to do with their reputation as inoffensive, harmless snakes. Some herpetologists raise questions about the safety of handling any opisthoglyphous species, but such considerations are generally ignored. Through the mid 1990s, only the boomslang and the twig snakes (Thelotornis sp.) were considered truly dangerous. By contrast, the eastern hognose was considered a pushover (when threatened, this species prefers to "play dead"), and the western hognose (Heterodon nasicus) remains a popular hobby animal in the worldwide pet trade.

As I learned more about this group of snakes, however, I became convinced that they were potentially dangerous. In the summer of 1998, I captured an eastern hognose in Williamsburg, Virginia, where I was attending college. I attempted to acclimate the reptile to captivity. I even caught Fowler’s toads (Bufo fowleri), a favorite prey of the species, but all to no avail. Whenever the snake detected movement near the enclosure, it would play dead or emit a foul-smelling musk. Two weeks later, I released the snake in the place where I had captured it. I had not been bitten, fortunately, but I had kept a venomous reptile! This was a source of quiet pride, as I didn’t dare tell those around me (especially my roommates) that I believed the eastern hognose was poisonous. (Note: Since this experience, I have learned that such capture-and-release practices are potentially problematic. Had this Heterodon platyrhinos been infected by any mites or pathogens carried by the other snakes that I kept at the time, it could have introduced these foreign threats to the local ecosystem with potentially devastating consequences.)

Around the same time, I read several articles suggesting that many more snake species carried venom than was generally accepted in the scientific community. Not all of these articles were current. “Beware: Nonpoisonous Snakes,” by Sherman A. Minton, Jr., was published in the November 1978 issue of Natural History magazine. In the article, Minton wrote,
“My father belonged to a generation that killed all snakes on the assumption that even if they were not poisonous, they could not be distinguished from kinds that were. I don’t advocate a return to that attitude, but I would suggest a little more caution in handling reputedly harmless snakes.”
Evidently, Minton’s warning was taken to heart by Dr. Bryan Fry (University of Melbourne, Australia) and Dr. Wolfgang Wuster (University of Wales, UK). Recent protein and physiology studies by the two biologists show that instead of 200 known venomous species, 2,200 of the world’s 2,700 snake species are venomous! (These numbers are estimates.) Not only are these 2,000 snakes venomous, but the venom itself is of “similar complexity to those of their most deadly cousins…albeit in small quantities and without the apparatus to deliver it efficiently.”

Snakes, as we know them, first appear in the fossil record during the Cretaceous Period (about 100 million years ago). These early snakes presumably evolved from lizards that spent most of their time in burrows underground. Legs were more of a hindrance than help, so evolution did away with them. Over millennia, the reptiles have continued to evolve. The more “primitive” snakes, such as boas or pythons, are not venomous and fewer evolutionary steps separate the boids from the earliest snake fossils. (In fact, most boids still have vestigial legs, called spurs, near the rear of the body.) In the late Cretaceous or early Tertiary Period, the so-called “modern” snakes evolve.

What Drs. Fry and Wuster have shown in their studies is startling, but sensible. Venom evolved prior to the various anatomical delivery apparatuses. The black rat snake (Pantherophis obsoletus) that fell on my head as a child has the venom, but a poor delivery system. The “simple, small teeth” of this aglyphous snake “make only a shallow wound in their prey into which some venom might seep.” The opisthoglyphous snakes have a better way of delivering the venom, with their “longer, grooved teeth at the back of the jaw.” The vipers, with their “long tubular fangs that are hinged,” are best equipped to deliver a fatal bite. Not surprisingly, the vipers are also the most highly evolved snakes.

Jennifer Daltry, one of Dr. Wuster’s colleagues, has made some curious discoveries of her own. Studying Malaysian pit vipers (Calloselasma rhodostoma), she learned that different populations of the same species tailor their venom to better target their primary food source. The populations of Calloselasma rhodostoma that feed primarily on lizards have evolved one variety of venom, while those with a diet predominantly composed of mammals have evolved another. The protein makeup is being altered in a relatively short timeframe, providing us with a nice window onto natural selection.

Of course, I find all of this incredibly exciting! I am worried, though, that most people who learn of this new information will only fear snakes that much more. I sincerely hope this won’t be the case, but given the animosity most humans show the beautiful reptiles presently, one imagines it will only increase once the vast majority of them can be labeled DANGEROUS / VENOMOUS REPTILE.

Photo credit: N. Sivasothi, 2004 (Mangrove snake, Boiga dendrophila)


Anonymous said...

If it allays your concern at all, I've always been scared totally shitless of all snakes, and your post hasn't made me any more afraid. I bear them no ill will, and pray that they're as scared of me as I am of them.

And I also run like a little girl when I hear things rustle in the grass.

Hungry Hyaena said...

And if it makes you feel any better... despite my love of snakes, I'm sure I've tied some high-jump world records when surprised by the not-so-proverbial "snake in the grass."

Just as long as you're not killing them, selling them to pet stores or arguing that the world is better off without them, you're alright in my book.

Sivasothi said...

Thanks for the citation - photo was contributed by Francis Lim, the snake expert at the Singapore Zoological Gardens.