There are species of snakes that derive toxins from prey for their own defence against predators. Rhabdophis tigrinus is an oviparous Asian snake that has unusual defensive glands on the neck (nuchal glands), which typically contain toxic bufadienolide steroids that the snakes sequester from consumed toads. A paper published in Journal of Zoology shows how this species may exhibit population-level variation in their chemical arsenal, reflecting the availability of chemically defended prey in their habitat. The study compared the chemistry of the nuchal gland fluid of R. tigrinus from two Japanese islands: Ishima, where toads are abundant, and Kinkasan, where toads are absent. The results showed that captive-hatched juveniles from toad-rich Ishima Island that had not been fed toads still possessed defensive bufadienolides in their nuchal glands, presumably due to maternal provisioning of these sequestered compounds. Wild-caught juveniles from Ishima also possessed large quantities of bufadienolides, which could result from a combination of sequestration of defensive compounds from consumed toads and maternal provisioning. Captive-born hatchlings from Kinkasan Island lack bufadienolides in their nuchal glands, reflecting the absence of toads on that island, but they can still sequester bufadienolides by feeding on toads in captivity.
It is likely that contact between the irritating nuchal gland fluid from these snakes and the eyes or mouth of its predators, such as raptors, giant salamanders, raccoondogs and other snakes, will serve as an effective deterrent. Therefore, the presence of large quantities of bufadienolides in the nuchal glands of R. tigrinus on the Ishima Island could provide an effective chemical defence and deterrent against predators, whereas R. tigrinus on the toad-free Kinkasan Island may experience more intensive predation due to the lack of defensive compounds in their nuchal glands. Indeed, this is probably why the Kinkasan snakes typically flee from perceived predatory threats as they are less well defended against predator attacks.
by Linda DaVolls