Irish researchers have revealed why the venom of some snakes makes them more deadly than others – and how venom has evolved depending on where they live and their most immediate threat in the form of prey.
Snakes are notorious for possessing potent venom, which makes them deadly predators. Species such as cobras, boomslangs and rattlesnakes have far more venom than they apparently need in a single reserve of venom, with the potential to kill thousands of prey animals and several adult humans.
Why venoms vary so much in ability to kill or incapacitate potential prey animals has long puzzled scientists. The marbled sea snake, for example, has only a tiny amount of weak venom, making it harmless to large animals such as humans.
In a study by researchers at NUI Galway, Trinity College Dublin, the University of St Andrews and the Zoological Society of London, published in Ecology Letters, they compare venom potency and quantity for more 100 venomous species.
Evolved
They found strong evidence that venoms have evolved to be more potent toward animals that are more closely related to their diet. They found potency in snakes which mainly eat fish, such as the aquatic coral snake, were highest when measured on fish and lowest when measured on mice, which are distantly related to fish.
The amount of venom a snake has depends on its size and the environment it lives in
Dr Kevin Healy, who conducted research in St Andrews and is now based at NUIG, said: "These results make sense from an evolutionary viewpoint, as we expect that evolution will have shaped venoms to be more efficient at killing the animals that are most often the target of the venom."
The research shows the amount of venom a snake has depends on its size and the environment it lives in. "Like all substances, venom is dosage-dependent," said Dr Andrew Jackson, associate professor in zoology at TCD. "Even alcohol, coffee and water can be toxic at high enough volumes so we needed to consider how much venom different species of snake produce and store in their venom glands."
Tree-dwelling
Big terrestrial species had the most venom, while smaller tree-dwelling or aquatic species the least. “This difference may be due to how often a snake encounters its prey in these different environments, with terrestrial species requiring a larger reserve of venom to take advantage of the rarer opportunities to feed,” he added.
Understanding how venom evolves may help scientists better identify the risks to humans from different snake groups, and also potentially from other venomous animals such as spiders, scorpions, centipedes and jellyfish.
