But an attack could break the snakes' skin near their neck glands and release the toad toxins, potent steroids called bufadienolides.
A large dose can stop the heart of a would-be predator, while a lower dose likely would affect the predator's muscles and nerves, slowing it down.
Unique Adaptation
There are many frogs and a few birds that get their toxins from their diet, but the source is usually invertebrates such as insects.
This Asian snake is the first vertebrate known to eat another vertebrate's toxin and save it to use in its own defense.
The adaptation is a potent survival tool, according to the new study.
Another population of the same species of snake lives on toad-free islands, which separated more than 10,000 years ago when sea levels rose at the end of the last Ice Age.
Since then, the toxin-deprived snakes have evolved into scaredy cats.
Rather than taunting predators, these snakes flee, the researchers show.
Family Heirloom
In addition, while a mother snake is carrying eggs in her body, she can pass toxins on to her offspring, the study demonstrates.
This gives the young enough of a protective dose of the toxin to last them until they can start eating toads on their own.
"That's the coolest part of the study," said Edmund Brodie III, an evolutionary biologist at the University of Virginia.
Instead of having to pass on genes to the hatchlings, mother snakes can give them toxins directly, a system that in theory is simpler to evolve, Brodie said.
"It makes it a lot easier to evolve toxicity when you've got a maternal effect."
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