Venom Delivery

In general, venom delivery in vipers and elapids can be thought of as a “high pressure system” (Kardong and Lavin-Murcio, 1993).  The mechanism of a hypodermic needle is a useful analogy for understanding venom injection (Greene, 1997). Contraction of jaw muscles (the plunger) act on the venom gland (the barrel) to drive a pulse of venom down through the fang (the needle) and into the prey (Kardong and Lavin-Murcio, 1993; Greene, 1997).  Kinematic studies on striking behavior in rattlesnakes find that the strike itself is extremely quick (usually less than 0.5 s) and consists of four distinct stages: extend, contact, release, and contract (Kardong and Bels, 1998).  Contrary to what many may think, snakes do not stab at prey with their fangs.  In fact, the lower jaw makes the first contact with the prey, which then acts as a pivot for the snake's head as the fangs are rotated down into the prey (Kardong and Bels, 1998).  Other studies on rattlesnakes show that juveniles and adults may have different envenomation strategies with juveniles holding onto prey longer, presumably to ensure sufficient envenomation, while adults often release the prey sooner, perhaps to avoid struggling with a large prey item (Hayes, 1991).  Rattlesnakes are also known to alter their behavior according to the prey type, holding onto prey that are more mobile (birds) while releasing prey (mice) that may be easily located using the chemosensory system even if they are allowed to travel after being envenomated (Hayes, 1992a).  Recent work suggests that the fang sheath may play an integral role in venom delivery since its inner membrane appears to block venom from flowing into the entrance orifice of the fang when the fang is retracted (Young et al., 2001).

Figure 1. Scanning electron micrographs of non-spitting cobra and spitting cobra fangs. From Young et al., 2004 with permission from the first author. Click on the image to see a larger version.

A very unique strategy for defensive venom delivery exists in the spitting cobras which eject venom through specialized fangs (Fig. 1) (Young et al., 2004).  Researchers have found that a two step process is necessary for the ejection of venom in the "pressurized horizontal stream" that is characteristic of spitting cobras (for examples of venom spits see Fig. 2 and 3) (Young et al., 2004).  First, the snakes flex their heads in such a way to displace the soft tissues that are responsible for restricting venom flow, then the snakes contract muscles that increase intragladular pressure on the venom gland which results in venom “spitting” out from the fang orifice (Young et al., 2004).  Interestingly, it seems the cranial movements involved in this process are similar to those used during prey ingestion and hence may have an evolutionary history in common with such behavior (Young et al., 2004).  Another study found that there are two distinct patterns of venom spit by cobras (Fig. 2) and low variation in the amount of venom ejected, indicating that these snakes may have fine control over their venom delivery system (Young and O’Shea, 2005). 

Figure 2. Dispersal patterns of spits from Naja mossambica. From Young and O'Shea, 2005 with permission from the first author. Click on the image to see a larger version.

Figure 3. Single spit from an individual Naja mossambica. See figure for details. From Young and O'Shea, 2005 with permission from the first author.