Spitting Snakes

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).

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.

Image

Naja sumatrana, Nick Weigner

Ontogenetic Shifts in Venom Composition

When venomous snakes are born they already possess the apparatus and venom to deliver a toxic bite. It is an essential ability in order for the young to feed and defend themselves immediately as this is when they are most susceptible to predation. As growth from young to old is closely linked to preferential prey it is likely a that this change corresponds to more accurately target its prey at that age (Gibbs et al. 2011).

Venoms are an efficient and targeted tool to aid in feeding, this specialisation of venoms leads to the diverse forms we see today. As the diet of the snakes change so must its venom to ensure it is effective. Investigations into the composition shifts in venoms over a snakes lifetime has been carried out on many species particularly the Crotaline snakes (rattle snakes and relatives). Many of these snakes feed on frogs or small lizards when young. In the case of Crotalus oreganus a higher percentage of myotoxins are found in the venom of older snakes that have progressed to feeding on small mammals.The younger specimens showed more neurotoxic venom while still at a similar overall toxicity to the adults (Mackessy et al. 2003). 

This adaptation can cause greater difficulty in producing an effective treatment for bites. The most effective antivenom, polyvalent antivenom, is produced to target specific component of the venom.Should the venom be varied due to ontogenetic shifts a bite from a juvenile may not be effectively treated by an antivenom produced for adults venom. Natural variation in populations and between populations in different localities and conditions also presents the same concern (McCue. 2006).


References 

Mackessy, S.P., Williams, K. and Ashton, K.G., (2003). “Ontogenetic variation in venom composition and diet of Crotalus oreganus concolor: a case of venom paedomorphosis?.” Copeia, Vol: 2003 No: 4, pp.769-782.
McCue M.D. (2006). “Cost of Producing Venom in Three North American Pitviper Species.” Copeia, Vol: 2006, No: 4, pp. 818-825.
Gibbs, H.L., Sanz, L., Chiucchi, J.E., Farrell, T.M. and Calvete, J.J., (2011). “Proteomic analysis of ontogenetic and diet-related changes in venom composition of juvenile and adult Dusky Pigmy rattlesnakes (Sistrurus miliarius barbouri).” Journal of proteomics, Vol: 74 No:10, pp.2169-2179.

Image 
http://www.stuartdahnephotography.com/keyword/crotalus%20tigris%20the%20tiger%20rattler/i-xttv4cn accessed: 13/4/16

Toxicofera

Phylogenetic of the venomous reptiles is a broadly debated. Many different classifications previously presented used morphological features to define clades. The Toxicofera theory was born after genetic testing of the Squamata order showed serious discrepancies between this morphological grouping and their genetic relationships (Vidal & Hedges. 2005). Further testing of this group gave greater support for the new classification of toxicofera based on the genetic similarities, primarily the presence of venom coding genes within three of the Squamata groupings (Fry et al. 2009a). 

The extant taxa forming this clade includes all Ophidia (snakes), Anguimorpha (lizards including monitors) and the Iguanians (lizards including dragons, chameleons and of course iguanas). All the known venomous reptiles belong to this group, however some families in the group are not venomous. It is thought that the non-venomous members have simply lost the venom production ability to some degree (Fry et al. 2009a).

New evidence is constantly being uncovered to support this clade. A study on anguimorphs showed a toxin homologous with those of snakes and functional venom glands have been discovered in the jaw of Komodo dragons Varanus komodoensis dispelling the previously held belief that it was only bacteria that forms the toxic bite by these gargantuous lizard (Fry et al. 2010; Fry et al. 2009b).

The common ancestor to all toxicoferan had a host of core venom genes. Along with other toxin recruitment, these are the original genes which have diversified into the venoms we see in many species today (Fry et al. 2009a).

References 

Fry, BG, Winter, K, Norman, JA, Roelants, K, Nabuurs, RJA, et al. 2010, ‘Functional and structural diversification of the Anguimorpha lizard venom system’, Molecular & Cellular Proteomics, Vol: 9, No: 11, pp. 2369-2390.

Fry, BG, Vidal, N, van der Weerd, L, Kochva, E & Renjifo, C. 2009a, ‘Evolution and diversification of the Toxicofera reptile venom system’, Journal of Proteomics, Vol: 72, No: 2, pp. 127-136.

Fry, BG, Wroe, S, Teeuwisse, W, van Osch, MJP, Moreno, et al. 2009b, ‘A central role for venom in predation by Varanus komodoensis (Komodo Dragon) and the extinct giant Varanus (Megalania) priscus’, Proceedings of the National Academy of Sciences of the United States of America, Vol:106, No: 22, pp. 8969-8974.

Vidal, N & Hedges, SB. 2005, ‘The phylogeny of squamate reptiles (lizards, snakes, and amphsbaenians) inferred from nine nuclear protein-coding genes’, Comptes rendus – Biologies, Vol: 328, No: 10, pp. 1000-1008.

Image

http://www.bbc.com/earth/story/20160226-the-islands-where-dragons-are-real, accessed 5/4/2015