Red-necked keelback snakes are highly toxic—mere drops of their pungent yellow poison could blind a mongoose and stop its heart within minutes. But the snakes don’t make that toxin themselves; rather, they steal it from the poisonous toads they eat.
After a red-necked keelback (Rhabdophis subminiatus) eats a true toad (a member of the Bufonidae family), the snake’s intestines soak up the toxic bufadienolide molecules from the amphibian’s skin. The toxins are then shuttled into more than a dozen pairs of storage pockets in the snakes’ necks called nuchal glands. Then the snakes act fearless. They rise and jut their necks at mongooses and other would-be predators as if to say, “Go ahead—I dare you.”
That brazen attitude doesn’t last, though. If dinner has been nontoxic recently—poison-free frogs or fish, for example—these reptiles often hurriedly slither away.
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Scientists used to think that these and other Asian snakes that “toxin sequester” were somehow gauging their toxin stocks to guide their defense choices. Pit vipers and rattlesnakes that produce their own poisons do this, probably by feeling more or less “full” in their glands—although researchers are still working out the exact mechanisms.
To see if this was the case in toxin sequesterers, too, Tomonori Kodama, a behavioral ecologist at Nagoya University in Japan, fed nontoxic frogs and toxic toads to 23 wild red-necked keelbacks. A few weeks later, he and his colleagues pinned them down with a foam-coated hook to mimic a mongoose attack. A few days afterward, the researchers used forceps to squeeze the snakes’ nuchal glands empty and then pinned them down with the fake mongoose again.
To the team’s surprise, the snakes didn’t seem to realize that their poison tanks had run out. The animals responded to the attacks with their classic, neck-showcasing, daredevil attitude at essentially the same rate, whether before or after squeezing.
The results, published recently in Ethology, suggest that red-necked keelbacks don’t have any direct feedback about their toxin stores—or at least, they don’t act on it if they do, says Deborah Hutchinson, a snake biologist based in Seattle, who wasn’t involved with the new research.
It seems that they might instead remember what type of prey they most recently ate, Kodama says.
More research could expose why these snakes don’t monitor their reserves, says Kurt Schwenk, an evolutionary biologist at the University of Connecticut. “Maybe snakes in the field replenish their supply of the toxin regularly enough that they are never sufficiently depleted for monitoring to matter,” he says.