If the diet of most snakes consists of smallish rodents and lizards, then why do some of them have venom potent enough to kill very large creatures (like us)? Why the overkill? Wouldn’t a less toxic venom serve just as well?
Just a WAG, but my guess is that it’s not about killing prey so much as protecting oneself from larger predators.
Another guess, but I seem to recall it was more a question of the speed of kill. A snake with more toxic venom will make the kill more quickly, meaning it doesn’t have to go as far to find its kill after inflicting the bite. I suppose it would also help reduce the possibility of the animal fighting back and causing the snake harm.
Just a guess, based on way too many nature programs from long ago.
AFAIK it’s just basic natural selection at work. Mostly a snake doesn’t need that amount of venom to kill its prey. However occasionally a snake will need more potent venom. It may miss slightly with a strike and only inject a tiny portion of the total venom volume, the rest going into the fur. It may miss altogether and inject the entire load into the feathers. The second strike will then be using smaller amounts of venom. Whatever the cause, just occasionally having more potent venom and/or larger amounts of venom will mean the difference between life and death. That tiny amount is sufficient to select for snakes with more potent venom. Given enough time snakes will evolve into forms with more potent venom.
It’s a bit like asking why peregrines can fly so fast when they only need to be marginally faster than their prey to survive. Of course the don’t just need to be faster than their prey, they also need to be faster than their competition. Same for snakes. They don’t just need to be able to kill their prey, they need to be able to kill more prey than their competitors.
When a large animal, such as a horse, sees a snake, it panics and runs away. The overly deadly venom is what prevents a snake from being stepped on by the horse.
Not really. Any venom is what is stopping it, and it’s quite obvious why snakes need venom. However all that the snake would need is sufficient venom to make a horse feel pain to provoke the same reaction. Ever been riding a horse when a bot fly is around? Same reaction. Horses also present the same reaction to hornets and bees.
Instead we have a situation where snakes produce enough venom to kill a dozen healthy horses. That’s clear overkill if all you need to do is make the horse leave you alone. Horses will leave almost any animal alone it it knows it can cause it pain. There’s no profit for a horse on taking on even a non-venomous snake.
In addition to Blake’s contribution, there is the point that for many snakes the venom also has other purposes and a barely lethal dose may not be enough to satisfy those other requirements.
For example a Western Rattlesnake ( Crotalus viridis ) typically injects three hundred times the lethal dose into a mouse in a bite. But viper venoms in very high concentrations seem to kill much more quickly by inducing massive internal clotting, reducing the energetic cost associated with trailing and perhaps extricating dieing prey. Further the venom performs a very significant digestive function, breaking down the skin and exposing the body cavity much more quickly than in an unvenomated prey item, potentially very significant in low temperture regimes in particular.
- Tamerlane
While we’re on the subject of snakes’ venom being an evolutionary development, has its prey also developed, to some degree, a level of resistance to such poisons? If a mouse (historic prey) was the same size as a human (not a prey of snakes) were both bitten by the same snake, would the mouse have a better chance or survival?
Not necessarily! Horses will often trample a snake rather than running away. Especially during springtime (foaling season).
Why? Well, I’ve always assumed this was done in the spirit of “protecting the herd”.
A couple of people have mentioned snakes trailing prey after biting them. Does this actually happen? Every time I’ve seen a snake attack a prey animal it bites and hold on for grim death until the struggels cease. Do some snake species actually bite and then release to wait for the prey to kick the bucket?
Not AFAIK.
The trouble is that resistance to snake venom isn’t likely to be selected for because the chances of being exposed and surviving are so slim. Every individual snake might hit prey with a sub-lethal dose at least once in its lifespan. That’s a pretty fair driver for the evolution of more potent venoms. In contrast only one in a million rats of any given species will ever be injected with a sublethal dose. An even smaller number will be injected and then go on to survive both the snake attack and the complications associated with poisoning. Not much chance of being selected for really.
Apparently with vipers this is generally the case. While venomous colubrids and elapids generally grab and hold, waiting for struggles to cease, vipers tend to strike and release ( maybe partially to protect their longer, more fragile fangs, though that’s just a WAG on my part ), then re-locate the prey, which at leastsometimes involves trailing a critter that has fled some yards away. I’ve seen some film of it with rattlesnakes.
- Tamerlane
Interesting. And strange that the vipers generaly have less potent venom than the colubrids and elepids. You’d think with behaviour like that they would have been under even greater pressure to evolve more potent and faster acting venom.
Hmmm…I suppose that’s true on average. Though there are certainly some very nasty vipers out there and Russell’s Viper probably causes more fatalities year in and year out than any other species of snake and in North America the most dangerous snake are probably certain neurotoxic populations of the Mojave Rattlesnake ( Coral Snaks being far more inoffensive critters than the very aggressive, highly toxic Mojave Green’s ).
I wonder if there is some correlation with diet. I know in the Northern Pacific Rattlesnake there is an developmental shift in venom with age and diet, with the lizard-eating juveniles having a greater toxicity, but the rodent-eating adults having more proteases to aid in digestion.
Well a lot of them are pretty bad. But I suppose the other argument is that a strike and release habit is more tolerant of slower-working venoms, because the snake doesn’t have to worry about a thrashing prey item injuring it. shrug
Also, a mouse or lizard probably doesn’t give a hoot if a given dose of venom has a LD50 10x greater than another, if it’s still enough to kill them 20x over :).
- Tamerlane
I have an interesting anecdote that I’m thinking the resident herpetologists could comment on. Anyway, I met a woman who grew up in the Austrian Alps on a dairy farm. She said snakes were a huge problem because they’d prey on cattle! It was common for snakes to wind up the legs of their cows, attach to the udder and drink the milk. Now they weren’t venomous enough to kill the cow entirely but it would kill the udder and make them useless for milking. The idea of it sends shivers through me but I’m wondering if anybody else has heard such stories and if it is known to happen.
The reason some snakes have more potent toxin then others is because of the need for speed and certainty. For example, many times the most potent venoms will be places where prey is not easily available (like Australia’s outback and the taipan), so the snake needs to be able to make sure the prey is dead every time it makes a bite. Overkill is the best way to accomplish that.
In addition, a more potent venom will ensure a bite kills prey quickly, making it less likely that the prey runs off after being bitten and making a meal for some other animal. Especially for those snakes that prey on larger animals that it might be dangerous to hold on to (risk of fang damage during struggles). So it’s best that the venom works extremely quickly.
In addition to being toxic and killing the prey, the rattlesnake’s venom is part of the digestive process. It breaks down the flesh of the victim so that it is partialy pre-digested even before being swallowed. Maybe the more venom the easier to digest and that might be an advantage.
I know that some venoms aid in breaking down tissues (cytotoxic) and make the prey easier for the snake to digest, but what about the cases with neurotoxic venoms? I would suppose it’s the onset of action that would be improved by having a stronger venom, (i.e. having the mouse stop wiggling sooner) but can anyone back me up toxicologically? Does the potency of the venom have an effect on the onset of action, or is it only going to affect the LD50?
Rattlesnakes can also control the amount of venom injected. From my reading, many rattlesnake strikes of large animals, which would be defensive strikes and not for food, are relatively “dry,” i.e. with little venom.
Apparently venom production is relatively “expensive” energywise for the snake and the whole supply isn’t necessarily used every strike.
This is an old story, dating back to at least the 1800’s. (See example here .)
But it isn’t true, as thi page about the Iowa milk snake shows.
In fact, it’s obviously untrue when you think about it. Snakes have not developed the enzyme needed to digest milk (why would they, when it’s not a normal part of their diet?). So snakes are in effect lactose-intolerant, as some humans are. The idea that they would risk their lives to climb up to a cows’ udder, in order to drink something that would make them sick is silly. And how, exactly, would they squeeze the teat to release the milk?
But it makes a fun story to tell to gullible city folk!
P.S. On another topic, the page cited above notes that Milk Snakes will eat other venomous snakes, and are at least partially immune to their venom. So some snakes have developed immunity to snake venom.