Toads on the Japanese island of Ishima seem to be losing their evolutionary battle with snakes. Most snakes, and indeed most other animals, avoid eating toads because of the toxins in their skin. Rhabdophis tigrinus snakes, however, not only tolerate the toxins, they store the chemicals for their own defensive arsenal.
Deborah Hutchinson at Old Dominion University in Norfolk, Virginia, US, and colleagues, found that snakes on Ishima had bufadienolide compounds - toad toxins - in their neck glands, while those snakes living on the toad-free island of Kinkazan had none.
The snakes are unable to synthesise their own toxins, so they can only have derived bufadienolide compounds from their diet. Hutchinson's team confirmed this by feeding snake hatchlings either a toad-rich or a toad-free diet. Toad-fed snakes accumulated toad-toxins in the nuchal glands on the back of the neck; snakes on a toad-free diet did not.
"Rhabdophis tigrinus is the first species known to use these dietary toxins for its own defence," says Hutchinson.
Fight or flight
What is more, when attacked, snakes on different islands react differently. On Ishima, snakes stand their ground and rely on the toxins in their nuchal glands to repel the predator. On Kinkazan, the snakes flee.
"Snakes on Kinkazan have evolved to use their nuchal glands in defence less often than other populations of snakes, presumably due to their lack of defensive compounds," says Hutchinson.
Moreover, baby snakes benefit too. The team showed that snake mothers with high toxin levels pass on the compounds to their offspring. Snake hatchlings thus also enjoy the toad-derived protection.
Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas0610785104)
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