Bite This: Borrowed toad toxins save snake’s neck

An Asian snake stocks its defensive arsenal by collecting toxins from poisonous toads, scientists report.

The tiger keelback snake stores toxins in glands behind its head, says Deborah A. Hutchinson of Old Dominion University in Norfolk, Va. An attacker biting the snake’s neck bursts the glands and gets a burning mouthful and sometimes a blinding squirt into the eyes.

Earlier studies had suggested that the snake doesn’t make the gland’s toxic contents but harvests those compounds from toads that it has eaten. In a paper published in the Feb. 13 Proceedings of the National Academy of Sciences, Hutchinson and her colleagues report a variety of experiments that confirm that scenario.

“The case is unique in that this is a vertebrate sequestering a vertebrate-prey toxin,” comments Edmund D. Brodie III of the University of Virginia in Charlottesville. In contrast, he says, although the common garter snakes that he studies eat poisonous newts and the poison lingers in their bodies, they don’t sequester it in special glands.

The glands of the poison-collecting Asian snake, Rhabdophis tigrinus, don’t play a role in the snake’s attacks on its prey. In its bite, the tiger keelback delivers slow-acting salivary toxins, which disable a clotting factor in the blood of its victims. People have bled to death from the snakes’ bites, but the snakes seldom deliver a lethal dose. “They really have to chew on something,” says Hutchinson.

The snake’s defensive behavior displays the glands when an attacker looms, Hutchinson says. However in the 1990s, biologist Akira Mori of Kyoto University in Japan found little displaying among tiger keelbacks on the Japanese island Kinkazan. The snakes there tend to flee their predators. Since the island has no poisonous toads, Mori proposed that the snakes need to eat toads to have the defensive toxins, called bufadienolides.

To test that idea, Hutchinson, Mori, and their colleagues analyzed gland fluids in snakes from various places in Japan. The researchers found high toxin concentrations in snakes from zones that contained poisonous toads but no toxins in fluids from snakes from toadfree Kinkazan.

The researchers also tracked gland contents from snakes hatched in the lab. The team found that mother snakes from high-toad zones passed along enough toxins to protect their offspring for at least 2 months. However, when the mother snakes came from the toadfree island, the youngsters hatched without toxins in their glands.

When researchers fed the latter juveniles a toxinfree diet of fish and harmless frogs, the young snakes’ neck glands remained toxinfree. However, such hatchlings fed toxin-bearing toads developed toxic fluids, the researchers report.

“The thing that I found most interesting was the provisioning of offspring by mothers,” comments Brodie.

Sequestering toxins is well known in invertebrates, poison arrow frogs, and a few birds that eat poison-bearing invertebrates, notes Becky Williams of the University of California, Berkeley, who has studied garter snakes. Many snakes eat toxin-bearing amphibians. Williams adds, “I would not be surprised to discover that other snakes like these sequester toxins.”