I recently saw a video of a garter snake (Thamnophis cyrtopsis) eating a Southern Leopard Frog. If you watch the video carefully, you will notice (~30 second mark) the snake appear to “chew” on the snake. The scene is then cut (probably 2-3 minutes elapse) and the frog appears to be only moderately “alive.” It is still breathing, but does not appear to be struggling. Did the animal just give up? Well, no, not exactly. Thamnophids (and many other “colubrids”*) possess a structure homologous to the venom glands of Elapids (cobras, kraits, coral snakes*, and other “hingeless front-fanged snakes”) and viperids (adders, pitless “true” vipers, and pit vipers). This gland is known as Duvernoy’s Gland, and it has some very interesting properties in many species. In Thamnophis, for example, the venom produced by these glands has (essentially) no delivery mechanism. Its rear teeth have no grooves for venom to flow into a wound and it has no fangs. It does have slightly larger teeth in the rear of the mouth, however, this is not uncommon to many reptile species. Anoles, for example, illustrate this wonderfully. This (and other evidence) suggests that the venom delivery systems via fangs developed some time after venom glands were around. The presence of venom-producing structures in monitor lizards and iguanids (as well as the “glass lizards) further compounds the issue of when these venom producing systems evolved.
Most of these venoms can’t kill humans, right? Well, not in the doses normally produced by these specimens; in some species (such as the Thamnophis), the LD50 is so high that it could truly be considered “nonvenomous” to humans. The LD50 in mice is in the 30+ mg/kg range with the average yield of 57 µg/snake, meaning one would need to get all of the venom from 125 snakes to kill a 1/2 lb rat. Not all of these are so harmless, though. Dispholidus, for example, is a very pretty snake which appears harmless enough, until you see inside of the mouth. The size of the fangs, however, are not what should concern you. The venom yield of this particular species (from that same Duvernoy’s Gland) exceeds one mg and can reach up to 8 mg. That doesn’t sound like much when you consider a typical plastic ballpoint pen cap weighs about one gram. This number, however, comes into its own when you consider the LD50 of this venom is roughly 0.07mg/kg. This makes it one of the most potent venoms produced by any snake species.
*Note: As a child, I was often told that the fangs of coral snakes were in the back of the mouth. Being the inquisitive child I was, I decided to test this by killing one, allowing it to decompose on top of a very friendly fire-ant mound, and then looked at the skull. Much to my surprise, the fangs were in the front of the jaw. This can be illustrated by this image. All elapids and viperids have fangs in the front while “colubrids” have fangs set further back.
I wouldn’t be a cladistically inclined biologist if I didn’t make the obligatory parting shot at “colubrid” as a term to describe a group of related snakes. It pains me to know that such a convoluted dumping ground exists for all snakes other than pythons, vipers, and cobras.