Sunday, April 5, 2020

Snake Venom Evolved for Prey, Not Defense



“Fangs for the Memories? A Survey of Pain in Snakebite Patients Does Not Support a Strong Role for Defense in the Evolution of Snake Venom Composition”
Harry Ward-Smith, Kevin Arbuckle, Arno Naude, and Wolfgang Wuster
Toxins journal 2020
https://www.mdpi.com/2072-6651/12/3/201

                A paper recently published in the Toxins journal shows evidence that snake venom evolved for prey capture and not as a defense mechanism. There are many creatures that use venom or poison as a means to deter predators – bees, scorpions, fish, and so on. In order to be an effective deterrent, the sting or bite has to inflict enough pain quickly enough to leave the potential predator incapable of continuing an attack. The authors of this article sought to examine the level of pain induced by venomous snake bite, and the trajectory of that pain through time. While some animals cause a distracting level of pain immediately, such as the aforementioned Hymenoptera, this did not seem to be the case with the majority of snake bites.
                The researchers collected information from people who had experienced snake bite either with a captive animal or in the wild, gathering a total of 584 bite reports. This encompassed 192 different snake species. Analysis of pain level was accomplished by having respondents note their pain at <1min post bite, 1-5min post bite, and max pain experienced at any time. Interestingly, over 50% of people never experienced a pain level that would be great enough to interfere with normal activity. Only about 14% of people noted this high-level distracting pain in the first 5 minutes post bite. Even more noteworthy, there was a great variance between pain levels of different individuals bitten by the same species. Research has shown that venom compositions of individuals amongst the same species can be highly variable, which is further supported by differences in envenomation experiences. This is an indication that venom has not evolved as a predator defense. 
 

               The incidence of late pain is likely caused by the harmful effects of venom over time. The tissue damage and swelling associated with some venoms, as opposed to pain causing enzymes (such as MitTx in Micrurus), are the direct sources of pain in late stages of envenomation.
                Phylogeny seems to be the most likely indicator of early pain in snake bite. New world pit vipers and Elapidae had the most painful early bite responses, though many species have transitioned to no pain or late pain states. This loss of early pain response for some venomous species is another indicator that predator defense is not a likely explanation for the evolution of venom. Two interesting exceptions may be the spitting cobras and some South American Micrurus, though our native coral snake does not seem to share the pain-inducing toxin of its southern counterparts.   
                Further supporting the hypothesis that venom has evolved for prey capture is the loss of venom in some species. Dasypeltis is part of a clade of rear-fanged venomous snakes. While there are vestiges of a venom delivery system, it is being lost through mutation, presumably because Dasypeltis feeds on bird eggs and does not need venom for foraging. Other research has found that many species possess a prey-specific venom toxicity, further supporting the prey-based evolution of venoms.
                While a venomous bite may offer some protection against predators, either as an individual deterrent or through social learning of predators, this seems to be a secondary benefit. The authors mention the “life-dinner principle,” which posits that “defense, where the snake is fighting for its life, should take precedence over foraging efficiency, where a suboptimal strategy would merely result in reduced energy intake.” This research provides an interesting counterpoint to this principle in the strategy of venomous snakes. 

P.S. if anyone is interested in some fun reading on venoms, and some very graphic stories of envenomations, check out Dr. Brian Grieg Fry's 'natural history memoir' - Venom Doc. Here's a free PDF to keep you entertained in your quarantine.
https://static1.squarespace.com/static/55a239e2e4b0b3a7ae106f25/t/5c85885b53450a49a40694d4/1552255122300/Venom+Doc.pdf
               
               



2 comments:

Allison Welch said...

Fascinating! Do the authors address the potential that the experience of pain may differ in humans vs. potential predators of snakes?

Nicole said...

They did mention it, but not delve into any details. Essentially, they theorized that generalized predators would have similar nociceptors, and therefore pain response, as humans. Obviously, there are some specialists adapted to eating venomous creatures, but the authors focused on generalists.