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| Naja sumatrana spraying venom. |
Some animals have developed the ability to spray chemicals on perceived attackers as a form of defence. While many of the species abilities are very efficient such as the skunk and bombardier beetle (which sprays a liquid at 100°C; Eisner, 1958), the capability of some snake species to accurately spray venom up to 2m takes the evolutionary cake (Young et al, 2004).
Some species of Naja (cobras) and the Rinkhals of Hemachatus genus have separately evolved unique fang structure to produce an accurate ranged defence. Some vipers, primarily the Mangshan Viper (Zhaoermia mangshanensis) of China have been reported to “spit” venom. These cases are put down to a larger venom yield and hissing action, they have not been shown to employ “intentionally aimed spitting” unlike the Spitting Cobras and Rinkhals (Young et al, 2004).Spitting is only used as a defence mechanism and not for hunting. These snakes still use their fangs normally to deliver a fatal bite whilst hunting (Wüster & Thorpe, 1992).
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| Fig 1. Fang structure of Spitting and Non-spitting cobras. Note the size and shape of the exit orifice (Wüster & Thorpe, 1992). |
Venom is produced as per normal in the venom glands but the ability comes from the small elliptical exit point at the terminal end of the fang (Fig 1.). The venom is pushed into the fang under great pressure, formed by the contraction of muscles around the venom gland (Young et al, 2004; Wüster & Thorpe, 1992).The head is tilted back and often with a shaking motion of the head the venom is pushed out the smaller rounder exit hole (Wüster & Thorpe, 1992).
The venoms from most of the spitting Naja contain Neurotoxins and Cytotoxins (Wüster & Thorpe, 1992).
References
Eisner, T. (1958). "The protective role of the spray mechanism of the bombardier beetle, Brachynus ballistarius Lec." Journal of Insect Physiology,Vol:2, No:3, pp.215-220.
Wüster, W. and Thorpe, R.S. (1992). "Dentitional phenomena in cobras revisited: spitting and fang structure in the Asiatic species of Naja (Serpentes: Elapidae)." Herpetologica, Vol: 48, No: 4, pp.424-434.
Young, B.A., Dunlap, K., Koenig, K. and Singer, M. (2004). "The buccal buckle: the functional morphology of venom spitting in cobras." Journal of Experimental Biology, Vol: 207, No: 20, pp.3483-3494.
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Naja sumatrana, Nick Weigner
Fascinating stuff! The size of the hole is related to the pressure, which is cool. But does this then mean that the animals deliver less venom when they bite some, or more because the pressure build up is higher? Are their venoms more potent as a consequence?
ReplyDeleteThere seems to be no effect on the venom delivery with significant variation in the venom yield from spitters relative to non spitters.Venom potency also seems independant to spitting ability although the 2nd lowest LD50 of a Naja species is a spitter.
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