Non-Venomous Asian Snakes 'Borrow' Defensive Poison from Toxic Toads
 

The Yamakagashi snake (Rhabdophis tigrinus) steals toxins from the toads it eats, according to a study published in the Proceedings of the National Academy of Sciences this week. After eating the toad and siphoning the toxins from the toad’s skin, the snake harbors the stolen toxin in glands on the back of its neck. When it is under attack, the snake arches its neck, and moves the poison glands toward the predator, in a move called “neck-butting.”

Researchers have long known that Yamakagashi snakes like to eat toads; on a tongue flick test, which is how scientists gauge a snake’s food preference, these snakes seem to favor toads over other foods. (They flick their tongue more for toads than fish, for example.) But this study suggests that toad appeal is not just a matter of taste.

Researchers first wondered if the snakes stole the toxin when they observed behavioral differences in snakes that lived with toads, compared to those that didn’t. The snakes living in toad-free places, such as the island of Kinkazan off of Japan, were less aggressive than toad-eating snakes—fleeing fights more often, and less frequently displaying the defensive arched-neck.

"The behavioral differences helped form the sequestration hypothesis,” says Deborah Hutchinson, a researcher at Old Dominion University in Norfolk, VA and lead author on the paper.

In this study, Hutchinson and co-authors confirmed that the snakes that didn’t eat toads had no toxins in their glands, explaining their propensity to flee; while the toad-eating snakes had levels of toxin that varied with the amount of toad in their diet.

How does the toxin get from the toad’s skin to the glands on the snake’s neck? The researchers know that the glands are infused with a dense cluster of capillaries, suggesting that the toxin is transported through the blood. But, Hutchinson says, “It’s not known whether the toxins are absorbed directly across the lining of the mouth or whether that transport takes place in the gut.” Hutchinson adds: “we’re not sure what it is about the glands that grabs onto the toxins and concentrates them.”

The study revealed not only that these snakes steal, but also that they pass the poison to their snakelings. The mother snake, called a dam, includes a concentrated dose of toxin in the yolk, which provides nutrients to her un-hatched snakes. Preliminary evidence also suggests that dams can pass the toxins late in the pregnancy, right through the leathery shell of their soon-to-be snakeling’s egg.

Link to Article