Wouldn’t the presence of venom glands favor snake’s survival and hunting? If yes then why non-venomous snakes are present? Pythons hunt by constricting their prey, so wouldn’t presence of venom glands ease their feeding, and also for self-defense?
Producing venom is energetically costly – if it’s not necessary, it may well not be favored. It’s also possible that venom simply didn’t evolve in some lineages, as a matter of chance. There are suggestions about the evolution of venom in reptiles that would argue against that, though – at least one researcher has argued that venom first evolved in an early lizard lineage, ancestors of monitor lizards, several other lizard groups, and the snakes.
A similar question recently spurred some interesting discussion of “why” questions
An analogy might be that there are many ways to make your bed…but we tend to do it the way our mothers taught us (inheritance). No one way is necessarily better than another, and it could simply be a matter of chance along life’s evolutionary path that led to one way or another of obtaining prey.
Venom is effective for taking down large prey that can’t be incapacitated in other ways. Pit vipers have the most developed mechanisms for venom delivery, which allows them to strike and back off to avoid injury. But constriction is also effective, albeit perhaps more dangerous to the snake, to incapacitate large prey. If the snake is mainly taking smaller prey that can be grabbed, held, and swallowed while still kicking, neither is really needed. Although there are smaller snakes that take rather small (by comparison) prey that also have venom that is delivered while the prey is held. A lot of evolutionary diversity in how a legless animal can safely get an intact prey item into its belly.
There are of course different potencies of venom in different snakes. I recall hearing there was some evidence that even gartersnakes (Thamnophis) have venomous properties in their saliva (which is the precursor to venom) but don’t recall if that was ever proven. Those are basically grab-it-and-swallow species.
It always intrigued me that the most potent snake venom belongs to a marine reptile, sea snakes; but I suppose it makes sense, given how quick and slippery a lot of fishes and other marine prey are, needing to be incapacitated quickly. The oceans have more venomous predators than any other ecosystem, likely for the same reason.
Quite a few rear-fanged snake (RFS) species outside of Elapidae and Viperidae that we traditionally think of as “non-venomous” do in fact produce venom (though gland, fang, and muscle structures differ considerably from elapids and viperids). In the US for example, Thamnophis, Heterodon, Hypsiglena, Tantilla, Trimorphodon, and Diadophis (just to name a few) all produce and utilize venom in prey capture. The majority of these pose little-to-no threat to humans, however.
Outside of the US, many RFS do pose a medical threat to humans, and have caused severe envenomations and/or deaths, including Conophis, Dispholidus, and Thelotornis.
A few good, recent reviews on RFS venoms are Saviola et al. (2014), Junquiera de Azevedo et al. (2016), and Modahl & Mackessy (2019), though these focus primarily on proteins in the venom rather than the snakes themselves. This is a burgeoning field so many papers have been published since that 2019 review (with many more to come) but they are still good starting points.
As someone already pointed out, “why”-questions are problematic in biology. You’ll never get a satisfying answer for these questions because they cannot be tackled experimentally. The only reason for why there are different strategies in snakes is because the different strategies work very well for the individual lineages.
Moreover, there is no simple “bifurcation of venomous and non-venomous snakes”. It’s apparently still under debate whether venom evolved once and got lost several times in different lineages or whether it evolved several times independently. If you want to know why different lineages don’t possess any venom you always have to specifically look at specific lineages. These lineages live in different habitats, have different lifestyles, different body sizes, different metabolic rates etc. and therefore there are countless different reasons for why they might not have a need for venom. To think that there are the same universal reasons for all of these events is a bit too simplistic.
Venom of course seems to be very beneficial but it also comes with a downside: it requires resources to be produced. When you are a gigantic snake like a python or an anaconda then there’s just no necessity to invest into the production of venom when you can easily overwhelm all of your desired prey items just by constriction. Also for snakes which exclusively feed on eggs it would be a waste of resources to still produce venom.
One of a more general driving force that I could think of could be a shift in behaviour. If there is an event in a lineage of snakes that possess venom which leads to a dietary shift it could render the venom unnecessary. One hypothetical scenario could be that a species that usually feeds on young birds in the nest suddenly starts to feed on the eggs instead. In this scenario there would be no positive selection pressure anymore that would act on the integrity of the gene regulatory network that is required to produde the venom. If this selection pressure is missing, random mutations (that occur all the time in the genome) happening in these particular genes will not be weeded out of the gene pool because it simple doesn’t matter if the snake can produce venom or if it can’t. Therefore, over a long period of time the venom might become weaker, the amount being produced might become less and at some point the species might completely lose the ability to produce venom.
There is a wikipedia article specifically on the topic of the evolution of snake venom (https://en.wikipedia.org/wiki/Evolution_of_snake_venom). It might be of interest to you and point you to some further literature on this topic.
Thank you so much sir for your detailed answer
Thanks for sharing a wonderful thread
Water plays bigger role imo, more venom is just lost from the wounds.
“Venom” is a pretty complicated thing with different snakes using different substances. Venom is also more than a means of incapacitating prey. Venoms of many snakes (e.g. rattlesnakes) aid digestion by breaking down tissues, which is why bites from these snakes can result in long-term or permanent damage in survivors. Not all snake venoms include such components and some are pretty much exclusively neurotoxic. For such species (e.g. coral snakes) survivors generally recover without lasting damage.
Any explanation of the evolutionary costs and benefits of venom needs to account for a pretty complex range of types and functions in a variety of ecological contexts.
This gives a sense of the complexity.
Self snake bites?
I would not know if this is an evolutionary pressure (or even how true it is). Yet, I have been told that rattlesnakes sometimes bite themselves, say accidentally during a struggle with another snake or animal; and that they are susceptible to their own venom.
I also read, though I cannot find a link to the reference at the moment), that gopher snakes (non-venomous snakes that resemble rattlesnakes) often have more resistance to rattlesnake venom than the rattlesnakes themselves have.
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