Front- and Rear-Fanged Snake Envenomation Systems
Morphology of Venom Delivery Systems
Generally speaking, there are four types of venom glands, more dependent on snake family than on whether the snake is front- or rear-fanged. Snakes belonging to family Colubridae (rear-fanged snakes) possess a Duvernoy's (venom) gland, which is often thought to be the evolutionary "precursor" to all other snake venom glands. This gland has little space for storing venom that is readily available for use and is primarily composed of serous cells which contain most of the venom. Among the front-fanged snakes, however, members of family Elapidae (Cobras, Coral Snakes, Sea Snakes) and family Viperidae (Vipers, Rattlesnakes) are known to have a main venom gland along with an accessory venom gland. Tubular secretory epithelial cells make up much of the walls of the main venom gland, with a large central lumen (cavity) that is capable of storing a great deal of venom for immediate use. Accessory glands contain serous cells and mucus-secreting epithelial cells, but do not appear to secrete any venom toxins. It has been hypothesized that the accessory gland helps "activate" venom components as they pass through it on their way to the front-fang.
Although the exact placement of the accessory gland may differ somewhat, viperids generally have their main venom gland connected to the accessory gland by the primary duct with the secondary duct connecting the accessory gland to the front-fang, whereas elapids have their main venom gland and accessory gland directly next to one another (virtually attached side-by-side) with just the primary duct running through the accessory gland to the front-fang. Snakes of family Atractaspididae (Burrowing Asps, Stiletto Snakes) have many venom system attributes that are similar to elapids and viperids, but lack accessory glands and secondary venom ducts. Although they have not been as extensively studied as the other front-fang families, the atractaspidid venom gland appears to have a similar histochemical structure to those main venom glands, with a central lumen and tubular cells lining the walls. The venom gland is very elongate and has a primary duct leading to the fang (whose position varies on the species involved, being either in the front of the mouth or towards the back; for simplification purposes, however, I will refer to them as being "front-fanged").
Colubrid (Rear-fanged) Snake Venom System
Elapid (Front-fanged) Snake Venom System
Efficiency of Venom Delivery Systems
The venom delivery system of front-fanged snakes is more efficient than that of rear-fanged snakes. This is due to a combination of factors. The first comes back to the structure of the fangs themselves. Since front-fangs are tubular in design (with the venom entry/exit holes sealed by the specialized fang sheath), they comprise a completely enclosed venom system. Rear-fangs, whether they are grooved or not, are part of an open venom system because the venom duct empties onto/above the rear-fang, with little to enclose the venom flow (even apart from the observation that some snakes have multiple, other venom ducts running to teeth near the pair of rear-fangs).
Secondly, front-fanged snakes possess a muscle that either overlays the venom gland (elapids/viperids) or is somehow associated with and inserts on the venom gland (atractaspidids). This muscle serves to compress the venom gland and encourage venom flow from the gland's lumen to the fang. Rear-fanged snakes do not tend to have any musculature associated with the venom gland, resulting in a significantly decreased ability to quickly deliver a bolus of venom. There is, however, a ligament which connects the Duvernoy's gland to the quadrate bone (a bone that connects the lower jaw to the upper jaw and skull) and this may permit tension created by biting/chewing to instigate venom flow to a small degree (in addition to the residual venom flow induced by capillary action).
Third of all, since a large volume of venom is stored in the lumen of the venom gland of front-fanged snakes, there is a large venom reservoir that is ready for immediate use. Rear-fanged snakes, on the other hand, only carry a small amount of venom in the lumen of the venom gland that is ready for use, meaning that a much longer period of time is needed to be able to inject large amounts of venom (the "reserve venom" within the secretory cells must be released via vesicles into the lumen, where it can be used). Overall, front-fanged snakes have high-pressure (enclosed) venom systems with the capacity to inject large volumes of venom in a short time frame, whereas rear-fanged snakes possess low-pressure (open) venom systems that are only capable of injecting small amounts of venom quickly. To give an example of the difference in venom delivery efficiency, one can extract most of the venom from a front-fanged snake in a matter of ~12 seconds, compared to the ~20 minutes it takes to extract most of the venom from a rear-fanged snake, making the front-fanged snake venom delivery system ~100x more efficient (relative to the total venom yield possible) than the rear-fanged snake venom delivery system. This means that while front-fanged snakes can inject a moderate volume of venom within a fraction of a second, rear-fanged snakes must hold on (and, in some cases, actually "chew") in order to be able to inject appreciable amounts of venom into their target. Although there is much variability among/within species, this predicted behavior is generally observed, with front-fanged snakes often biting and releasing their prey, and rear-fanged snakes biting and holding onto their prey.
You may take the quiz below to test your knowledge of the two snake envenomation systems discussed here. You can also check out the video below, which illustrates the speed with which a venom extraction from a front-fanged snake can occur, especially when coupled with a manual massage of the venom gland. If you would like to learn more about the envenomation symptoms elicited by rear-fanged snakes, please see the Amazon links below for some useful book resources. If you have further questions about snakes that are not addressed by this article on front- and rear-fanged envenomation systems (or any other articles in this snake venom series), please see my article on FAQs About Snakes.
Viperid (Front-fanged) Snake Venom System
Atractaspidid (Front-fanged) Snake Venom System
Rear-fanged Snake Capturing Prey
Front-fanged Snake Capturing Prey
More in This Series About Snake Venom
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Front-fanged Snake Venom Extraction
This hub is intended to educate people ranging from snake experts to laymen about the particulars of front- and rear-fanged snake envenomation systems. This information contains generalizations and by no means encompasses all exceptions to the most common "rules" presented here. This information comes from my personal experience/knowledge as well as various primary (journal articles) and secondary (books) literature sources (and can be made available upon request). All pictures and videos, unless specifically noted otherwise, are my property and may not be used in any form, to any degree, without my express permission (please send email inquiries to firstname.lastname@example.org).
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© 2012 Christopher Rex