viperids

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The West African Gaboon Viper (Bitis gabonica rhinoceros) is distinguished from the East African subspecies (B. g. gabonica) by its striking nasal scales, not unlike those of the Rhinoceros Viper (B. nasicornis). Gaboon Vipers possess the longest fangs of all viperids, reaching up to 5cm (2 inches) long, and produce the highest venom yield. 

 Photos by Patrick Jean at the Museum de Nantes

anonymous asked:

Can you explain why some snakes evolve to be not venomous while some have really powerful venom?

This is an excellent question.

The simple answer is ecology: the niche the snake occupies and the prey it eats are the two biggest driving factors. Predation may also be key.

But for a more detailed answer, we have to look further back down the evolutionary tree.

Snakes are deeply nested within lizards - that is to say, all snakes are really one giant radiation of legless lizards. Dr. Brian Fry has produced a lot of research pointing toward one conclusion: the ancestral condition of the clade containing snakes that also contains iguanids and anguimorphs, known as the Toxicofera, is the possession of venom-like proteins. In some lineages, these have been modified to form the powerful venom known from viperid snakes etc. In others, it has been almost or completely lost.

Fry et al. 2013 (downloadable for free if you have a ResearchGate account) discussed the loss and adaptation of venom proteins across the Toxicofera, and this and other papers make a really good foundation for understanding the evolution of venom in reptiles.

So the phrasing of your question is actually just right: from an ancestral position of having some venom proteins (that does not make them venomous - venom proteins were probably associated with oral hygiene rather than for their current, more macabre implementation), some snakes have lost these proteins, while others have modified them extensively to make them true killing machines.

So why? Well, consider an arboreal snake. When it kills prey, it needs to minimise the struggling of that prey so that it itself is not thrown from the tree by its dying prey. Venom can greatly decrease this risk.

But venom is not the only answer to this problem. Two other strategies exist: Arboreal booids (Pythons, Boas, etc.) have extremely long, curved teeth, that help them latch on to any prey they capture, and these, coupled with their immensely strong muscles, allow them to stay up in the trees with even the most violently struggling prey. Many true tree snakes are non-venomous and also not quite as muscular as booid snakes. Typically these will take smaller prey that is less likely to struggle and throw them from the tree. They also often have specialised teeth that give them good grip on their prey - hyacynthus is probably one of the best people in the world to talk to about this right now, as she just finished her MSc project on the dentition of Madagascar’s snakes, one radiation of which are predominantly arboreal.

Why are there groups of arboreal snakes that are venomous and non-venomous then? The key is phylogeny. While the possession of venom is able to change from presence to absence and the reverse over evolutionary time, these transitions are not always enacted every time a snake takes to the trees. There is more than one way to live among the branches, and the evolutionary history of a snake is more predictive of whether or not it will possess venom than its ecological niche.

Prey is also not the only reason for evolving venom. There is a school of thought that venom evolved for defensive, rather than offensive, purposes. If defence is the main objective, the strength of the venom could perhaps be proportional to the level of predation suffered. By way of example, a mamba’s bite can inject enough venom to kill a young elephant - it takes something like 15 mg of venom to kill a human, the bite can inject up to 400 mg (100 mg is apparently typical). Why would any snake need to inject so much venom, when a tiny fraction of the amount would be more than sufficient to kill any prey item? Defence.

I rather think venom has been co-opted for defensive purposes, after initially having been derived from hygiene proteins to serve a predatory function. The jury is largely still out on this though. There may be an unlimited directional selection on the volume and strength of venom - sure, it’s good to have a minimum that is able to subdue your prey, but prey items don’t get any more dead the stronger your venom after that threshold. However, there are also no caveats on having stronger venom, and it is better to be more defensive.

Armillifer grandis

…is a species of Pentastomid crustacean (commonly known as tongue worms) distributed throughout tropical Central and West Africa . Like other pentastomids A. grandis is an internal parasite on a range of Viperid snakes (Notably Bitis gabonica, B. nasicornis, and Cerastes cerastes). Armilifer grandis will infect the nasal passage and feed on the blood. I'ts possible for humans to be accidentally infect by the eggs if they come in contact with infected snakes, this will then cause porocephalosis, which can be debilitating or even lethal in some cases.

Classification

Animalia-Arthropoda-Crustacea-Maxillopoda-Pentastomida-Porocephalidae-Armillifer-A. grandis

Image: Lajos Rozsa