|Puff-adders probably cause more human snake-bites than any
snake, but are rarely fatal. This is a juvenile, but don't think it's harmless.
This is a topic that's puzzled lots of people for quite a long time, and a number of different answers have been proposed with no one clear winner. The two main threads of answers have been proposed: firstly, that there's no effective natural selection on snake venom, once evolved, venom toxicity just drifts along. Others suggest exactly the opposite: that it must be a trait that is subject to extremely high selection pressures to have evolved as it has. Now, the argument for a neutral drift stems mainly from the observation that non-venomous snakes exist and, indeed thrive: if one species manages without venom, but another has it and both do just fine, it would seem clear that evolution has nothing to do with it. On the surface it's a pretty convincing idea, but I wonder too if there's some slightly dubious thinking going on here. Remember the thorn story: many plants here have thorns, but the animals eat them anyway, so why should they have evolved? I think the same sort of idea is happening here: we're looking at a situation that may seem daft today, but wasn't before.
|Female Boomslang - you'll probably die painfully |
in 2-5 days if not treated. Happily, they're rather shy.
|Boomslang showing rear fangs typical of colubrid snakes. |
Don't try this at home folks...
Just as bacteria can evolve resistance to our antibiotics, so too can resistance evolve in prey to snake venom. Now, resistance is most likely to evolve in bacteria when the doses aren't kept high enough for long enough to kill everything - that's why it's crucial to complete a course of antibiotics (if you don't, there might be some bacteria that were nearly resistant still alive when you stop, and you've just selected very strongly for those strains that are close to evolving proper resistance). It's surely the same with snake venom: it's much harder to evolve resistance to a massive dose of a toxic nasty, than it is to a smaller dose that some individuals might just survive. We also use multiple antibiotics when we're trying to protect against bacteria developing resistance (it's much harder to simultaneously evolve resistance to two antibiotics than just one), which seems like a plausible reason for why venom is full of a whole range of toxins too. Despite this, resistance can still evolve in prey animals, so the doses need to keep getting larger and larger - just like thorns do - with the result that when used in self defence against an animals that has no evolved immunity because it's so rare to get bitten, the dose is incredibly toxic. All good reasons for why evolution might well lead to what looks like 'overkill' in snake venom.
|Tiger snakes are only the tiniest bit venomous (no human risk), and use both |
constriction and venom to subdue birds, bats, and other retiles.
Now, what we haven't talked about is what it actually feels like to be bitten by one of these, and it so happens that I know there's at least one person involved the blog who could give us a first hand account if he were feeling inspired in the comments? [He's in good company, by the way - estimates suggest about 43,000 venomous snake-bites each year in East Sub-saharan Africa.]
PS a word of advice if you do get bitten by a venomous snake - if you can identify it, great, if not please don't got thrashing about after it to kill it or get a better look. Most snake-bites can be identified sufficiently from their toxic effects and many anti-venin these days can target a group of snakes. If you (or your friends) go thrashing around after a venomous snake that's already scared the most likely outcome is someone else getting bitten too which doesn't help things at all. Do get yourself somewhere with anti-venin as fast as possible though...
Mebs, D. (2001). Toxicity in animals. Trends in evolution? Toxicon, 39 (1), 87-96 DOI: 10.1016/S0041-0101(00)00155-0
Barlow, A., Pook, C., Harrison, R., & Wuster, W. (2009). Coevolution of diet and prey-specific venom activity supports the role of selection in snake venom evolution Proceedings of the Royal Society B: Biological Sciences, 276 (1666), 2443-2449 DOI: 10.1098/rspb.2009.0048