Why is Snake Venom so variable?

Based on a presentation by Dr. Wolfgang Wüster (Bangor University) – 12/03/13

Saw-scaled Viper (Echis carinatus) (Photo credit: Frupus)

Saw-scaled Viper (Echis carinatus) (Photo credit: Frupus)

I hate snakes. I’m just going to say it from the start; they scare the living daylights out of me. I’d have been living with one if my girlfriend hadn’t noticed the colour drain from my face when she mentioned buying one. And yet, for reasons I cannot explain, I went along to a seminar yesterday all about venomous snakes! I’m glad I did though – Dr. Wolfgang Wüster talked about them with great energy and enthusiasm, getting quite a few laughs along the way, and, most importantly, piquing the entire lecture theatre’s interest. I found the talk so engaging that I’ve decided to share what I learned here.

Snake venoms are mixtures of toxins, usually consisting of tens to hundreds of the poisonous proteins. This obviously allows for a great degree of variation in nature as different venoms contain different combinations of toxins and quantities thereof. As you’d expect, lots of different species of snake have different toxins; however, the variation can go all the way down to differences between members of the same species. In fact, in some species, an adult’s venom can be different to its venom as an infant. This wide range of venoms has an equally diverse range of effects on prey, resulting in paralysis, haemorrhages, and massive cell death and tissue damage, amongst other things. Upon explaining this in the talk, Dr. Wüster took great pleasure in showing some truly disgusting images – remember; I go through the pains of Science so you don’t have to!

Common symptoms of any kind of snake bite poisoning (Photo credit: Wikipedia)

Common symptoms of any kind of snake bite poisoning (Photo credit: Wikipedia)

The main question of the talk was that of what causes this variation in venom composition. It’s probable that this all depends on what individual venoms are used for, which, in the majority of cases is overpowering and killing prey. Diet in snakes is an example of a ‘selective pressure’. This is where something affects the survival of a population, thus encouraging evolution of that population to overcome the stress.

Diet, as a selective pressure, acts upon many characteristics of venom. For example, the volume of venom stored in a snake’s glands is usually only enough to kill enough prey to survive. As such, snakes requiring a greater food intake or those that kill larger prey will produce more venom than those that consume less food. The overriding reason for this is that producing venom requires energy, so the minimal amount necessary is made and used.

Dr. Wüster’s group saw an interesting example of this in a model system of 4 species of Saw-scaled Vipers. Whilst most snakes eat vertebrates (animals with a backbone), these vipers also eat arthropods (invertebrates with exoskeletons and segmented bodies, such as scorpions and spiders). The 4 species differ greatly in the ratios of arthropods and vertebrates that they eat, yet all 4 species take 2 to 3 bites to kill scorpions, taking their time to see how much venom is necessary to subdue their prey. This may be evidence of economy of usage of venom, meaning that these model organisms have evolved to favour potent, rather than voluminous, venom to reduce the amount required.

Anatomy of a venomous snake’s head (Photo Credit: How Stuff Works)

Anatomy of a venomous snake’s head (Photo Credit: How Stuff Works)

Prey resistance also plays a role in determining the volume of venom a snake produces, as well as the potency of that venom. For example, Moray Eels that live in the same regions as Sea Snakes have evolved resistance to the snakes’ venom. As a direct consequence of this the snakes have evolved to produce and release more of it to compensate.

In conclusion, Dr. Wüster presented compelling evidence that venom composition differs based on dietary requirements. Different combinations of toxins affect different preys, and different snakes need their venoms to have different harmful effects. The ‘arms race’ that develops from predator-prey relationships, whereby the prey evolves to resist the venom and the snake evolves to counteract this, also drives diversification. Finally, using venom economically seems to be a very important factor in these predators. Dr Wüster explained that future work would take a detailed look at the genetics behind venom variation, studying the genes encoding toxins and the variation that exists therein. I, for one, look forward to hearing about their findings, even if it does mean spending more time looking at pictures and videos of snakes…

This post, by author Ian Wilson, was kindly donated by the Scouse Science Alliance and the original text can be found here.

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