Structure of venom glands in snakes

Snakes are one of the most feared species on the planet. They are widely distributed throughout most continents and are known for being extremely deadly. This stems from the large amount of snake species that are venomous. These snake species have evolved special apparatuses to inject venom and complex venom glands that produce the toxins. The structure of venom glands are not similar with every snake family though and only share a commonality in that they are encased by a fibrous sheath of connective tissue allowing muscle attachment . The family's Viperidae, Elapidae, and Atractaspididae have the most highly developed venom glands and are mostly what I will be focusing on in this blog. The Colubridae family is known to have a type of venom gland, but is said to be less developed than other venom glands and is called a Duvernoy's gland (Jackson 2003).

The viperid family venom gland is large with a triangular shape that has the longest side along the upper lip. The main gland is a complex tubular structure divided into smaller sections by in-foldings. Large quantities of venom are stored in a structure called the lumen and is the primary duct to the mucous accessory gland that extends to the fangs (Jackson 2003).

The Elapid family venom gland is an oval shape. The main gland is made up of simple tubules and the lumen is narrow, but the venom is stored within cells than in the lumen. The mucous accessory gland surrounds the entire lumen  and continues to the fangs (Jackson 2003).

The Atractaspidid venom gland is cylindrical and extends back along the body well behind the head. The lumen extends the length of the main gland with tubules radiating outward. There is no accessory gland, but mucous cells line the lumen (Jackson 2003).

These are the main differences between the venom glands and they have evolved differentially over time. It is believed that the Viperidae, Elapidae, and Atractadpididae family lineages evolved their venom glands from the Duvernoy's gland. The venom glands for the family's are hypothesized to have had three separate evolutions and the Duvernoys gland only had one at an early point in time (Jackson 2003).

Jackson, K. (2003) 'The evolution of venom-delivery systems in snakes', Zoological Journal of the Linnean Society, Vol. 137, Iss. 3, Pgs. 337-354

Venom Metering for Prey

For snakes it was found that venom was expended by coordination between various muscles controlling the jaw, fang, and venom gland (Harrison et al. 2008 ). These muscles that control the amount of venom expended are believed to be controlled by the central nervous system allowing for the snake to meter the amount of venom given (Gennaro et al. 2001). This could also be how many other species are able to meter venom for predatory use but has not been fully studied. Venom metering for predatory strikes has been linked to the size of the prey, the amount of struggle, and the type of the prey species.

Prairie Rattlesnakes (Crotalus viridis) were found to control the amount of venom given for different species. For mice it would bite and release to envonmate it, but for songbirds it was found to hold the prey. This is believed to reduce the risk of the songbird flying away and the rattlesnake losing its meal, but it also increases the prairie rattlesnakes risk for injury. To reduce this risk, the rattlesnake will increase the amount of venom given to immobilize the songbird before it can hurt it (Gennaro et al. 2001).

The wandering spider (Cupiennius salei) was found to be capable of injecting precise venom quantities in order to immobilize its meal. The wandering spider will inject less venom in insect species that are un-problematic and have less of a chance of escape. Stick insects and crickets are easily overcome, but beetles and blowflies have higher risks and are found to receive more venom because of it. In the case of blowflies it was found that they produce high frequent vibrations with their wings when trying to fly away, which cause the spider to give more venom so as not to lose its prey. Beetles were found to receive the highest amount of venom most likely due to their hard sclerotisation and chance of hurting the spider (Kuhn-Nentwig et al. 2002).

The Cottonmouth (Agkistrodon piscivorous) was found to regulate the amount of venom given due to the size of its prey. A study found that the cottonmouth would inject similar venom into mice and rats, but higher quantities into guinea pigs. This was also found in the Northern Pacific rattlesnake (Crotalus oreganus) between large and small mice. This show us that there is an intrinsic ability for the snake to control the amount of venom given (Gennaro et al. 2001).

Gennaro, J. Hayes, W. Herbert, S. Rehling, G. (2001) 'Factors that influence venom expenditure by viperid and other snakes during predatory and defensive contexts'. In Biology of the Vipers (ed. Schuett, G. Hoggren, M. Greene, H). Traverse City: Biological Sciences Press

Harrison, J. Hayes, W. Herbert, S. Wiley, H. (2008) 'Spitting versus biting: Differential Venom Gland Contraction Regulates Venom Expenditure in the Black-Necked Spitting Cobra: Naja nigricollis nigricollis'. Journal of Herpetology, Vol. 42, Iss. 3, Pgs. 453-460.

Kuhn-Nentwig, L. Nentwig, W. Wigger, E. (2002) 'The venom optimisation hypothesis: a spider injects large venom quantities only into difficult prey types'. Toxicon, Vol. 40, Iss. 6, Pgs. 749-752.

Venom Metering for defense

In my first blog I touched a little about how metabolic cost can be a restraint for the use of venom. In this blog I am going to go into further detail about how certain species use venom metering to not exceed their metabolic limits for defense. Venom metering is the use of venom in the most economically way possible for both defensive and predatory actions (Hayes et al. 2014).  In many venomous species it is believed that due to the high metabolic rate many species will deliver a 'dry' bite, which is a bite that contains no venom, to defend itself (Hayes et al. 2014). This is a behavioural adaptation to conserve venom for prey and high risk situations. In my next article I will talk about venom metering for offense.
It was shown in the Western Black Widow (Latrodectus hesperus) that it uses venom metering in defense. Hayes, Kelln, and Nelson (2014) found that spiders in low threat levels many spiders either tried to play dead or escape and in high threat levels would bite 60% of the time. It was also found that a majority of single bites when provoked were 'dry' bites. When pinched on the body the Black Widow would deliver 1.8-fold more venom in a bite then if pinched on the legs (Hayes et al. 2014). This shows that the Black Widow is metering its amount of venom due to the higher risk of danger to the body then legs.
Another study on the Dark scorpion (Parabuthus transvaalicus) found that in low threat situations the scorpion would deliver 'dry' stings, but also found that if the threat persisted it was more likely to envenomate (Hayes and Nisani 2011). When in a high threat situation the scorpion would produce 2-fold more venom per sting then during a low threat (Hayes and Nisani 2011). Some scorpion species also has an extra ability in being able to produce a pre-venom. The study found that scorpions with low threat situations delivered the pre-venom first and if the threat persisted it would switch to its other venom, but during a high threat situation the scorpion would skip the pre-venom and use its main venom (Hayes and Nisani 2011). This is due to the pre-venom being less energetic costly then its main protein-rich venom.

Hayes, W. Kelln, W. Nelsen, D. (2014) 'Poke but don't pinch: risk assessment and venom metering in the Western Black Widow spider, Latrodectus hesperus'. Animal Behaviour, Vol. 89, Pgs. 107-114. 

Hayes, W. Nisani, Z. (2011) 'Defensive stinging by Parabathus transvaalicus scorpions: risk assessment and venom metering'. Animal Behaviour, Vol. 81, Iss. 3, Pgs. 622-633.

Diet and Venom evolution

Most venomous species use their venom for the main purpose of prey capture. Over time venom composition and behaviours associated with venom producing species have evolved for specific functions, conservation of energy, and to be prey-specific. In certain venomous species they are found to be particularly more toxic to one type of prey. This is found in the mangrove snake (Boiga dendrophilia) with bird-specific toxins. Whereas the mangrove snake has specific toxins in its venom to be more lethal to one type of prey, Some venomous species diet will change their venom composition. An example of this is the Malayan pit viper (Calloselasma rhodostoma). In many venomous species it was found that envenomation has a high metabolic cost, which could be an important constraint on make-up and use of the venom. This would require the organism to either require more nutrients or adapt behavioural and physiological mechanisms due to the high energetic cost. Some examples of these mechanisms are: in several rattlesnakes the amount of venom injected correlated with the size of the prey; in certain spiders the amount of venom given is not associated with the size of the prey, but with the intensity or duration of the prey's movement; and in scorpions they have developed a pre-venom that is highly painful for low-threat encounters which requires low amounts of energy. Another challenge faced is prey-species becoming resistant over time to a particular venom. This was found in the California ground squirrel (Spermophilus Beecheyi) becoming resistant to the Northern Pacific rattlesnakes (Crotalus viridis oreganus) venom in that area (Benjamini Et al. 1987). With the all of these showing changes in individual species behaviour and venom composition it points to diet being one of the major drivers for the evolution of venom.

Benjamini, E. Coss, R. Poran, N. (1987) 'Resistance of California ground squirrels (Spermophilus beecheyi) to the venom of northern Pacific rattlesnake (Crotalus viridis oreganus): A study of adaptive variation' Toxicon, Vol 25, Iss. 7, Pgs. 767-777. Viewed 09 March 2014, < http://ac.els-cdn.com/0041010187901279/1-s2.0-0041010187901279-main.pdf?_tid=f9a02e3a-a770-11e3-bfde-00000aab0f27&acdnat=1394359121_c2a34a75d23ebbd3682b3bf2c1aec5e7>

Casewell, N. Fry, B. Harrison, R. Vonk, F. Wuster, W. (2013) 'Complex cocktails: the evolutionary novely of venoms' Trends in Ecology & Evolution, Vol 28, Iss. 4, Pgs. 219-229. Viewed 09 March 2014, <http://ac.els-cdn.com/S0169534712002935/1-s2.0-S0169534712002935-main.pdf?_tid=aaa0568a-a72e-11e3-81b3-00000aab0f6c&acdnat=1394330642_c5b63a6ff8feea5a4907351e6a08d153>