Boomslangs and Blade Fangs: An introduction to the weird world of snake dentition

The study of reptiles and amphibians, known as herpetology, is rife with interesting and unusual characters, all with stories to tell. One of the saddest belongs to Dr. Karl Patterson Schmidt, an intrepid researcher who undertook expeditions across Central and South America, took a brief interlude to serve in the US army, named over 200 new species, and once described maintaining his hold on the snout of an angry crocodile in much the same way you might hold a hot potato. As with any good story, the specifics have become a little distorted over time, but the version I heard first goes like this.

Dr. Schmidt was a skilled herpetologist and curator of the Field Museum in Chicago, and authored a wealth of natural history books and articles in his time, including titles such as Lizards by Parcel Post, Crocodile-Hunting in Central America, and Crocodiles, Perilously Collected by Hunters in a Canoe, Now Exhibited. As such, you can imagine that when he arrived at work one morning to be presented with a small snake, and a polite request for identification from the director of Lincoln Park Zoo, his only dismay may have been that it wasn't a large crocodile. The snake was, in fact, a boomslang, a tree-dwelling species with large, beguiling eyes, native to sub-Saharan Africa. As Schmidt picked it up to examine it, the snake hooked a single, lazy fang into his thumb. Schmidt, unbothered, continued with his examination.

Over the next 48 hours, he documented in extensive detail the onset and progression of a concerning variety of symptoms in his journal, culminating in the final entry “Mouth and nose continuing to bleed. Not excessively.”

Roughly eight hours later, at 3pm on September 26th, 1957, Dr. Schmidt was pronounced dead. His autopsy report would reveal immense internal bleeding from his lungs, eyes, kidneys, brain – even his heart. The official cause of death was listed as respiratory suppression. What neither the report nor his journals suggest, however, is why he never sought medical help. The nature of boomslang venom was well-known amongst herpetologists at the time; as he described his fever, nausea and bleeding, he must have been aware of the biological war being waged. Some have claimed he refused intervention to avoid disrupting his documentation of the symptoms. Others have pointed out that while boomslang antivenom had been first manufactured some twenty years earlier, it was available only in Africa – a fact Schmidt would have been only too aware of. 

Nonetheless, as my skin began to tingle in response to the bite, I looked into a set of dark, reptilian eyes and couldn't help but think of Dr. Schmidt, standing in his laboratory with a young boomslang hanging off his finger.

There was certainly no chance of this bite being fatal – it was the fact that I was experiencing any kind of symptom at all that surprised me! The snake in question was a young, captive grass snake, well-regarded as a non-venomous species. They're also reluctant to bite, but having spent her whole life around humans, she had found a cognitive sweet spot: smart enough to associate fingers with food, and not quite smart enough to differentiate the two. To be fair to her, I was also not quite smart enough to feed her with tongs. I'd watch her eat twice a week, familiar with her strategy: seize hold of her prey (usually a chunk of fish, or a frozen-thawed pinkie mouse) in a clumsy burst of speed, then slowly work her jaws around it until she could swallow it.

For something with such a simple and consistent body plan, snakes are amazingly variable, and no group of snakes is more amazing or more varied than the colubrids. The Colubridae family can be found on every continent bar Antarctica, in deserts and rainforests, in ponds, up trees and even burrowing underground. Some lay eggs, while others give birth to live young. Many are non-venomous, and these are sometimes – but not always! - constrictors, coiling muscular bodies around their prey and using pressure to induce cardiac arrest. The strangest of them all, the egg-eating snakes, have lost their teeth completely over time. They engulf bird's eggs in inky, cavernous mouths and force them down into their throats, where the shells are crushed by modified vertebrae and regurgitated. While there are venomous colubrids, most of them don't pose much danger to humans; the boomslang is one in a handful of notable exceptions.

Venomous snake tends to call to mind two specific families: Viperidae, which unsurprisingly represents the vipers, and Elapidae, a diverse group whose most famous members include the cobras and mambas, amongst others. Exact delivery mechanisms vary, but both families store their highly potent venom in glands, surrounded by powerful muscles. When these muscles contract, the venom is forced out of the gland and administered into the wound through a pair of hollow, syringe-like fangs.

Even the boomslang, one of the most lethally venomous colubrids, doesn't have a true venom gland. No colubrid does. What about 40% of colubrids do have is a Duvernoy's gland, which is, at first glance, exactly the same thing: a gland in the face that secretes a toxic compound into the mouth. The mechanism of secretion, and lack of storage space, are enough, in the eyes of science, to differentiate the colubridine Duvernoy's gland from the venom glands of vipers and elapids.

Colubrid teeth are also very different to the fangs of vipers and elapids, and tend to fall into two broad categories: aglyphous and opisthoglyphous. The opisthoglyphous boomslang does have two fangs, with a groove in each to deliver the Duvernoy's venom. These fangs are situated at the back of the month, which is why opisthoglyphous snakes are also called rear-fanged, and chew the venom into the wound rather than injecting it. Some aglyphouse snakes also have a Duvernoy's gland, although not all; they chew the venom in with rows of identical teeth running down the length of the jaw.

One of the most well-known examples of an aglyphous venomous snake is the garter snake, a term encompassing 35-odd species in the genus Thamnophis and spanning two continents, from Canada to Costa Rica. They are a common port of call for reptile keepers looking for something more interesting than a ball python, and a staple of any outdoorsy American childhood. Unless you're a particularly literate frog, a bite will rarely cause more than a mild burning, swelling or itching. In my time working with snakes, I have personally offended a fair few of these 35-odd species, and for me, envenomation presents as a distinctive tingling sensation, localised roughly in the vicinity of the bite.

Interestingly, this is exactly the symptom that followed my grass snake bite – and not something I'd ever experienced from the (many, many) other times I've been caught out at the sharp end of a snake. Clearly, with this grass snake now happily attending to her chunk of sprat, it was time to do some research.

I quickly found a paper from 2011 that confirmed my suspicions – a close relative of my grass snake, the dice snake Natrix tessellata, does have a Duvernoy's gland. A 1983 report claimed that all members of the Natrixgenus have a Duvernoy's gland, but the source for this was an article I haven't been able to find (Venomous or Not? By Harold F. DeLisle, 1981). The DeLisle article is also cited for an incident where a frog was bitten by Natrix natrix, survived, and then died two hours later. This could be the result of internal damage from the bite, or even unhappy coincidence, but it does add some credence to the idea of venomous Natrix.

While to my knowledge, this gland has only ever been confirmed in one species, it doesn't seem like a leap to suggest that my grass snake may well have a Duvernoy's gland too...Famous last words (if not quite as poignant as Schmidt's “Not excessively.”) - taken from the original version of this article, which I had written, edited, sent to Eleanor and vowed never to look at again. Unfortunately the prolific herpetologist Alessandro Paterna, armed with two roadkill grass snakes and a publication in the Phyllomedusa Journal of Herpetology, had other ideas, and a few weeks after I emailed my article to Eleanor, a paper titled Analyzing and comparing the buccal anatomy of European colubroid snakes: A reassessment of dentition models appeared on my Facebook feed.

This paper was my vindication, making a conclusive argument for Natrix as a rear-fanged snake. By dissecting and comparing the jaws of several European colubrid snakes, Paterna proposes an entirely new category of dentition: blade-fanged. Like other opisthoglyphs, the grass snakes have enlarged rear teeth, but the cutting edge is larger in the back (the distal carina) than the front (the medial carina), forming a blade-like shape. While these teeth aren't grooved, and don't meet the criteria for classification as opisthoglyphous, they are very capable of delivering venom.

One of my favourite things about snakes is the sheer number of questions that are still to be answered, and while Paterna's paper answered a question that has bothered me for the five years since that bite, it also demonstrates how much there is left to learn in a field as niche as snake dentistry. Rear-fanged traits have been found in several species previously thought to be aglyphous and totally non-venomous, like another British native, the smooth snake. Perhaps even more interestingly, remnants of Duvernoy's gland have been found in truly aglyphous constrictors, including the corn snake – a wildly popular pet, native to north America but found in living rooms across the globe. This begins to answer another question: where did rear fangs come from? Rather than appearing independently in the many rear-fanged colubrid groups, they must have arisen in a distant ancestral colubrid to be refined in the boomslang, adapted in the grass snake, and slowly lost by the corn snake in favour of constriction.

Clifford Pope comments in his official publication documenting Schmidt's death “The following account of the bite and its effects was written by Dr. Schmidt out of scientific curiosity and in a characteristic spirit of optimism”. Colubrid or otherwise, venomous snakes are the herpetologist's equivalent of a lion or chimpanzee: immensely charismatic, inspiring fascination and fear in equal measure. With snakes across the globe under threat from loss of habitat and conflict with humans, we can only hope that they persist to keep inspiring these questions for generations to come.

Sources

Hendrickx, C., Mateus, O. and Araujo, R. (2015) A proposed terminology of theropod teeth (Dinosauria, Saurischia). Journal of Vertebrate Paleontology, 35(5), e982797.

Kardong, K. V. (2002) Colubrid snakes and Duvernoy's “venom” glands. Journal of Toxicology Toxin Reviews, 21(1), pp. 1-19.

Ovadia, M. (1984) Embryonic Development of Duvernoy's Gland in the Snake Natrix tessellata (Colubridae). Copeia, 1984(2), pp. 516-521.

Paterna, A. (2024) Analyzing and comparing the buccal anatomy of European colubroid snakes: A reassessment of dentition models. Phyllomedusa Journal of Herpetology, 23(2), pp. 111-124.

Pope, C. H. (1958) Fatal Bite of Captive African Rear-Fanged Snake (Dispholidus). Copeia, 1958(5), pp. 280-282.

Westeen, E. P., Durso, A. M., Grundler, M. C., Rabosky, D. L. and Davis Rabosky, A. R. (2020) What makes a fang? Phylogenetic and ecological controls on tooth evolution in rear-fanged snakes. BMC Evolutionary Biology, 20, pp. 1-15.