The venomous fangs of the Amazon puffing snake are a double-edged sword. This South American tree snake has developed a venom with toxins that target different prey – one for killing small mammals like rodents, and another that targets birds and lizards.
“Other tree snakes, when they feed on mammals, they use constriction,” says Stephen Mackessy at the University of Northern Colorado, who studied the venom produced by Spilotes sulphurous. “These Amazon puffing snakes are not very good constrictors, so they’re at a disadvantage,” he says.
But he found that the snakes’ venom gives them a different kind of advantage. Mackessy and his team extracted the venom from three Amazon puffing snakes – no easy feat, as they can grow as long as 2.7 metres in length and their venom flows fairly slowly – and then analysed the toxins present. They also tested the dosage lizards or mammals could withstand using house geckos and mice.
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They found both sulditoxin, which is highly toxic to lizards and birds, but non-toxic to mammals even when the dose is 22 times higher than what would be delivered by the bite of an Amazon puffing snake. They also found sulmotoxin 1, which works the opposite way around: it is lethal to mammals but not birds or lizards.
While other snakes have developed prey-specific neurotoxins in their venom, this is the first time such a pattern has been shown, says Bryan Fry at the University of Queensland in Australia, who was not involved in this work.
Mackessy says that over time, this snake’s prey may have been evolving resistance to its venom. In turn, the snake is evolving new toxins.
He and his colleagues constructed an evolutionary tree to trace both toxins across both front-fanged and rear-fanged snake families.
They found that the lizard-specific toxin likely evolved first, and the mammal-specific one came later. Fry says that makes sense, as lizards are more common prey for these snakes.