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An Indian kraits snake (Bungarus caeruleus). They are notorious for biting sleeping people at night. While highly lethal, the bites are so painless that they are often dismissed as trivial until it is too late. This indicates no defensive function for this venom. Credit: Wolfgang Wuster
It is estimated that every year, over 100,000 human deaths can be attributed to snakebite from the world’s 700 venomous snake species – all inflicted in self-defence when the snakes feel threatened by encroaching humans. However, new research from a joint Bangor/Swansea team concludes that snake venom did not evolve as a defence mechanism.
Senior Lecturer Dr Wolfgang Wüster, a world-renowned expert on snake venom evolution at Bangor University’s School of Natural Sciences, explained:
“We know that snake venom is used primarily for foraging; for overpowering and killing prey. However, we also know that snakes use their venom in self-defence - that's why so many people get bitten, and sometimes killed, by venomous snakes worldwide. We wanted to investigate whether defence was a driver in venom evolution.”
To provide efficient defence from a predator, the snake venom would need to provide sufficient instant pain to deter the predator and enable the snake to escape, much as a bee-sting hurts immediately.
A new paper published in Toxins reveals that surprisingly few venomous snake bites cause immediate pain, implying that the venom make-up has not evolved for a defensive primary purpose.
Bangor University student Harry Ward-Smith tested this hypothesis under Dr Wüster’s supervision. He gathered online survey responses from reptile keepers, herpetologists and fieldworkers, as part of his undergraduate degree in Zoology with Herpetology.
His survey, which gained nearly 400 responses world-wide, asked people working with snakes about the time-course of pain after bites they had sustained.
The results of around 600 experiences revealed that only a minority of bites and species regularly cause rapid, severe pain. Moreover, where early pain evolves during snake evolution, it is likely to be lost again in descendant lineages.
Dr Kevin Arbuckle, Senior Lecturer in Biosciences (Evolutionary Biology) at Swansea University’s Department of Biosciences and a co-author of the paper commented:
"Our results suggest little evidence for widespread evolution of venoms driven by their use in defence, though interesting exceptions likely exist such as the defensive use of venom 'spitting' in some cobras, and these specific cases deserve further study."
Dr Wüster added:
“Even though we might have expected defending your life to be more important than feeding, it turns out that natural selection for diet does seem to be the main driver of venom evolution in snakes”
Harry Ward-Smith said:
“I’m proud to be in the minority of scientists who published their undergraduate work; writing this up took a lot of work even after graduating.
I hope studies like this start to encourage further research into the function and natural ecology of snake venom, particularly rear-fanged venomous snakes which we still know so little about.”