The Wonderful World of Snake Venom
Venom is not unique to the snake world and has evolved many times throughout the animal kingdom, but the evolutionary origins can be shrouded in mystery. The great irony with snake venom is that it has been highly studied for its medicinal properties, but the question of why it popped up has been largely ignored until recently. In a recently published paper researchers Harry Ward-Smith, Kevin Arbuckle, Arno Naude, and Wolfgang Wüster talk about their exploration into the evolutionary history of snake venom.
Their study began by looking at venomous species from other branches of the animal family tree, how that venom is utilized, and what might indicate the primary function of the venom. In species that evolved venom for defensive purposes, like honey bees and most venomous fish, the injection of their venom is accompanied by an almost immediate pain provided by specific toxins in the venom itself. Now, this phenomenon has also been observed in animals that use their venom for both prey capture and defense, like centipedes and scorpions, but the key thing here is the presence of toxins that have no other function than the stimulation of pain receptors in the envenomated creature. Of course, pain responses are subjective and will vary from case to case, but defensive venom at least inflicts a spike of pain immediately after injection which would allow for the venomous creature to escape from potential danger.
Looking at the snake world, one will find many examples of venomous species, as well as a dizzying array of defensive adaptations that have evolved to help snakes avoid predation. Some of the most recognizable defenses also happen to be seen in venomous species, like the hood display in cobras, rattling in rattlesnakes, and the vibrant coloration of coral snakes. These displays are often interpreted as a warning of the danger hidden in the mouths of these animals by nature programs on the television and researchers alike. But biting and injecting venom into a potential predator puts the snakes in more direct danger, and is generally an absolute last defense if all else fails, which led to the development of the study.
The basic hypothesis that was being tested was that venom developed primarily (or even partially for defense) would inflict an almost immediate intense pain that would follow a similar pattern of intensity even when taking subjective pain tolerance into account. To test this, the researchers surveyed professionals that routinely handle venomous snakes to determine if and when the pain from the venom became “too distracting for normal activity” where a time of five minutes or less would be defined as “critically early.” A small portion of respondents indicated intense pain in the “critical period” and nearly double responded that their intense pain did not develop until much later. Interestingly, when they compiled the results, the researchers found that a majority of the respondents never experienced intense pain and only the venom of two groups of snakes consistently produced early pain; new world vipers and the Elapidae family.
The new world vipers have toxins that break down tissues, which does correlate with more pain, but the pain in the respondents was highly variable without a consistent pattern. The Elapidae family, however, tends to have highly neurotoxic venom which also correlates with more pain, but the spitting cobra specifically is the only snake with a purely defensive use for their venom. It is also worth noting that some species of snakes in venomous families that have specialized in feeding on things like eggs have subsequently lost their venom, which is evidence that the primary function was for prey capture.
Ultimately, the conclusion that was made in this study was that it is highly unlikely that snake venom evolved primarily for defense. Of course, much more research is needed to conclusively determine the evolutionary reasons for the development of snake venom, but defense does seem to have likely been a happy accident in the serpent world.
Reference:
