Tiger snake venom is so effective that it stopped evolving more than 10 million years ago, because it just didn't need to get any better for its target. It's been more or less the same ever since.
The venom is a particularly unusual case in snake evolution. Ordinarily predators and prey take part in an arms race, with the predator – in this case the tiger snake – evolving a weapon to kill its prey, and the prey evolving to resist it.
But tiger snake venom hasn't been part of an arms race for more than 10 million years, finds a study by researchers at the University of Queensland. That's because its target – a clotting protein called prothrombin – plays such a crucial role in the blood that it's virtually impossible for it to be changed.
The blood clotting cascade is made up of a very long and elaborate series of enzymes that function in perfect balance. When genetic mutations happen that upset the cascade, you quickly end up with a potentially life-threatening condition, such as haemophilia. This means that proteins like prothrombin are under immense evolutionary selection pressure to stay just as they are.
Tiger snake venom taps into this. Once the ancestor of the snakes gained the mutations that let it target prothrombin, the prey was unable to evolve resistance – that would involve upsetting the careful balance of the clotting cascade system.
A further benefit of attacking prothrombin is that it is shared across a huge range of animals, giving tiger snakes a very versatile weapon.
"Our clotting cascade is incredibly conserved, more so than any other part of our physiology," study author Bryan Fry told IBTimes UK. "It's shared almost identically between many species – whether from amphibians to birds and mammals."
As well as being an extraordinarily unusual evolutionary case, the findings have positive implications for treating snakebite, too.
A total of 16 populations of tiger snake across southern Australia were sampled for the study and all of them share this same venom. Three other groups of snake closely related to the tigers also share this venom, indicating it was their common ancestor that originally evolved the strategy. This means that antivenom that works on one type of tiger snake will work on all types – and three other groups in addition to that.
This could make treating certain types of snakebite easier in the clinic, when there is limited time to save someone's life. Although there have been no clinical trials to test this, the finding confirms anecdotal reports from doctors who treated snakebite patients with tiger snake antivenom because no others were available, with positive results.
"Because the venom isn't changing, the antivenom will work against this entire group of snakes," Fry said.
"It's a great evolutionary story, with venom locked down on ice because its target is on ice. So we could use this type of antivenom on anything that has this kind of toxin."