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Snake "Steals" Toad Toxins for Defense Against Predators

Mason Inman
for National Geographic News
January 30, 2007
 
Sometimes the best defense isn't a good offense—it's a good diet.

A new study shows that some snakes can store toxins from toads they eat and use the debilitating chemicals as a defense mechanism.

The toads make the toxins to protect themselves against predators, which learn not to eat the deadly amphibians.

But the Asian snake Rhabdophis tigrinus has evolved a way to cosume the toxic meal safely.

Instead, the snake stores the toad toxins in glands in its neck, making it too poisonous to eat.

These snakes even taunt enemies to attack, according to the new research, which appears in this week's issue of the journal Proceedings of the National Academy of Sciences.

The snakes also pass the toxins on to their offspring to protect them while they are too young to eat toads themselves.

Deadly Meal

Asian snakes are hunted by a number of predators, including hawks, salamanders, and other snakes. But the species responds with stylized movements that take advantage of its stored toxins.

"The snakes bend their head over and expose the glands on the back of their neck," said Deborah Hutchinson of Old Dominion University in Norfolk, Virginia, who worked on the study with colleagues in the U.S. and Japan.

"They also ... do neck-butting, where they actually move the neck toward the attacker."

The snakes are apparently goading predators into taking a swipe or a bite. (Related: "Snake Threat May Have Spurred Evolution of Primate Eyes" [August 10, 2006].)

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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