These snakes steal poison from their prey – here

Snakes that poison themselves by eating toads seem to know when they are poisonous by keeping track of what they last ate

By Christa Lesté-Lasserre edited by Andrea Thompson

Red-necked keelboat snakes are highly toxic: single drops of their pungent yellow poison could blind a mongoose and stop its heart within minutes. But snakes don’t make this toxin themselves; rather, they steal it from the poisonous toads on which they feed.

After a red-necked keel (Rhabdophis mined) eats a real toad (member of the Bufonidae family), the snake’s intestines absorb toxic bufadienolide molecules from the amphibian’s skin. The toxins are then transported to more than a dozen pairs of storage pouches located in the snakes’ necks, called nuchal glands. So snakes act without fear. They stand up and crane their necks towards mongooses and other potential predators as if to say, “Go ahead, I dare you.”

But this brazen attitude does not last. If the dinner has been non-toxic recently (poison-free frogs or fish, for example), these reptiles often scurry away.

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Scientists believed that these and other Asian snakes that “sequester toxins” somehow assessed their toxin stores to guide their defense choices. Pit Vipers and rattlesnakes that produce their own venoms do so, probably by feeling more or less “satiated” in their glands – although researchers are still working on the exact mechanisms.

To see if this was also the case for toxin sequestrants, Tomonori Kodama, a behavioral ecologist at Nagoya University in Japan, fed non-toxic frogs and toxic 23-keel toads to wild red-necked toads. A few weeks later, he and his colleagues attached them with a moss-covered hook to imitate a mongoose attack. A few days later, the researchers used forceps to empty the snakes’ nuchal glands, then stuck them again with the fake mongoose.

To the team’s surprise, the snakes did not seem to realize that their poison reservoirs were depleted. The animals responded to the attacks with their classic posture, showcasing the neck and daredevil, at essentially the same rate, whether before or after the compression.

The results, recently published in Ethology, suggest that red-necked keels don’t have a direct feedback on their toxin stores—or at least, they don’t act on it if they do, says Deborah Hutchinson, a Seattle-based snake biologist who was not involved in the new research.

Instead, it seems they remember the type of prey they most recently ate, Kodama explains.

Additional research could explain why these snakes don’t guard their reserves, says Kurt Schwenk, an evolutionary biologist at the University of Connecticut. “Perhaps snakes in the field replenish their toxin stores regularly enough that they never become depleted enough for monitoring to be important,” he says.