Venoms from snakes are often associated with intense pain. This is one of the strongest reasons why many people fear them.
However, a study led by scientists from Bangor University (United Kingdom), in collaboration with European and Australian institutions, reveals that, at least in this case, the venom of European vipers did not evolve to cause pain.
The team analysed the venoms of four viper species – Vipera ursinii, Vipera ammodytes, Vipera aspis and Vipera berus – to try to answer a question that has long remained unanswered: how do these snakes balance the defensive function of venom (to protect themselves from predators) with the function of capturing prey (which requires the rapid immobilization of the prey)?
Pain makes victims writhe in agony, which may increase the likelihood of the predator being injured, and seriously, and of the prey escaping. On the other hand, venom designed to subdue prey may not serve as a strong warning to the snakes’ predators.
“We expected to find evidence that some vipers evolved venom components specifically to cause pain quickly, as a defence mechanism,” recalls Wolfgang Wüster, the study’s lead co-author.
“Instead, we did not find any evidence that venoms directly activate pain-sensitive nerve cells. This suggests that, in European vipers, venom evolution is, primarily, driven by the need to subjugate prey,” he explains.
The snakes’ venom glands can produce only specific types of venoms depending on the species, which means that some produce venoms with particular functions and effects and others with different ones.
The researchers began the study convinced that vipers feeding on animals with fewer defences, such as insects, might have developed toxins to produce pain for defence.
Not needing a large venomous effort to subdue the insects they feed on, these snakes may have oriented their venom toward protection against predators and other threats. The opposite would be expected for species that feed on prey that can cause more damage to snakes, such as rodents, with sharp claws and teeth.
However, that is not what they found when testing the venoms of the different European vipers in the laboratory on pain-sensitive cells. Regardless of their diet, none of the venoms of these reptiles evolved specifically to cause pain.
Thus, the authors of this work conclude that natural selection gave priority to hunting efficiency over the production of pain as defence during the evolution of European vipers’ venoms.
“This challenges the idea that defensive and predatory functions drive venom evolution in different directions in snakes,” says Bálint Üveges, the article’s first author. “Our results indicate that prey capture is the dominant force shaping venom composition in these species.”
